Some years ago I decided that I wanted to build a Valve amp. Perhaps I was overconfident at the time but I searched forums, fingered through books and at last came up with a design posted on Lundahls page and made/modified by a man named Claus Byrith.
The Link
After a time I stumbled over a pair of transformers with similar specs as the ones from lundahl (opt) and bought them. Along with these came some EL34s which turned out to be quite valuable and those I sold to finance the rest.
Somewhere along the way I was talking about this amp and got into my head that 6550 could be a more fun choise and that these tubes where an almost exact replacement to EL34. Atleast most EL34 amps could be run on the 6550 without and modifications. This caused me to buy a matched quad 6550.
Now some years later I think to my self (didn't keep notes earlier) where did I get the more or less stupid idéa to switch tubes and was it as simple as that?
There must have been some modifications involved but which ones?
So, is there any pitfalls involved with this plan? The bias is probably different but do I need to change the supplyvoltage or size of any resistors? Obviosly I have three options
1: Ask you guys or
2: Read enough to be able to design an amp from scratch and check the entire circuit.
3: Be a pest and ask the designer.
Obviosly I chose option nr1 and hopefully I will learn some more in doing so.
The circuit is posted later in the appendix at the above mentioned link.
Kind regards!
The Link
After a time I stumbled over a pair of transformers with similar specs as the ones from lundahl (opt) and bought them. Along with these came some EL34s which turned out to be quite valuable and those I sold to finance the rest.
Somewhere along the way I was talking about this amp and got into my head that 6550 could be a more fun choise and that these tubes where an almost exact replacement to EL34. Atleast most EL34 amps could be run on the 6550 without and modifications. This caused me to buy a matched quad 6550.
Now some years later I think to my self (didn't keep notes earlier) where did I get the more or less stupid idéa to switch tubes and was it as simple as that?
There must have been some modifications involved but which ones?
So, is there any pitfalls involved with this plan? The bias is probably different but do I need to change the supplyvoltage or size of any resistors? Obviosly I have three options
1: Ask you guys or
2: Read enough to be able to design an amp from scratch and check the entire circuit.
3: Be a pest and ask the designer.
Obviosly I chose option nr1 and hopefully I will learn some more in doing so.
The circuit is posted later in the appendix at the above mentioned link.
Kind regards!
The 6550 is actually better suited to the OPT that Byrith was using than the EL34. However, it is NOT better suited to his splitter/driver.
You can often use EL34 in a circuit designed for 6550 but not necessarily the other way around. This is because the 6550 needs a much lower grid-to-ground resistance than the EL34 for its bias to be stable, especially with fixed bias. That makes the 6550 harder to drive, and a 12AX7 is definitely NOT suitable for that purpose (it's arguable that it is unsuitable even for the EL34 with fixed bias).
What you can do is to reduce the grid 'leak' resistor of each 6550 to 100k and use a 12AT7 instead of a 12AX7 for tjhe splitter/driver. You'll need to reduce the plate load resistors of the 12AT7 to 47k. The tail (common cathode) resistor should be replaced with a constant current sink to achieve acceptable balance. You can use a cacode transistor circuit for that purpose, set to draw 6mA.
You can often use EL34 in a circuit designed for 6550 but not necessarily the other way around. This is because the 6550 needs a much lower grid-to-ground resistance than the EL34 for its bias to be stable, especially with fixed bias. That makes the 6550 harder to drive, and a 12AX7 is definitely NOT suitable for that purpose (it's arguable that it is unsuitable even for the EL34 with fixed bias).
What you can do is to reduce the grid 'leak' resistor of each 6550 to 100k and use a 12AT7 instead of a 12AX7 for tjhe splitter/driver. You'll need to reduce the plate load resistors of the 12AT7 to 47k. The tail (common cathode) resistor should be replaced with a constant current sink to achieve acceptable balance. You can use a cacode transistor circuit for that purpose, set to draw 6mA.
kmj said:Ok, it sounds like I got it a bit wrong
I might as well sell the tubes and buy a quad of EL34. Thanks for the info!
It's not quite that simple. And it's certainly NOT black and white. The last thing I would do at this point is sell and purchase a third set of tubes before beginning construction. Keep the 6550s... if you decide to switch, do it later.
The beauty of tubes is it's a big ball park we are playing in. And by that I mean you don't have to be dead on spec to get a good sound.
Claus has put together a couple great papers on amps - PP and SE - and they are a recommended read. His PP is basically a modified Mullard 5-20. A google will give you lots of hits on that.
You could build his circuit as is and use 6550 or EL34 - just rebias. It may not be optimal, but it will work. With some modification, it could readily switch between the tubes without much impact on sonics. A different driver tube is not required but would work better.
I personally would skip the pentode input and definitely change the 12ax7 - I am not a fan of this tube for hi-fi at all. And as much of a change as that sounds, it's not something a few posts here couldn't help you overcome.
So don't fret, dive in.. even if you build his amp verbatim and then swap tubes and mod later on.... put your dollars to construction at this point.
Well, what do you know? Here I was starting a new thread and found my old one insted.
A few things has happend since I started this thread. First of, I bought a quad of EL34, another of 6L6 and finally two sets of PCBs made by Mr Byrith himself (or rather, ordered by him), the sets consisted of one PCB for the amplifier and one for the PSU.
I got the other parts and populated the PCBs, connected everything together and as of yesterday I am at the stage of testing them out. Here the problem starts.
On page 24 of the amplifier-PDF the writer states that
and it is the final part that makes me a bit confused, perhaps it is my english but I can't figure out where to put the probes of my multimeter to take measurements.
Since I couldn't figure it out and to me, that part had nothing to do with the accual setup I skipped along to the part containing the fun stuff. Plug in the tubes and set bias, DC-balance and AC-balance.
So, I plugged the tubes in (EL34) connected an old speaker and fired it up. During the time that the tubes heated up the bias stayed at sub 100mV but after a few seconds it quickly rose to about 1.0V and at this point I threw myself at the powerswitch. I tried a few times to turn the bias pot clockwise and anticlockwise and taking new measurements between adjustments but it still rose to high to fast. Along with the rushing bias the amp started to humm/buzz, a bit like oscillating, but not to howl. Claus states that if it does that the amp has positive feedback and that I should switch places with the feedback and ground on the secondary side of the output transformer.
Now I removed the tubes, disconnected the speakers and powered it up again, this time to make sure that the powersupply gave the correct voltages. It does, or at least as close to a difference of 20VDC.
I then switched to AC-measuring and redid the procedure. No AC exept between ground and the centertap of the opt, here I got almost 1000VAC.
And at this point I'm stuck. Is there anyone who can lend me a bit of support? I admit that I'm not that experienced but that is why I decided to use the designers PCBs after all.
I have built the channels as fully separated monoblocks and the schematic I have used is the one on page 20 of the Appendix-pdf complemented with the gridstoppers, screen grid resistors and the small 0,47uF polypropylene cap between the primary centertap and ground on the seconday side.
A few things has happend since I started this thread. First of, I bought a quad of EL34, another of 6L6 and finally two sets of PCBs made by Mr Byrith himself (or rather, ordered by him), the sets consisted of one PCB for the amplifier and one for the PSU.
I got the other parts and populated the PCBs, connected everything together and as of yesterday I am at the stage of testing them out. Here the problem starts.
On page 24 of the amplifier-PDF the writer states that
and it is the final part that makes me a bit confused, perhaps it is my english but I can't figure out where to put the probes of my multimeter to take measurements.
Since I couldn't figure it out and to me, that part had nothing to do with the accual setup I skipped along to the part containing the fun stuff. Plug in the tubes and set bias, DC-balance and AC-balance.
So, I plugged the tubes in (EL34) connected an old speaker and fired it up. During the time that the tubes heated up the bias stayed at sub 100mV but after a few seconds it quickly rose to about 1.0V and at this point I threw myself at the powerswitch. I tried a few times to turn the bias pot clockwise and anticlockwise and taking new measurements between adjustments but it still rose to high to fast. Along with the rushing bias the amp started to humm/buzz, a bit like oscillating, but not to howl. Claus states that if it does that the amp has positive feedback and that I should switch places with the feedback and ground on the secondary side of the output transformer.
Now I removed the tubes, disconnected the speakers and powered it up again, this time to make sure that the powersupply gave the correct voltages. It does, or at least as close to a difference of 20VDC.
I then switched to AC-measuring and redid the procedure. No AC exept between ground and the centertap of the opt, here I got almost 1000VAC.
And at this point I'm stuck. Is there anyone who can lend me a bit of support? I admit that I'm not that experienced but that is why I decided to use the designers PCBs after all.
I have built the channels as fully separated monoblocks and the schematic I have used is the one on page 20 of the Appendix-pdf complemented with the gridstoppers, screen grid resistors and the small 0,47uF polypropylene cap between the primary centertap and ground on the seconday side.
With the output tubes removed, adjust the bias adj pot to put out maximum voltage; this will make the output tube grid voltages very negative and not allow much current to flow. If the voltage is negative enough, no current will flow at all. Your bias supply voltage adjustment needs to exceed -35V or so at the tube grid to be able to reduce the bias current to workable levels (with EL34's). Measure the voltage from the output side of RVg1 (at least -60VDC) to ground or pin 5 of the tube socket to ground.
The DC voltage at the transformer center tap should be around 450 volts with the tubes installed (and warmed up/conducting) and about 500V without the tubes in.
Then stick the tubes back in and measure the bias voltage across each 10R resistor in the output tubes cathode circuits. I=E/R so you want about .6V across the 10R resistors to give around 60 ma per tube. For EL34's this requires about negative 30-35V or so out of your bias supply.
On another note, answering your questions from last year, here is a thread that uses KT88's in a modified Mullard 5-20 circuit:
http://www.diyaudio.com/forums/tubes-valves/156699-mullard-5-20-kt88-pp-blocks.html
You could use your 6550's here. Also note that the LTP ha been modified and does not use the 12AX7.
The DC voltage at the transformer center tap should be around 450 volts with the tubes installed (and warmed up/conducting) and about 500V without the tubes in.
Then stick the tubes back in and measure the bias voltage across each 10R resistor in the output tubes cathode circuits. I=E/R so you want about .6V across the 10R resistors to give around 60 ma per tube. For EL34's this requires about negative 30-35V or so out of your bias supply.
On another note, answering your questions from last year, here is a thread that uses KT88's in a modified Mullard 5-20 circuit:
http://www.diyaudio.com/forums/tubes-valves/156699-mullard-5-20-kt88-pp-blocks.html
You could use your 6550's here. Also note that the LTP ha been modified and does not use the 12AX7.
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Well, no. I have 110VAC and 40mW as stated in the datasheet of the recommended powertransformer. The schematic says 130V but in the text he recommends using a transformer with a 110V secondary winding. Since he was asked to design/modify the amp to be used by Lundahls transformers.
Last paragaph on page 23 of the building instruction.
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Yep, 20K
Ok, I'll se if I can find a few resistors around 150-180K to put in parallel with the 68K. It's easier to solder another one in place than it is to remove the existing one. Since the 68K is of a higher wattage than stated I should get away with a regular resistor there, unless I'm mistaken.
Well, of to bed and then tomorrow I will do a bit of diving in my good-stuff-to-have box. I'll post the results when I have them.
For now, thank a lot for the help!
I added a 220k resistor in parallel with the Rvg1 and was able to raise the negative voltage on the input of the screen resistor to -57VDC bu turning the bias pot all the way clockwise. Measuring between pin5 and ground at this point gave me about -10VDC and turning the bias all the way counter clockwise gave me about -27VDC.
Something isn't adding up here, I am in the specified range but it is still wrong. At least if I'm mesuring in the same place as Mr Byrith wants me to.
Something isn't adding up here, I am in the specified range but it is still wrong. At least if I'm mesuring in the same place as Mr Byrith wants me to.
Assuming you mean the grid leak resistor.................also confirm that the grid leak R and grid stopper R's are correct.
Sounds like you are getting close, you may need to get Rvg1 down to around 33K to get the pin 5 voltage to go lower than -35V.
Putting a 10K-20K R between the 20K pot and ground will also raise the voltage between Rvg1 and the pot.
As far as not having the same #'s as Mr. Bryrith, you are losing at small amount of adjustment by using a 20K pot, and your transformer secondary voltage is a little lower than that shown on the schematic, so things are not that far off (at least from where I'm sitting!)
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I added a 220k resistor in parallel with the Rvg1 and was able to raise the negative voltage on the input of the grid stopper to -57VDC by turning the bias pot all the way clockwise. Measuring between pin5 and ground at this point gave me about -10VDC and turning the bias all the way counter clockwise gave me about -27VDC.
The grid stoppers value is correct, 4k7 and what I assume is the grid leak resistor is also correct, 390k.
I'll try the rest of your recommendations and get back, I might not be able to do it today thou.
Sounds great!
The grid stoppers value is correct, 4k7 and what I assume is the grid leak resistor is also correct, 390k.
I'll try the rest of your recommendations and get back, I might not be able to do it today thou.
Sounds great!
Almost this time. I Parallelled two 68K to get 34K and by this I got -29,5VDC at pin5 (-63VDC just before the gridstopper). The result was still the same, a rushing bias a few seconds after power up. I'll se if I can find some more resistors and thus get perhaps a bit under -35VDC.
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