| Shoog |
I am about to recieve some 6080's and some 6AS7's and I was just starting to think what I might do with them.
I had a crazy idea to build a 2 tube PP amp. The idea would be to have a beefy single pentode driver stage (choke/CCS loaded) and then to do the phase splitting by taking the signal for one of the 6080 triodes from the cathode of the other. In this way it would be possable to build a complete stereo PP amp with just 2 driver tubes and two 6080's. If there wasn't a suitable driver pentode, how about using a ECL86 as the driver and maintaining the two tube approach.
What do you think of my chances of success. I understand that the 6080 and the 6AS7 can be very variable, even within the same envelope. I also understand that cathode bias is mandatory and that these tubes can bias up at wildly different voltages - sounds like a bit of a long shot to get this working, but what do you guys think of my minimalist idea?
Shoog |
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| Fuling |
| Nice idea, but I believe self splitting PP output stages works best with tubes that has a little higher µ that 6AS7 |
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| kevinkr |
Yes, that would be my experience with the 6080/6AS7 as well. Might try an interstage transformer and cathode bias the output tubes independently of each other. Some means of trimming the operating point of one against the other is advisable. A single 6080 in pp should be capable of at least 7wrms..
Fixed bias is much more efficient than cathode bias in terms of required HT (HV) but is risky due to warm up issues and the variability of the tube's transconductance making it very sensitive to minute variations in grid bias voltage. A mix of both might be a good compromise.
I believe James, Sowter, and several others make suitable driver/splitter transformers.
Aim for a pk-pk swing of at least 300V to assure that you can drive them to full power.
Several of my friends have built pp amplifiers with these tubes using talema, ulveco or similar toroidal power transformers for the output transformer. As I recall a model with two 120V primaries and two 18V secondaries works well to match to an 8 ohm load, and they're cheap.
Connect the primaries in series, with center tap connection, and connect the secondaries in parallel. If you want to match a 4 ohm load I think a 12V secondaries is close enough.
This equates to a primary Z of about 1.42K ohms plate to plate to an 8 ohm load. This trick probably won't work as well with EI types due to the much higher leakage inductance so use a toroid.
Kevin |
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| Yvesm |
Just an idiot idea not yet checked :
Almost no DC in the OPT so toroîds should be usable.
Yves. |
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| Shoog |
I was toying with the idea of using toroidals as outputs, but was a little concerned about DC with such variable tubes. Seperate cathode resistors would seem a must.
I saw listed on a datasheet a completely different operating point for these tubes. 135V anode, 250R cathode resistor, giving a current of 125mA. This means that the grid is at -35V which presumably would lessen the variability of bias. Since my speakers are 4ohm, I would need a 800ohm load which would mean about an 18V:240V which coincidentally is exactly the toroidals I have. The driver design would presumably be much more straighforward at these voltages, how about an improved futterman style driver with the cathode returned to the output.
Good to get some feedback.
Shoog |
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| Yvesm |
Hey Shoog,
I'm not so far from that you suggest.
It's designed so that each half 6AS7 draw 100mA and has 115V Vak, bias being 40V.
This already set the plate dissipation at near 12W.
This sleeps in cartons for some month, I plan to build a preamp based on the same topology but with smaller 6n6p triodes before, but I can't tell when :rolleyes:
Yves. |
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| Shoog |
It looks interesting and sort of the same, but isn't it running as something like a SEPP. Are the phase splitter and first stage two halves of ECC83, as this maintains the 2 tube philosophy.
I like the idea of running mine at just 135V because I have a big toroidal power transformer which would probably do power supply duty. Mine would be running at just over 0.5A in total.
When the output tubes grid is at -35V, whats the drive voltage which the phase splitter need to supply (70V PP maybe) ? This would mean that the phase splitter would need at least double that across it, so at least 140V.
How would I go about biasing this into more class AB territory. I have never done PP before so don't know about things like this.
Shoog |
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| Shoog |
How about using a mini power pentode to do current amplification, putting it through a toroidal interstage transformer with a 1:10 Voltage ratio. This would do the voltage amplification and phase splitting and would drive 6as7 directly. Simple and elegant.The only issue that would arise would be stability issues if global feedback was needed.
Shoog |
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| kevinkr |
I wouldn't use a 1:10 step up unless the rp of the driving tube was extremely low as this equates to a 25 fold increase in source impedance to each of the driven grids.. (say 3K rp to 75K effectively at each grid) Mr. Miller would come to town and you would be lucky to have any response above 10kHz.
The designs I have seen all use the toroid as a conventional pp output transformer. Bear in mind the low voltage configuration is going to give you somewhat fewer watts as well. Some means of adjusting dc balance may be advisable. |
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| Shoog |
Good point on the impedence step up. I was thinking that by running a driver tube at 40mA, after stepping up I would still be left with about 7mA to drive the grids of the 6AS7. This would allow me to use the same +B and a low gain driver tube. I would have to go down a different rough and use a a higher voltage input to the transformer and step down the voltage to the grids instead of stepping it up. This would require a voltage doubler at least to get the required voltage to the driver.
I have used toroidals as outputs and they work well. I don't see why they wouldn't work as interstages. The difference between them and a proper (expensive) interstage transformer is that they wouldn't accept DC and so need a RC stage. Other than that there hi bandwidth should make them ideal. I can't imagine that they would produce a worse result than any of the more conventional tube based phase splitters, and there slight imbalances. The only way to find out is to try them.
The output tubes have a 100R pot to balance them.
Shoog |
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| Shoog |
Had another idea. What about making the driver a cathode follower. Give responsability for voltage gain entirely to the interstage transformer. This reduces the output impedance to acceptable levels. I could use another 6080 to do the driver duty, which would be nice because then we are down to a 3 valve amp. Linearity would be less of an issue when run as a cathode follower. However 2.5A heater current is a hard one to swallow for just a driver.
How about a triode strapped EL86, which has a low Ra and is happy to run at relatively low voltages.
Shoog |
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| kevinkr |
| I think a triode strapped EL86 is a good idea and one possible approach, perhaps an ECC99 or 6H30 would be another. (dual triodes both) |
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| Yvesm |
HI,
You should apply drive to the upper tube between its grid and its cathode, not relative to the ground.
So, using an interstage tranny, one winding should be referenced to ground, (for thr lower triode) and the other to the cathode of the upper one.
In the idea I showed, this is done by returning the plate load of the phase spiltter to the output and having the primary of the OPT returning to B+.
So the concertina plate is "bootstraped" and enjoy from an apparent supply voltage almost twice the B+.
This IS the trick ;)
Yves. |
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| rcavictim |
| quote: | Originally posted by Shoog
G
The output tubes have a 100R pot to balance them.
Shoog |
I'm sure a 1R pot between the tubes should have enough angular resolutioin to provide easy balance adjustment. |
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| Shoog |
I've been thinking about this to much today (my head hurts). Using the full 10x voltage gain of the interstage would require 100mA of drive current to produce 1mA at the grid of the 6as7, which is not good really. So if I go for 5x voltage gain from the interstage we get a more reasonable current requirement from the driver (I think 20mA for the same 1mA at the grid). However this now isn't enough voltage gain for my needs, so the idea of a cathode follower needs an additional stage with a gain of about 5x. Doing this we could still have everything hunky dory with the "dream of a two tube PP, by using something like a ECL82 or ECL86 to drive it. The ECL82 I have, and can be made reasonably linear by applying plate to plate feedback between the two stages.
Am I on the right track here? How much current do I really need on the grid to keep the top end.
Shoog |
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| Shoog |
ECL82 is the way to go. Taking the output from the anode, but applying local plate to plate feedback, we have an output impedance approaching that of a cathode follower, but with the advantage of the gain we need. Infact we could probably reverse the phase splitting to reduce the the voltage rather than increase it, and still have enough voltage gain.We also have lower distortion to boot. The only thing there would be is that it would definately need a higher +B for the driver to achieve this.
2 tubes, perfect phase splitting, only one cap in the signal path. Sounds as if it might even be good.
Shoog |
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| Dave Cigna |
Personally, I wouldn't use any step-up at all in the IT. The chances of it working well are slim.
I think a single 6SL7 would be able to produce the necessary voltage swing when driving a 1+1:1+1 interstage with your supply voltage of 135V with plenty of voltage gain. It would probably not be able to do it if loaded by plate resistors, but the IT puts (nearly) the full supply voltage on the plates.
Connect the cathodes of the 6SL7 together and put a simple CCS under them. You would need a negative supply of -10V or so, but it only needs to deliver a few milliamps.
The grids of the 6080 are not hard to drive because the Miller capacitance is low. |
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| panos29 |
| I used the following schematic with excellent results, very dynamic with good and fast slam and very analytical mid and highs, power was around 6 watts limited by driver swing. On my horn poaded Tannoys 6 w were more than enough. |
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| Shoog |
I have the solution.
A 5687 driver with CCS or Choke load. Works quite happily at 120V Anode and 15mA.
Put this through a 55V:110+110V to do phase splitting and 2X voltage gain. This leaves me with 7mA drive for the grids of the 6080, which should be more than adequate.
So three tubes, two CCS and two interstages. One coupling cap. I will probably have a seperate valve rectifier for the 5687 to get the best out of the critical driver stage.
Shoog |
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| rcavictim |
Shoog,
You have to derate the voltage on that 60Hz transformer you are planning for an interstage if you want response below 60Hz. Dropping the winding voltages to 1/2 will get you to 30 Hz. Dropping to 1/3rd gives 20 Hz.
I think you can do this with a 6SN7GTB on a separate HV supply using the first stage as input gain and the second stage as a concertina splitter and driver. |
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| Shoog |
The transformers will be 50hz rated and are 50VA. Since I will be driving them with about an 18V signal off the 5687's with only about 15-20mA per transformer I don't think there will be issues with saturation. There may be issues with HF roll of due to the high inductance of the driver side - though I doubt it.
I don't think anyone has documented trying this approach so I am voyaging into uncharted territory. It may or may not work, but I don't think I will lose much by trying (apart from the cost of 2 transformers). It has an elegant simplicity which appeals to me, though of course it may be to simple. I am more concerned with resonant tanks than anything else. sizing the coupling cap is a real suck it and see scenario.
Shoog |
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| rcavictim |
OK, sounds like you have this covered. I think the Z of a 50 volt winding will be totally too low for use as an IT if plate coupled but you are right, using an IT does end up with a simpler looking schematic than other methods.
The beauty of DIY is that you can find this stuff out yourself and get a practical and valuable education along with the fun, all at the same time. In rare instances you even save money. .) |
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| Shoog |
I am going RC coupled to the interstage so the 5687 will have its own load ala parafeed.
Shoog |
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| rcavictim |
That won't solve your Z problem. Your efficiency will be in the toilet, and the distortion up. Don't try to sell ME on it though. You are the one this is for. Go for it. You have uncharted territory (to you) to explore.
If it was my house, I'd be trying the 6SN7 as I suggested above without iron. |
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| Shoog |
"That won't solve your Z problem. Your efficiency will be in the toilet, and the distortion up. Don't try to sell ME on it though. You are the one this is for. Go for it. You have uncharted territory (to you) to explore."
Excuse my ignorance if I am wrong, but is my thinking on this correct.
If I go for the maximum grid circuit resistance of 1meg ohm for a 6080, and I am using a 1:2 ratio I get a reflected resistance of 250K. If this is in parallel with a CCS I should be loading the 5687 by about 240K which is a good load is it not.
If my logic on this one is wrong can you explain to me how, so that I can learn.
Shoog |
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| rcavictim |
| quote: | Originally posted by Shoog
"That won't solve your Z problem. Your efficiency will be in the toilet, and the distortion up. Don't try to sell ME on it though. You are the one this is for. Go for it. You have uncharted territory (to you) to explore."
Excuse my ignorance if I am wrong, but is my thinking on this correct.
If I go for the maximum grid circuit resistance of 1meg ohm for a 6080, and I am using a 1:2 ratio I get a reflected resistance of 250K. If this is in parallel with a CCS I should be loading the 5687 by about 240K which is a good load is it not.
If my logic on this one is wrong can you explain to me how, so that I can learn.
Shoog |
If you need a mathemagical explanation I'm not your best tutor. I do a lot of thinking like this with my hands which I can use to pick up my handy reactance chart. It works for me. For any given frequency as you increase the number of turns and increase L, you also increase the AC impedance. For a fixed number of turns on a given core, as you lower frequency the Z will drop. The plate circuit of your driver stage wants a pretty high plate load Z, otherwise the dinky amount of plate current available won't be capable of creating a sufficient voltage drop across the coil.
Forgetting about frequency or any loading, if you have a plate swing of 100 volts at 10 mA, that primary winding has to look like 10K ohms. Now consider bass frequencies which require the most L for a given current. Looking at my handy reactance chart I see that 10K Z= a 30 Hy coil at 60 Hz. To swing 100 volts at 20 Hz from a 10 mA source would require about 85 Hy's.
As a power xfmer, if your 50 volt winding is rated at 50 watts at 50 Hz that is 1 amp. According to Ohms Law (not R's Law) that is a Z of 50 ohms. According to my handy reactance chart that coil is about 0.16 Henries. You are way on the short side of not enough L.
By cap coupling this winding to the driver tube plate you will get whatever voltage across it that can be developed with 10 mA. At 50Hz that would be about 0.5 volts. The waveform will be extremely distorted. Now add secondary windings and loading and this gets even worse. Try to reduce the frequency to 20 Hz and it gets even worse again.
You might be able to use this xfmer as an interstage but it would have to be cathode driven by a tube with a low output Z. Maybe another 6080 with sections parallelled. |
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| Shoog |
Good explanation. I have learn't. Does the same thinking apply to output transformers. I have successfully used toroidals as SE parafeed output transformers, and the bass response is satisfactory.
Shoog |
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| rcavictim |
Absolutely the same applies to output xfmers, both directly in the plate circuit DC path or cap coupled only to the AC component. With a power stage however the plate Z is much lower, so lower L may be tolerated.
My PSET 6080 amp design has a really low plate Z with 8 triodes in parallel. I discovered that a certain PS toroid I found at a surplus store was great with two (120 volt primaries in series, low voltage secondaries in parallel) as an output xfmer for this amp, so I bought up as many as I could, enough to make a few of these amps eventually. I use two ~80 VA cores (~160 VA) at 60 Hz to handle 20 Hz at about 20+ watts with negligible waveform distortion. |
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| Shoog |
I put my hands up and declare, I am enlightened of my ignorance. So the games up and the idea has to be abandoned.
But I am not keen on spending €200.00 on interstage transformers, and I'am not keen on using tube based phase splitting. I am not ready to abandon the idea yet.
But wait a minuite, there is a factor of any parafeed circuit which can be turned to our advantage. A parafeed circuit has a tuned resonance, usually we tune that resonance to be as near to DC as possable, but it is possable to tune it up and use it as a bass boost. This is quite common practice in parafeed output stages. The fequency of the resonance is dependent on the primary inductance, the parafeed capacitance and the output impedence of the driver circuit.
The primary inductance is fixed.
The capacitance can be varied, but is a pain to fiddle with and it might require a electo capacitor.
The output impedence of the driver is seemingly fixed, but on closer examination it is possable to varie it by using a two stage driver and applying partial feedback between the plates. So starting with a ECL82, we can apply the plate to plate feedback with a 200K pot, which would allow us to tune the resonance by ear to somewhere around 10-20Hz.
Sounds like a possablity. Care to express an opinion.
I just went back and compared my Partial Feedback parafeed SET amp to my best gainclone, and did notice that the bass was definately rolled off. So I applied the above principle and reduced the 190K Plate to Plate feedback to 115K. Result and significant improvement in frequencies bellow 50hZ.
Unfortunately this is not an option I have with getting the mains toroidal output transformers in my proposed PP amp working. I may just have to look for a 1K2 PP output transformer.
Shoog |
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| Shoog |
I decided to try an experiment to see how a transformer would perform. I put a 10V signal into a little 30VA RS toroidal with a ration of 24V:110V and applied the signal to be stepped up. I loaded the secondary with a 470K resistor.
I swept a signal down from 20Khz down to about 10hz. There was a slight fall in response down from the KHZ range, but no significant drop off below 100hz. The sine wave came through clean.
I also applied a square wave signal and got sever spiking and ringing, in the order of 3 times the main signal. This is to be expected when a secondary is so lightly loaded. Hopefully the grid capacitance and a bit of capacitance over the secondary would calm this.
I also tried a much larger toroidal with a ratio of 48V:240V. This showed much more pronounced drop off in the bass, with a -3db point at about 20hz. The sine wave signal was heavily distorted below 20hz, and lightly distorted below 50hz.
Anyone care to duplicate my experiment to confirm my findings.
Just my finding. Makes me think the little toroidals will work if the ringing can be calmed. Anyone else got any useful observations.
Shoog |
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| Shoog |
I went back and checked my figures. The input voltage was nearer to 6V. Also I have just learned that the -3db point is at 0.707 x the midrange voltage, and not half as I though. I therefore have to correct myself and say that the -3db point is at 10hz. This seems comparable to most commercial interstage transformers.
I will get some 110+110V:55+55V tranformers. This will allow me to configure them as either a 1:2+2 voltage stepup which should give me an input sensitivity of about 1V to drive the 6080's fully using a 5687. This will only put about 18V onto the primary which is well bellow their 55V rating.
Or I can configure them as a 1:1+1 which should give me an input sensitivity of 2V to drive the 6080's to full ouput. Again I will only be putting 36V onto the primary which is 1/3 of there rated voltage.
If this does show to much low frequency roll off I will replace the 5687 with ECL82's with plate to plate feedback so that I can tune the resonance as I suggested before.
I am prepared to take the risk that this won't work out, for an outlay of only €35.00 on transformers. I will do measurements when I do, so that I can establish whats going on. It might be a few months before I have any results. I will build it with mains toroidal outputs and if they show to much low frequency roll of I will look for better.
Shoog |
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| Shoog |
Hi there,
I just thought I would give a status report on how the project has been going.
I have built the power supply. I can report that a microwave oven transformer performs very well as a filter choke and doesn't seem to saturate at up to 1/2 amp of DC current. I have a final +B of about 115V which is a tad shy of what I would want. The choke has a DCR of 100R so it drops about 50V all by itself. Unfortunately my DMM has died and so I am having to do readings from my scope.
I built the cathode bias network. The 6AS7 is very variable, I am using individual resistances for each cathode with a paralleled 1K plus a 1K pot for bias adjustment, this gives me a an adjustment range of about 280R to 330R, which is barely enough to get currents balanced. May need a rethink on this.
Got the toroidals for the interstage today. They are 0-55V,0-55V:115V,111V at 50VA. I used my signal generator to do some basic tests,with a 6Vpp input signal (unknown current). With the secondaries in series to give a 1:1+1. I tried the "created" primary wired in one direction and got terrible results. It rolled off below 200hz and rolled off at the top end at about 30khz, before rising again after 40khz. There also seemed significant ringing. Not good at all I though.
I then tried reversing the primary connection. This produced markedly different results. There was no real low end roll off down to 10hz, at the top end the -3db point was at about 20khz and it continued rolling off from there without any rise. With a sine wave the signal came through increadably cleanly. With a square wave it stayed good down to about 200hz and then went somewhat triangular. At the top end the leading edge was rolled off after about 10khz (no ringing at all surprisingly). Of course I will be putting in at least 6 times this amount of voltage in the final design, so things may not be quite so rosy, but things look very promising so far. Also there was no load on the secondary, but also I would say that the signal generator would have nothing like the current output of my 5687 driver.
Will keep you all posted.
Shoog |
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| Andrewbee |
| Since you are far along with your design I thought I would post this for you:angel: PP 6AS7 |
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| Shoog |
Progress report.
Got one channel lashed up on the breadboard. Had the radio playing through it. There are a few problems though. Seems not to have anywhere near enough gain (Ie unity voltage gain). Also the interstage transformer is performing reasonably well, but has a dip in response at about 10KHz and rises after that. This is not good.
The main issue I am having though is with my CCS loaded 5687 drivers. I am using SY's jiggy CCS. I had it set up originally with 125V at the top of the CCS, giving me a plate voltage of 100V and a bias point of 3.7V and about 9mA of current. This caused sever clipping with anything more than about 0.5Vpp of input. So I tried taking the driver supply off before the PS choke, which gave me about 165V on the top of the CCS, the plate voltage has now risen up to 135V and the clipping is still there. Don't know how to force the CCS to consume more voltage. If I can't get this working I will have to try resistive loading.
Shoog |
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| ErikdeBest |
Hi Shoog
If I understood Morgan Jones book (valve amplifiers, 3rd edition, page 137) well you can set the anode voltage of the tube by looking at the plate curves (pretty logic, actually).
For an anode voltage of 100V at a 9mA draw you need the grid to be biased at a little less than -4V (something around -4,2V). MJ uses a zenerdiode to accomplish it. What voltage are you measuring at the cathode?
If I am wrong, there will be sufficient people around to correct me.
Erik |
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| Shoog |
There is a just over 4V on the cathode. The CCS just doesn't seem to want to pass that little current so, whatever setting I have on the CCS's adjustment pot the voltage stays stubbornly at about 140V(or higher). I have since discovered a few wiring errors in the CCS, but correcting them still doesn't change this 140V fact. 25V seems to be the most I can get it to drop.
Shoog |
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| Ryssen |
| quote: | | but correcting them still doesn't change this 140V fact |
wasnt some parts destriod during the wireing errors then.:) |
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| ErikdeBest |
Hi Shoog
I have no idea other than fiddling with the bias adustment. Set the CCS for the desired current, put a pot across the cathode resistor (if you are using one) and check if there is a change in plate voltage when adjusting the pot. At least it is easy to try...
Erik |
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| Shoog |
"wasnt some parts destriod during the wireing errors then."
No I checked the integrity of all components. If one of the transistors had failed the voltage across the CCS would have collapsed to about 15V rather than the 25V it has at the moment.
" have no idea other than fiddling with the bias adustment. Set the CCS for the desired current, put a pot across the cathode resistor (if you are using one) and check if there is a change in plate voltage when adjusting the pot. At least it is easy to try..."
I don't think this will work. The reason is that there are a range of plate voltages which can marry to the cathode voltage, the plate voltage seems to want to drift sufficiently high to collapse the correct functioning of the CCS. I have it working at the moment, but its still clipping, but not so badly as before.
I have LED bias with two red LEDs, I tried shorting out one of the LED's and it had little effect on the plate voltage.
Any more suggestions would be appreciated.
Shoog |
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| Shoog |
I worked out what was wrong. I had installed the transistors as if they were NPN's (ie with the Emitter and the Collector reversed). Fortunately the transistors hadn't suffered. Everything is now working. Since the Red LED's have a voltage drop of 2.1V I took out one from the reference chain. I also replaced the 2K2 with a green LED.
The plate voltage now adjusts itself to the correct point for the current set.
So it is now playing and sound good(ie not horribly distorted). However there are still a few problems. There is a resonance centered on 70hz which seems to have little to do with the parafeed cap. its about +3db.
Now that everythings working, overall it has a voltage gain of 2x which suggests a further driver stage.
The main problem is with the 5687 driver and the interstage transformer. The -3db point is at about 5Khz, this is with about 15mA through the 5687. I didn't expect transformer capacitance to be the limiting factor, rather the bottom end flaking out - which is fine.
Really the transformer needs about 30mA to drive its capacitance. The other problem is that the 5687 has headroom problems. With the 165V on the top of the CCS and a current of about 15mA, the plate has about 105V on its plate, this clips at about at about 110Vpp which gives a final output of about 15Vpp. Also in order to try to increase the current though the 5687 I drop the bias point to 2.1V, which clips well before full output. Therefore the 5687 could do with 150V on its plate with a grid bias of -4V. This would require about double the +B I have.
I would also say that the operating point I have chosen for the 6080 can only give about half of its potential output power (though i'am a little uncertain of the relationship between peak to peak input signal and bias points).
Still the circuit would be servicable if I could find a way of doubling the driver tubes +B without introducing another transformer. Is there a way of creating a voltage doubler off the same secondary as the main supply.
Shoog |
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| Shoog |
Heres my latest thinking. The 5687 just won't hack it, it just can't give enough gain and current without melting.
So re-enter the ECL82, not the most ideal candidate because of it excessive gain - but I have some so I will try. My ideas to use the ECL82. Split the load on the anode of the triode to reduce the 60x gain by half. CCS load the pentode at about 35mA, but triode strap it. Run everything off the 165V supply. Attempt to use the interstage for a 2x step up( which will lower the -3db point by about a half octave at the top). Rap a bit of global negitive feedback around the interstage transformer back to a split cathode resistor in an attempt to flatten its response. Try a bit of plate to plate feedback from the triode to the triode strapped pentode.
Any thoughts on this as a way to go.
Shoog |
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| ErikdeBest |
Hi Shoog
Why are you planning to reduce the gain of the triode and than using a step-up interstage trafo? Something with maximum swing or so? - I am quite a newbie..
Bob Danielak uses an ECL82 to drive an EL509.
http://www.geocities.com/bobdanielak/technoteNo33.html
With some adaptations it would probably do a nice job driving an interstage trafo for PP 6AS7.
still so much to learn...
Erik |
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| Shoog |
The gain would be at least 300 if untamed. The reason I want to try stepping up is because it should allow me to generate a bigger output voltage without clipping the driver stage.
Shoog |
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| ErikdeBest |
I understand it! A 165V supply really does not allow a lot of swing without clipping.
Sorry I can't be of much help - I am mostly learning from this thread.
Erik |
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| Shoog |
That OK.
The only thing I don't like with the ECL82 is that it only allows a bias point of -1.5V for 1mA through the triode - which means that its easy to clip the triode. Also with only 1mA through the Triode its not easy to apply much plate to plate feedback. If plate to plate feedback is used its possible to make an anode follower respond much like a cathode follower.
By the way uping the current through the 5687 to 25mA does improve the upper response. So far all I can get out of it is about 3.5W of power before clipping.
Shoog |
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| Shoog |
This is my design for the interstage transformer design using the ECL82. Any comments are welcome.
I had some ideas about how to overcome the limitations of the torodial interstage transformer approach. The first and obvious solution is to lower the output impedence of the driver triode. This will allow it to better drive the interwinding capacitance. Choosing a triode with high enough gain and low enough output impedence would be a problem. Alternatively simply push 40mA through the driver, but again achieveing this with only 170V of supply would be tricky.
Another solution which adds another dual triode would be to use a transformer with a step down ratio, this will have two benefits. It is likely to have lower interwinding capacitance. Secondly the step down ratio will increase the effective impedence of the load presented to the driver, by that ratio. The disadvantage is that we then need to add another pair of triodes after the transformer in order to regain the voltage we lost in the transformer. If we use a dual triode for this - mismatch shouldn't be a real issue. This could potentially introduce more distortion as we have introduced another gain stage. This however is probably the best solution overall as it stops the interstage transformer becoming the limiting factor.
The third solutions is what I am attempting, which is to lower the effective impedence of the driver so making it more able to drive the interwinding capacitance. There is plate to plate feedback which should level out the frequency response (the loading down at high frequency is present at the plate of the driver and doesn't appear within the transformer). it will also make the whole stage behave more like a cathode follower- ie lower output impedence. This is local feedback so should be stable. There is also global negative feedback which should help to reduce any distortion introduced within the transformer, and also reduce the output impedance. If the gNF proves unstable it will be removed - but the plate to plate is the important form of feedback. Because the CCS consumes 70V it should allow a voltage swing of 60V.
If this proves inadequate I will attempt a step up arrangement. If I can get the response flat to 10Khz I will be reasonably happy.
Another reason that I like my solution is that it maintains the two tube per channel idea.
Shoog |
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| Shoog |
Hi there,
I lashed up the ECL82 driver today.I made one or two simple changes. The split cathode on the triode is now made up of 630R on the top and 470R on the bottom. The anode load is 22K at the top and 33K at the bottom. Works really well. For a 2.5Vpp input I am getting about 8Watts of output. The pentode is producing about 110Vpp output which is about its theoretical limit with 65 volts over the CCS( (65-10)*2=110). At this point the input stage clips. It would therefore be pointless to try to get more output from the input triode as it would drive the pentode into clipping.
The frequency response has gone up from -3db at 4khz to -3db at 8khz, and response is sustained well beyond that point. The 70hz response peak is still there, but maybe slightly reduced. The plate to plate feedback is working and stable, the cathode feedback is working and stable. Sine wave response is good until about 8khz when it starts to look a bit odd. Square wave response seems to be a little more spiked.
There is one strange aspect in that the 6080's are giving about 2/3 of their grid input voltage as output. For some reason they are not even achieving unity gain. The cathodes are unbypassed. Would bypassing the cathodes solve this or is there another reason for the loading down. If I bypass - what value should I use?
Shoog |
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| coresta |
| Hi again Shoog , my past experiences with PSU transformers pushed me to move to OTL , especially with 6080 :D These are very affordable , in good nos cheap versions . Just find 6 of them for each chanel and .. enjoy :cool: |
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| Yvesm |
Probably too hi cathode degeneration ?
The sum of cathode resistors is appoximately equal to the anode load !
And the same current flows thru both.
If you drive'em with a dual widings IST, you could reference (AC only ! ) the 'cold' side of each winding to the respective cathode rather than to the ground.
If you can do that, you could also move the OPT between the 6080 cathodes and put a common bias resistor at CT. :xeye:
I wonder if this kills Miller caps ?
Yves. |
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| Shoog |
I think I have the solution.
The optimum plate to plate load for the operating point I have chosen is 1000ohm. The load I infact have is 1300ohm. This would account for a drop in output of about 1/4 or 1/3.
I like the idea of referencing the centre tap of the interstage to the respective cathodes of the output tubes. This presumably would introduce a bit of positive feedback for a bit more gain. Am I right ?
I have done similar on the cathode return from the primary side of the interstage and its driver pentode.
I am now getting about 8watts out in pure class A. Am I right in assuming that 50% efficiency is the max I can expect from a class A Push Pull. If so then this circuit as efficient as it can get without pushing it into class AB.
Shoog |
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| Yvesm |
| quote: | Originally posted by Shoog
I think I have the solution.
The optimum plate to plate load for the operating point I have chosen is 1000ohm. The load I infact have is 1300ohm. This would account for a drop in output of about 1/4 or 1/3.
I like the idea of referencing the centre tap of the interstage to the respective cathodes of the output tubes. This presumably would introduce a bit of positive feedback for a bit more gain. Am I right ?
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At least, this remove negative feed back from unbypassed cathodes.
| quote: |
I have done similar on the cathode return from the primary side of the interstage and its driver pentode.
I am now getting about 8watts out in pure class A. Am I right in assuming that 50% efficiency is the max I can expect from a class A Push Pull. If so then this circuit as efficient as it can get without pushing it into class AB.
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Look acceptable to me !
Yves. |
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| Shoog |
On the basis of that here is the final circuit, minus Yves mod, which I will try on the hoof.
If anyone has any useful suggestions on how to improve this design I am all ears.
Shoog |
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| Shoog |
I just wanted to correct some statements I have made.
The drive to the 6080's is 60Vpp which gives a final output of 5.5Vpp. This should give about 7.5Watts of clean output in 4ohm load.
The measurements I stated before were down to problems with interpreting my scope. I apologies for any misunderstanding.
Shoog |
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| kevinkr |
Did you mean 5.5vpk or 5.5Vpk-pk, there's a world of difference between the two.. :D By my calculations 5.5Vpk into 4 ohms is about 3.75Wrms..
I recommend either adding cathode bypass capacitors or alternatively a bipolar/non-polar cap between the cathodes of the 6080. Right now your unbypassed cathodes are reducing gain to less than 1 and also increasing rp to something like 1.3K if I haven't goofed the calculation.
You'll get at least twice the power, better bass extension due to the lower effective rp, and less distortion for the same reason at low frequencies.
I suggest trying a cap across the 6080 cathodes because in theory this will make the output pair a cheesy LTP and should result in some additional cancelation of even harmonics, and the net dc voltage difference will be small as will be the ac voltage across the cap as long as Xc is relatively small compared to the resistance present at the cathodes. I would think a 63V-75V cap would be adequate here which is good because it will need to be big to be effective - several hundred uF minimum. Something to try anyway, you may or may not like the results.. Bass performance would probably be better with straight cathode bypass caps. |
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| Shoog |
Thanks for that analysis. I was think that I was going to add cathode bypasses. I did a little experiment by adding 1000uf caps. The gain went up to just less than unity and the sound seemed to open up somewhat. Because of the high dissapation through the 330R cathode resistor I had to use wire wound, which seems to effect the high frequency response. I will bypass with 1000uf 63V electros paralleled with 10uf 63V polys.
Starting drilling the case. Will be about a week before I can test anything again.
Shoog |
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| Sheldon |
I'm looking at a hybrid design with an interstage phase splitter. Since you are cap coupled, you don't need a transformer, an autoformer will do. Should be easier to get wide response, especially if you can go 1:1 with the drive voltage. How about a center tapped choke? Connect both driver and opt grids to each end, R load across the ends, and ct to ground or bias.
Sheldon |
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| Shoog |
Sheldon,
The autotransformer idea sound interesting. Care to sketch me a plan as i'am having a little difficulty visualising your scheme.
The only thing I would be concerned about is the unused primary of the toroidals I have. In my small experience an unterminated winding tends to ring with large intermodulation distortion as a result.
Shoog |
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| Sheldon |
This shows the general idea. I think Electra-Print has some phase splitter autoformers. Others may have them as well, but might as well try the toroids.
http://www.philcoradio.com/tech/audio.htm
Sheldon |
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| Shoog |
"Probably too hi cathode degeneration ?
The sum of cathode resistors is appoximately equal to the anode load !
And the same current flows thru both.
If you drive'em with a dual widings IST, you could reference (AC only ! ) the 'cold' side of each winding to the respective cathode rather than to the ground.
If you can do that, you could also move the OPT between the 6080 cathodes and put a common bias resistor at CT.
I wonder if this kills Miller caps ?
Yves."
Tried that. Bad results. The current through the 6080 went up considerably, dropping the plate voltage availble to 80V. Overall much distortion and heat. May have done something wrong. took it out. Added cathode bypass to the 6080's.
In its case and final test configuration tried. I reduced the plate to plate feedback resistor to 75kohms. At 25mA driver current i have flat response down to 15hz (gone by 10hz) and now it extends flat to 30khz and tails off slowly to about 50khz. Very satisfactory. Can't tell you its final output power as I have only tested it on crappy speaker which distort badly above a few watts. Sine wave goes through absolutely clean. Square wave has a bit of overshoot and goes triangular above about 15khz. Still I am happy with the overall performance. The whole output stage amplifies what the driver stage gives it without adding any significant amount of distortion, and thats without any global feedback. Got about 5x voltage gain at this stage.
Still got plenty of case work to do so will be a few weeks before its completely finished and on test in my main system.
Got some nice russian coke bottle shape 6H13C and ECL82's.
Will keep you posted. |
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| Yvesm |
| quote: | Originally posted by Shoog
"If you drive'em with a dual widings IST, you could reference (AC only ! ) the 'cold' side of each winding to the respective cathode rather than to the ground.
If you can do that, you could also move the OPT between the 6080 cathodes and put a common bias resistor at CT.
I wonder if this kills Miller caps ?
Yves."
Tried that. Bad results. The current through the 6080 went up considerably, dropping the plate voltage availble to 80V. Overall much distortion and heat. May have done something wrong. took it out. Added cathode bypass to the 6080's.
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Hi Shoog,
I've enlighted in red what I suppose to be the cause of the failure :xeye:
Yves. |
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| Shoog |
Thanks for the tip.
That would require a blocking cap in the secondaries return leg - which I didn't have. What sort of value would you think. What benefits would you expect to achieve as a result. If its down as low as 0.47uf I suppose that it would behave better than the cathode bypass cap which is 1000uf + 10uf.
Do you think it would work with the cathode bypass, or would it be better alone?
I am quite willing to try again if I can expect an improvement.
Shoog |
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| Yvesm |
Yes, a blocking cap, in any leg, hot or return.
A grid leak resistor to ground is necessary of course.
The cap and the grid leak resistor form an hi pass, as in conventional cap coupled amp, here perhaps 0.5µ and 200K ?
The "good" is that this replace cumbersome and always critical cathode bypass.
The "bad" is that you no longer can push the 6080 in AB2.
To be checked :cool:
Yves. |
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| Shoog |
"The "bad" is that you no longer can push the 6080 in AB2."
Mine is working in purely class A so its not really an issue.
By going down this path you would be losing one of the supposed advantages of an interstage transformer, namely an ultra low resistance earth reference for the grids, and the elimination of overload recovery of the blocking cap should the grid go positive. Though i'am not entirely certain the grid can draw any DC current from the interstage anyway.
On balance I think the cathode bypass might have fewer evils and allow the interstage to perform at its best.
Shoog |
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| Shoog |
Moved the amp into the main system for final testing. Initial impressions are very favourable. Bass is deep and well defined and the top end is clean and well balanced. Good wide sound stage. I listened to some acoustic music first and I was impressed at how natural and detailed it all sounded. Putting on some Godzilla and the bass impressed. However if driven hard it did sound a little harsh on the top end. Could be overdriving my speakers. I will have to scope it up to find out whats going on. Kate Bushes Arial sounded fantastic.
I would almost go as far as to say that it sounds better, in almost all departments, than my 807 Parafeed SEPP amp. I will do a switch back tonight to confirm this.
O-yes very little hum to speak of, less than the SEPP
Shoog |
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| Shoog |
I just did a side by side comparison between the new amp and my 807 SEPP amp. I was staggered by the difference. The SEPP sounds good and clean but the bass just doesn't have any punch and definition to it comapared to the 6080 amp. I took the SEPP down to my bench to see what was happening on the scope. The frequency response of both amps was roughly comparable, with the SEPP going down to a respectable 15hz and up to 50khz without significant roll off. Under 70hz and the SEPP was showing significant crossover distortion where as the 6080 was showing now.
This doesn't explain the lack of punch though. The only thing that the SEPP might have over the 6080 amp is slightly cleaner highs. Though the SEPP went into distortion much earlier than the 6080 PP.
I can only attribute this qualitive difference to the intrinsic difference between PP and SE. On the strength of what I have found so far I will have to improve the 807 SEPP by a huge margin or decomission it and build a 807 PP instead. The 6080PP sounds fantastic.
Shoog |
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| ErikdeBest |
Hi Shoog
Congratulations with the good results! SY will be in heaven knowing there is another one converted from SE to PP!
Erik
PS. I received the small mosfets for your version of the FVP5 buffer , but at the moment my whole 'hobby' room is a mess because of some changes in placement of tables and so... DOnt'even know where i put the mosfets... :cannotbe: |
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| Shoog |
I'am glad that your going to give my preamp circuit a try. I hope it works out as well for you as it has for me. Any questions just ask. Do you have any other valve preamps which you can compare it to. I recently built a CCS loaded 5687 preamp. The FVP5 clone is significantly better, though the 5687 could probably be optimised more. I will be very interested in your results and inmpressions.
I was thinking about what I could do to improve the SEPP amp. It occurred to me that probably the best thing would be to convert it to a Class A tetrode 807 PP amp. This would give me a 36.5W output for about twice the input power. The power transformer I have has both a 900V-0-900V winding and a 350V-0-350V winding. This would happily supply supply the 600V plate if choke loaded. The second winding could happily supply the 300V screen supply. Exactly the same driver stage could be used as the 6080 PP amp, so retaining the benefit of the interstage transformer. The valve complement would go up by two overall. Because I would have a bit of +B to burn, I could implement a garter bias arrangement. This uses twice the cathode resistance to balance out current draw between the PP valves.
Practically the only thing I would need to replace would be the 10K a-a output transformers.
The only thing is I don't really need 36W of output power, or the 8x more gain !
Shoog |
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| ErikdeBest |
| quote: | | I'am glad that your going to give my preamp circuit a try. I hope it works out as well for you as it has for me. Any questions just ask. Do you have any other valve preamps which you can compare it to. I recently built a CCS loaded 5687 preamp. The FVP5 clone is significantly better, though the 5687 could probably be optimised more. I will be very interested in your results and inmpressions. |
Hi Shoog. I am building an active crossover http://www.glass-ware.com/tubecircu..._Crossover.html with bipolar supplies. I have physically separated the crossover circuit from the buffer. So it will be possible to combine a certain buffer with a certain crossover. So far I have concluded some crossover circuits and some buffer circuits (with PCF80, the triode as buffer, the pentode as CCS.) I will make this one play first, and when it works fine (I am trying to make a 'universal circuit', but I have still to test some issues) I plan to compare buffer circuits: yours and the buffer circuit from the Aikido are the first that come to mind, but others are also possible (all will be running from from bipolar power supply).
now let 's get back to work
Erik
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| Shoog |
Hi there all,
I discovered why the SEPP wasn't performing to its best. Some time ago I had configured it as UL. On most material this was fine, but when there was heavy bass the OT was saturating below 70hz. I have now reversed this mod and things are a lot better in the bass region.
I SEPP has all of the classic SET virtues (clean sweet mids and treble) but I still have a gut feeling that the overall balance of the 6080PP is better.
Shoog |
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| ja2dhc |
I had a crazy idea to build a 2 tube PP amp. The idea would be to have a beefy single pentode driver stage (choke/CCS loaded) and then to do the phase splitting by taking the signal for one of the 6080 triodes from the cathode of the other.
I have made 6080/6AS7 single tube PP. I used two pentodes for the driver/phase splitter. See my Web page below.
http://ja1cty.servehttp.com/6080/6080pp.png
http://ja1cty.servehttp.com/6080/
Sorry written in Japanese. |
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| Shoog |
I like your idea, its how I started at the concept stage of my PP amp. However I have to tell you that there are a few good reasons why it won't work particularly well. The 6080 is a very low MU valve and the self splitting arrangement you are suggesting really needs a medium MU valve to work satifactorily. It might work but you should consider it a SE amp with current compensated PP output transformer (ie the second tube is simply there to keep the PP output transformer from saturating). It might work and and it might not - my hunch is not. If it worked it would behave much more like an SE amp but probably with a punchier bass. If you go for it then seperate LM317 CCS in the tail of each output triode would be essential, in this case the capacitor coupled differential output stage becomes really attractive.
The other problem you will face is that the 6080 is a very hard driven tube, all but the meatist driver stage will sacrifice the top end. You might just get away with driving a 6AU6 hard into it but most small signal pentodes will wimp out, but again only experience would tell. Maybe a EL84 as a driver would work, but it seems a bit of a waste.
I finally dropped the ECL82 from my 6080PP amp. It just wasn't a happy fit and tended to sound a bit harsh. I replaced it with a parallelled 5687 with CCS anode load. I run this at a wopping 40mA. This still only gives 20mA per grid of drive current per output grid. Doing anything with a 6080 has to be considered as a brute force exercise. Well worth it in the end though.
If you are interested I can send you a copy of the schematic for the final version of the amp, I consider it a very optimised version of what is possible with this tube. Drop us an email.
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
all but the meatist driver stage will sacrifice the top end. You might just get away with driving a 6AU6 hard into it but most small signal pentodes will wimp out, but again only experience would tell. |
I built a parafeed 6080, using a 12at7 driver. At first, I had the 12at7 with 100K plate load, but the 12at7 can't drive the 6080 satisfactorily.
I don't have any other tube that is better than the 12at7, so I lowered the B+ for the 12at7 and used plate choke for the 12at7, sounds a bit better. In the parafeed circuit, the 6080 is very promising!
I am still in the hunt for a better driver for the 6080.
Would a triode strapped 6688 work? |
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| Shoog |
| quote: | | Would a triode strapped 6688 work? |
Couldn't find much data, but doesn't look a very good current champ. I would say not then.
I was toying with the idea of building a choke loaded parafeed SE 6080 amp. Only two dual triodes for a stereo amp. Horrendous weight in iron though !!!!!
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
Couldn't find much data, but doesn't look a very good current champ. I would say not then.
I was toying with the idea of building a choke loaded parafeed SE 6080 amp. Only two dual triodes for a stereo amp. Horrendous weight in iron though !!!!!
Shoog |
My parafeed 6080 is not that heavy. I used a 2.54cm stack of 76 EI core for the plate chokes and a 50VA 230V pri, 12V sec Talema toroidal for the OPT. The power transformer is also 76 EI core with 5cm stack height. In all, I have EIGHT transformers in the amp, (power, psu choke, four plate chokes and two OPTs), but none of them is bigger than a 76 EI core. I wonder how it would sound with an interstage? :devilr:
I got the idea of using toroidals from Shoog's post of using toroidal for OPT. |
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| Shoog |
I have a pair of Microwave Oven Transformers which I was already using as chokes - so they would be pressed into service rather than splashing more cash. These are big and heavy.
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
I have a pair of Microwave Oven Transformers which I was already using as chokes - so they would be pressed into service rather than splashing more cash. These are big and heavy.
Shoog |
Yes those are HEAVY!
If the circuit does not work, donate it to the Cunard Line for boat anchor... :D
Shoog,
Thanks for your work on the toroidal OPTs, the toroidal OPT on my parafeed works better than the EI cored transformer! :D |
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| Shoog |
| quote: | | Thanks for your work on the toroidal OPTs, the toroidal OPT on my parafeed works better than the EI cored transformer! |
Where you using main power EI's or proper SE transformers. The main transformers perform very poorly in this capacity.
I was thinking that if I did build this amp I could use dinky little 30VA transformers as the outputs. What voltage and current are you running your 6080's at. I have found they work very well at low volts high current. How is it sounding and have you got any other SE amp to compare it t.
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
Where you using main power EI's or proper SE transformers. The main transformers perform very poorly in this capacity.
I was thinking that if I did build this amp I could use dinky little 30VA transformers as the outputs. What voltage and current are you running your 6080's at. I have found they work very well at low volts high current. How is it sounding and have you got any other SE amp to compare it t.
Shoog |
I was using an EI mains transformer at first for parafeed OPT, then switched to a toroidal mains transformer, and big difference.
I also built a 12b4 parafeed using an EI mains transformer for OPT, bass and treble rolloff (but still sounds good). I might get some more toroidal mains transformer to use for OPT on this circuit.
I have built Mikael's SE KT88 amp and I think I like my 6080 parafeed better. The vocals on the 6080 parafeed have more clarity, highs are better, but the SE KT88 has a bit better bass (not a lot better, just a bit better).
I am thinking of building a parafeed EL34 early next year.
:D |
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| Shoog |
| quote: | | I am thinking of building a parafeed EL34 early next year. |
From what I gather the EL34 will have a mighty hard job matching up to the sound of the 6080. I personally wouldn't bother.
Your efforts would be better spent on a 6080 PP with interstage phase splitting. This can be done cheaply with main toroidal outputs and phase splitters. I guarantee you won't be sorry you did. The bass will be a huge step up on the SE 6080. Alternatively lash it up with the self splitting to start with and see how it sounds - if its not as good as hoped add in the interstage phase splitting later.
Shoog |
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| Steve Cresswell |
Hi Guys
I've only just caught this thread
Here's a schematic of the current version of my 6AS7 push pull amp.
It uses a transformer phase splitter on the input, a differential voltage amp and driver stage that runs at a very high voltage but low current.
The output stage is also a differential amp sitting on an EL34 constant current sink and feeds a custom wound Sowter 2k5 A-A output transformer.
The EL34 sink was changed this morning for an ITT 12E14 series pass regulator tetrode and it sounds very good indeed: dynamic, clean and detailed with a large soundstage and a tight, extended bass.
The whole scheme took a lot of thought and experiment over about a six month period before finally settling on the high voltage/ low current drive scheme. It is the total opposite to most drive setups, but that is because the 6AS7/6080 has quite different drive requirements than other more "sensible" choices of valve.
I get away with the low current because the very low mu and gain of the 6AS7 does not multiply up it's Miller capacitance to anything like the degree that most power triodes do: and of course chiefly because there are no step-up interstage transformers in the amp. The high voltage drivers negate the need for them.
The whole thing was inspired by Gary Dahl and Lynn Olsen's Amity PP project.
Each section has its own power supply; the power valves and tube constant current sink supply being solid state rectified, cap input PSUs and the voltage amp driver stages tube rectified with a CLC input using a pair of 8mA 150H Hammond chokes in parallel.
Steve |
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| Shoog |
Very nice looking amp.
I have a few questions;
What is the voltage at the anode of the EL34 ? How do you find current balance over time. Are you using a EI output transformer ?
Have you tried any other type of CCS in the tail of the first two stages? I have used the LM317 but only fully bypassed. I was lead to believe that the its impedance was to low at high frequencies.
What voltage swing are you getting out of the second stage? Are the grids of the 6080's loading this stage down at all.
What voltage is across the 6080's, you seem to be running them a lot more conseratively than I am. What power output are you getting?
I was thinking of moving my input phase splitting to the front end with an input transformer as you have, and then turning my paralled 5687's into a differential pair. However the overload behaviour would likely suffer if I removed the interstage transformer driving the grids of the 6080's
Shoog |
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| Steve Cresswell |
Hi
I'm getting around 55V at the anode of the EL34
The current balance seems to be spot on.
I'm currently looking into adjustable grids so that the voltages can be balanced between the sections. However there is little DC offset at the optx primary and the amp sounds so good that I can't seem to be bothered to mess about too much.
The output trafos are EI cored from Sowter, capable of handling 250mA
The LM317 CCSs I'm using are unbypassed to keep the Ra as high as possible on both stages of the driver circuitry.
Because I'm only operating the plates of the second stage at 2.75mA each the flat load produced by the CCS is capable of allowing the plates to swing from approx 20V to 500V with no problem. Halving this for the two sides and then adding back together at the output of the stage means I can get around 480V peak to peak.
The driver stage is cruising at 300V p to p when it is under full drive
I certainly can't detect any loading down of the driver by the grids of the 6AS7s. As I mentioned the whole idea behind the way the amp works is about hi-voltage drive, which is only possible in this instance due to the lack of interstage transformers, which of course need current if they are going to overcome the Miller capacitance caused by the transformer magnification.
The whole philosophy of the amp is to exploit the apparent disadvantage of the low gain of the valve by arranging the drivers to swing lots of volts without the need for accompanying high current. This keeps the noise right down to almost silent operation at idle.
There is about 120 volts across the valves and yes they are running very conservatively. Though many people would argue otherwise, I am convinced I can swing lots more volts through the outputs, without entering class AB because of the flattening load effect of the pentode CCS.
In fact with the amp being differential end to end It was necessary to be extremely careful at each stage of the design that there was no possibility of any of the stages entering grid current or straying out of class A operation even for an instant. Under these conditions the clipping behaviour would be extremely nasty.
So to summarize the main design considerations
a) I needed massive voltage swing to get the 6AS7s to move without an interstage transformer between.
b) there had to be strictly enforced class A operation throughout the amp if it was going to be differential from input to output otherwise overload operation would be nasty. This by necessity would limit output power but it was a compromise worth making in this particular instance.
Running the driver stage at high voltage took care of the first consideration
The low current operation went a long way to addressing the second, made possible, ironically, due to the low gain of the 6AS7 itself. As for power output, all I know is that it will go very loud indeed with no signs of distress.
The way this thing has been put together has caused a lot of raised eyebrows on another forum but the bottom line is it works beautifully. It seems to possess much of the treble air, space and midrange presence of a single ended amp combined with the bass slam of a push pull amp.
Here's a pic of the amp in situ
Steve. :) |
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| Shoog |
It look lovely, much nicer than mine. I see you also have the Russian coke bottle valves - nice sounding but a bit softer than the RCA's which to my ear sound the best. I also have some Mullards which are nice, but a bit thinner on the bass.
The only thing I'm still a little puzzled over is the need for the high voltage driver stage. In mine the driver is running off a 170V B+ with a CCS on top of the 5687's. This gives me a swing of about 60V. This is more than adequate because I have the 6080's biased at 30V. Of course my 6080's are running at 100V and 100mA so I am effectively dragging twice the power output out for about the same voltage across the 6080. I can't imagine that your second stage is ever asked to deliver more than a fraction of its potential voltage swing - which I suppose keeps it nice and clean.
As you observe the use of an interstage transformer massively amplifies the Miller capacitance, and I push 40mA single ended into the phase splitter transformer - this gives 20mA into each transformer/grid.
I run my amp for about 18hrs per day. I found that my first set of 6080's lasted about 1yrs before they fell so badly out of balance that the amp started to oscillate. Because I am using toroidal output transformers I can not tolerate any DC imbalance and therefore I put LM317 CCS's in the tails of each triode section. I keep it differential by capacitively coupling the cathodes together.
I have zero noise issues just as yourself.
I know what you mean about people not coming on board with these unusual designs, there is a lot of prejudice against the 6080 and running these valves at low voltages. Still thats their problem not ours.
My next amp is going to be based on Gary Pimms Tabor amp, but there are some design issues which I have been scratching my head over for a few months and they just won't quite resolve themselves !
Good work.
Shoog |
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| Steve Cresswell |
| quote: | Originally posted by Shoog
The only thing I'm still a little puzzled over is the need for the high voltage driver stage.
I know what you mean about people not coming on board with these unusual designs, there is a lot of prejudice against the 6080 and running these valves at low voltages. Still thats their problem not ours.
My next amp is going to be based on Gary Pimms Tabor amp, but there are some design issues which I have been scratching my head over for a few months and they just won't quite resolve themselves !
Good work. |
Thanks for the comments Shoog. :)
The driver stage as you have observed is merely cruising at normal listening levels. I deliberately over engineered it so that there was no chance of it straying out of class A or running into grid current. Also the design keeps the drive signal nice and clean.
I have read a lot of comments about the 6080 saying "why bother with it?'
Well it is cheap and easily obtainable for a start. If one is prepared to put in the design work to support its requirements then it can be made to sound superb much better than its on-paper performance would suggest.
Good luck with the new amp.
Steve |
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| alexg |
| quote: | Originally posted by Shoog
From what I gather the EL34 will have a mighty hard job matching up to the sound of the 6080. I personally wouldn't bother.
Your efforts would be better spent on a 6080 PP with interstage phase splitting. This can be done cheaply with main toroidal outputs and phase splitters. I guarantee you won't be sorry you did. The bass will be a huge step up on the SE 6080. Alternatively lash it up with the self splitting to start with and see how it sounds - if its not as good as hoped add in the interstage phase splitting later.
Shoog |
Thanks for the idea, Shoog.
I have done a self-splitting 832A PP amp and tried interstage splitting with the same amp, I found no sonic difference between the two, so I removed the interstage and use self splitting (less parts).
You suggested mains toroidal for PP OPT, what size should I get? I can also have a custom PP OPT wound.
How about this: B+ for the 6080 of around 250VDC, 75ma across the 6080, 80V bias, self-splitting, driver is 12at7 at 200VDC B+, plate choke loaded. Toroidal mains for OPT or a custom wound PP OPT.
Thanks. |
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| Shoog |
| quote: | | I have done a self-splitting 832A PP amp and tried interstage splitting with the same amp, I found no sonic difference between the two, so I removed the interstage and use self splitting (less parts). |
Interesting comment. I have a quad of EL86's for just such a project I will have to build it.
I would guess that the 832A probably has higher gain than the 6080, which would make it a better candidate for self splitting.
| quote: | | How about this: B+ for the 6080 of around 250VDC, 75ma across the 6080, 80V bias, self-splitting, driver is 12at7 at 200VDC B+, plate choke loaded. Toroidal mains for OPT or a custom wound PP OPT. |
I get about 5.500hrs out of my 6080's at 10Watts. 12.5Watts is a little harder run than I would choose.
If you can get custom wound toroidals then go for it.
I still think the 12at7 is a little weak for the job.
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
If you can get custom wound toroidals then go for it.
I still think the 12at7 is a little weak for the job.
Shoog |
I can't get custom wound toroidals. I can get an EI core wound for a PP OPT.
In lieu of the 12at7, what would be a nice driver? Or should I go for a two stage driver?
Thanks. |
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| alexg |
I have not done this topology before, I will look into it.
Thanks. |
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| Miles Prower |
| quote: | Originally posted by alexg
I have not done this topology before, I will look into it.
Thanks. |
I used the differential cascode for my last project. In that case, I used 6BQ7As with a CCS as an active tail load. It works just great: enough gain to support gNFB without having to add a second gain stage. Low distortion as well.
However, you have to be very careful with the layout since this is a high gain design, and is more susceptable to going unstable. That means make certain you keep all leads as short as possible, just as if you were doing an RF amp. Also, that glass ware schemo is missing a bypass cacacitor across R2. Since the upper triode is operating as a grounded grid, you need to make the grid-to-ground impedance as low as possible, otherwise, it'll oscillate. |
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| Shoog |
I would say that either of the two schemes would be more than suitable for a high voltage low current design using the 6080.
However if you decided to follow the approach I took, as in a low voltage high current, then a different type of driver is needed. Somewthing with a medium Mu and higher current capability. The ECC88 family would be a good start (though I prefer to run these at below 8mA for long lifes sake) or the 5687 makes an exccellent candidate. Another good option for a two tube stereo parafeed design might be the 6H30 which is both meaty and good.
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
I would say that either of the two schemes would be more than suitable for a high voltage low current design using the 6080.
However if you decided to follow the approach I took, as in a low voltage high current, then a different type of driver is needed. Somewthing with a medium Mu and higher current capability. The ECC88 family would be a good start (though I prefer to run these at below 8mA for long lifes sake) or the 5687 makes an exccellent candidate. Another good option for a two tube stereo parafeed design might be the 6H30 which is both meaty and good.
Shoog |
I did try your approach, 160V B+, 100V on the plate of the 6080, biased at 31V, 90ma through the tube. Am still using my 12at7 driver (with plate loaded with a choke).
Much better sound.
I don't have a 5687 nor a 6H30, but I will try to get them and use them.
Shoog, thanks for the low voltage, high current tip on the 6080. Makes a whole lot of difference.
Thanks. |
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| Shoog |
| quote: | | Shoog, thanks for the low voltage, high current tip on the 6080. Makes a whole lot of difference. |
Good to hear positive confirmation on that.
Shoog |
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| alexg |
| quote: | Originally posted by Shoog
Good to hear positive confirmation on that.
Shoog |
Thanks Shoog.
There is just one thing though, the power is a bit lower now. I will try to increase current across the tube. |
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| Miles Prower |
| quote: | Originally posted by Shoog
However if you decided to follow the approach I took, as in a low voltage high current, then a different type of driver is needed. Somewthing with a medium Mu and higher current capability. The ECC88 family would be a good start (though I prefer to run these at below 8mA for long lifes sake) or the 5687 makes an exccellent candidate. Another good option for a two tube stereo parafeed design might be the 6H30 which is both meaty and good. |
How about a Source Follower? |
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| Jeb-D. |
| This project sure has changed since the original, minimalistic idea. :D |
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| Akita |
| For the Troid OPT 110+110:18+18V? What is the speaker impedance you connected? |
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