| G |
| As I was shown and taught by Sy (haven't heard from him in a while) a simple cathode follower with a single sided power supply has some limitations because of the grid being referenced to ground and sitting essentially at zero volts. In a direct coupled two stage circuit where an anode follower is direct coupled to a cathode follower does the cathode follower still suffer from those limitations? It would seem to me that it does not as the grid is lifted quite a ways above ground by the plate voltage of the anode follower. Am I looking at this correctly? |
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| fdegrove |
Hi,
| quote: | | Am I looking at this correctly? |
You do...Not that direct coupling all of a sudden turns the humble CF into a super CF but, yeah...it does rid it of that particular problem.
Cheers,;) |
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| G |
| So I could direct couple a 5687 CF to the anode follower of my 5687 preamp and lower the output z significantly. How do you think that would sound sonically Frank? |
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| fdegrove |
Hi,
| quote: | | So I could direct couple a 5687 CF to the anode follower of my 5687 preamp and lower the output z significantly. |
You mean the other way around, right ?
Does that lower the Zout of the CF ? I don't recall it doing that, can't see how it would really.
| quote: | | How do you think that would sound sonically Frank? |
This is the kind of circuit we've been trying to avoid for the past ten years, maybe for the wrong reasons...
Anyway, from previous posts I gather you plan on building a preamp with good sonics and low Zout.
Why not try something with either the 12B4A or EL84/6BQ5 trioded with either choke or resistive load?
While certainly not the last word in Zout it's undoubtedly hard to beat on sound.
If you want lower Zout with that type of tube and still need some gain then you can also try a WOT with this.
Or, go for one like this one if you listen to headphones as well:
Cheers,;) |
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| tubetvr |
| quote: | | a simple cathode follower with a single sided power supply has some limitations because of the grid being referenced to ground and sitting essentially at zero volts. |
Yes, a CF connected this way is not very linear and can not handle large voltage swing.
| quote: | | In a direct coupled two stage circuit where an anode follower is direct coupled to a cathode follower does the cathode follower still suffer from those limitations? |
No it doesn't if you allow enough voltage over the cathode resistor, (this has to be larger than in an ordinary CF of course). the output impedance of the CF will be the same but will be shunted by a higher resistance.
Another way of increasing grid voltage and allowable voltage swing is to use 2 resistors in series between cathode and ground and then connect the grid leak to the connection point between those resistors. With this connection you can use capacitor coupling.
In any case if you allow 100V or more between cathode and ground of the CF using one of the above methods the linearity will in practise be the same as when using a CCS in the cathode, the only thing you gain by using a CCS is that you can use somewhat lower supply voltage but you dont really gain anything in linearity.
BTW, please don't call it anode follower, a follower is an amplifier with gain of ~1, call it cathode grounded amplifier that is the usual name.
Regards Hans |
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| Sch3mat1c |
Or, it's more stable with grid leak taken as a voltage divider from B+ to GND. Which is the same grid-leak-DC-operationage-ism as direct coupling to a plate.
Tim |
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| tubetvr |
| quote: | | Which is the same grid-leak-DC-operationage-ism as direct coupling to a plate. |
Except for the lower input impedance, if you want to maintain the very high input impedance of the CF the solution with 2 series connected cathode resistors works very well, stability for any alternative is not of any serious concern except if you want to DC couple the output.
Regards Hans |
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| Sch3mat1c |
You can bootstrap the divider too. :D
Tim |
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| tubetvr |
| quote: | | You can bootstrap the divider too |
I think it is what I have already described... the input impedance of a CF with dual cathode resistors is RG/(1-A*RL/*RL+Rk()where RL is the resistor closest to ground and Rk is the resistor closest to cathode, quite a difference to your variant using a potential divider between B+ and ground... I try never to waste gain or power in resistors....
Regards Hans |
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| Sch3mat1c |
Not like a few hundred K is much lost impedance. No worse than a light grid leak on any other grounded cathode stage.
I'd set up a voltage divider (from +300V to bias at 150Vk, two 1M resistors would do it), cap couple to cathode and run a resistor to grid. Impedance should be something like Rg/(1-gain) or however you'd express it. Which is electrically identical to the cathode biased method, except the voltage divider loads the output slightly. (It'll notice a gnat flying around the envelope before it notices 500k load, so, yeah.)
I don't know how a triode would tend to behave when self-biased as you advocate, but it doesn't feel like good, stable practice. Certainly wouldn't cut it in direct-coupled circuits like my active preamp design.
Tim |
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| tubetvr |
| quote: | | I don't know how a triode would tend to behave when self-biased as you advocate, but it doesn't feel like good, stable practice. |
Maybe you have to test things in reality to find out what works or not... the circuit i describe I have described is completely stable and the grid cathode voitage is defined by the drop over the cathode resistor, same circuit is for instance used in the Marantz 7, I have used the method described before with good result. I noticed that you have now changed your preferred circuit to include bootstrapping but i fail to see any advantages over the more traditional circuit I described...
Regards Hans |
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| Sch3mat1c |
Of course, neither has its advantages (wrt input impedance) because everything we use isn't going to care one way or the other whether that grid leak resistor "appears" or not. ;)
Tim |
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| SY |
Keep stuff easy. First, define what you need the circuit to do in terms of Zout, required swing, and what you expect it to drive. KISS.
In general, I don't like schemes which require big coupling caps, especially big HV coupling caps with lots of dielectric in the way, so I've tended to recommend circuits where the cathode sits at a relatively low voltage. And I really, really like direct coupled outputs- inputs can use nice, small caps because of the high Z. Let's start with defining your requirements and I'll go through a step by step, culminating in a finished design, with a few options about coupling. |
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| tubetvr |
| quote: | | In general, I don't like schemes which require big coupling caps, especially big HV coupling caps with lots of dielectric in the way, so I've tended to recommend circuits where the cathode sits at a relatively low voltage. |
Agreed, but a CF with low voltage between cathode and ground is not very linear, 2 ways to making it more linearis either to use higher voltage say 100V or using a CCS. I have compared these 2 solutions and found no practical benefits with the CCS solution except maybe for less requirements on the power supply.
The variant of CF with dual cathode resistors have the advantage of having very high input impedance, much higher than the grid leak which allow for use of small caps, the output is maybe at 100V or so which is acceptable I think when it comes to finding a suitable cap. If you don't like caps you can use a negative voltage to feed the cathode resistor and put the cathode at 0V making it possible to eliminate the cap or at least relax the requirements on it.
In my own preamp I dont use any coupling caps at all, I use DC coupling, (A SRPP connected to a cathode grounded stage connected to a cathode follower) this works very well with wide bandwidth and low distortion, I use AC and DC feedback to stablise the operating point and gain but I am among those who believe in global feedback if done correctly.
Regards Hans |
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| SY |
| quote: | | 2 ways to making it more linearis either to use higher voltage say 100V or using a CCS. |
Three more ways are using a pfet as a pseudo-complement run down to a negative rail, using a CCS to a negative rail, and using a large resistor to a large negative rail. |
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| G |
| quote: | Originally posted by SY
Keep stuff easy. First, define what you need the circuit to do in terms of Zout, required swing, and what you expect it to drive. KISS.
In general, I don't like schemes which require big coupling caps, especially big HV coupling caps with lots of dielectric in the way, so I've tended to recommend circuits where the cathode sits at a relatively low voltage. And I really, really like direct coupled outputs- inputs can use nice, small caps because of the high Z. Let's start with defining your requirements and I'll go through a step by step, culminating in a finished design, with a few options about coupling. |
Hi Sy. Long time no hear. Is this post in response to what I need to drive my amp or are you anwering someone else? Let's assume that you are responding to my post. I would like to have a preamp with about a gain of 6 - 8 with a output Z of around 100 - 200 ohms. I was reading about the Bottlehead Foreplay and discovered that they direct couple the gain stage to the output stage of the Foreplay. I then started thinking about direct coupling and what it meant in reference to the buffer that you so kindly helped me with a few weeks back. One of the drawbacks of the common CF is that the grid sits at earth potential and it's difficult to get it more than a few volts negative to the cathode without using a negative rail. After thinking about direct coupling a bit I saw that it would sit the grid at a high voltage above earth which would in turn enable you to bias the cathode voltage higher without concern of negative clipping due to the grid being essentially at earth. I may be looking at this wrong. Like that's new but I thought it would enable me to build a relatively simple circuit and still get the performance I am looking for. Am I wrong? |
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| SY |
No, you're not at all wrong. Floating the grid up to a voltage that's an order of magnitude more than you want to swing will work fine as long as you're willing to deal with getting rid of the inevitable DC on the output. And running my signal through a 10u/200V cap just esthetically bothers me, more so than running it through the 0.01-0.1u cap you'd need to couple into the grid of the output stage if that grid is biased to null out the DC component on the cathode.
But floating up the circuit and using that big output cap is absolutely doable and I can't claim that the methods I prefer (which have the tradeoff of requiring a negative rail) would show any better in a controlled listening test. I'll draw up a couple candidate circuits for you and we can discuss. |
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| G |
| quote: | Originally posted by SY
No, you're not at all wrong. Floating the grid up to a voltage that's an order of magnitude more than you want to swing will work fine as long as you're willing to deal with getting rid of the inevitable DC on the output. And running my signal through a 10u/200V cap just esthetically bothers me, more so than running it through the 0.01-0.1u cap you'd need to couple into the grid of the output stage if that grid is biased to null out the DC component on the cathode.
But floating up the circuit and using that big output cap is absolutely doable and I can't claim that the methods I prefer (which have the tradeoff of requiring a negative rail) would show any better in a controlled listening test. I'll draw up a couple candidate circuits for you and we can discuss. |
Thank you Sy. I would enjoy that. I look forward to hearing from you. |
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| SY |
OK, I drew a few things up and now I'm trying to get the #@!% scanner to work! While that's going on, let's take step one and pick a CF tube. That's the easy part- we start with your requirement Zo = 200 ohm. I'm arbitrarily not going to use an output transformer. I like to avoid them where possible for reasons of expense, bulk, and low frequency performance. Galvanic isolation is nice, but if that's what I want, I'd rather do it at the input where the signal level is lower, the transformer load is under my control, and where dodgy grounds from signal sources abound.
OK, then. The source impedance of a simple triode CF (and I'm being arbitrary here, you could use a pentode, too) will look like ~1/gm. So the required gm is 5000 umho or higher.
The required swing for a preamp output will be roughly 4-5 volts peak. And the capacitance of the load will be 500 pF or less- assuming you're not driving hideously long cables. So... if we want to avoid the complication and expense of paralleled tubes and stick to classic tube types, a 6DJ8 family member would be quite adequate. As an alternative, you could also use paralleled 6SN7s at the cost of doubling up tubes. But octals do look cool.
OK, make your choice and we'll go to the next step. |
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| G |
| I'm kinda torn between an octal (6BL7) and a 9 pin (5687). Opinions anyone? |
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| SY |
For a preamp output with the Z you're targeting, understanding that you don't need scads of current or voltage, the transconductance is the most important spec. A 5687 will barely get you the transconductance you need, but you'll have to run it pretty hard (12 ma or more). I don't have the 6BL7 data sheet in front of me, but I recall the transconductance as also being pretty marginal. These are probably not optimum tubes for this application.
Save the 6BL7 for a tiny triode amp output stage- or as a cathode follower driver for an AB2 amp, where you can make use of its major virtue, the ability to drive in lots of current. |
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| G |
| quote: | Originally posted by SY
For a preamp output with the Z you're targeting, understanding that you don't need scads of current or voltage, the transconductance is the most important spec. A 5687 will barely get you the transconductance you need, but you'll have to run it pretty hard (12 ma or more). I don't have the 6BL7 data sheet in front of me, but I recall the transconductance as also being pretty marginal. These are probably not optimum tubes for this application.
Save the 6BL7 for a tiny triode amp output stage- or as a cathode follower driver for an AB2 amp, where you can make use of its major virtue, the ability to drive in lots of current. |
Well it appears that the ECC88 is the one to go with. I've searched my little preamp tube database and I can't find a better candidate. High transconductance and low voltage requirements. Although a 5842 would work also but they are much harder to come by. |
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| SY |
| Among commonly-available tubes, it's a great choice. If you want more exotic, there are frame-grid RF triodes with high transconductance and mu (like 6HA5-types), and even Nuvistors. But the 6DJ8/ECC88/7308 family will get you there efficiently and with no fuss. |
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| G |
| quote: | Originally posted by SY
Among commonly-available tubes, it's a great choice. If you want more exotic, there are frame-grid RF triodes with high transconductance and mu (like 6HA5-types), and even Nuvistors. But the 6DJ8/ECC88/7308 family will get you there efficiently and with no fuss. |
Well I guess that's settled. I would like to keep the plate voltages below 200v as I have some dual section Elna Cerafines that I would like to use. They are rated at 350 volts. Would you like me to start the calculations and go from there or do you have something specific in mind? I'm by no means a direct coupling expert and there are still a few things that I'm not sure about as far as the CF stage and biasing it go but I can certainly design the gain stage. I'm not ebtirely sure that I want to use tube rectification either. I have a couple of IXYS integrated hyperfred bridges that I would like to try. Even though they are grossly overrated for the task. |
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| SY |
| quote: | | Would you like me to start the calculations and go from there or do you have something specific in mind? |
I had a few options in mind, but by all means, throw out some proposals. They'll either be good as-is or at least be useful as starting points or as pedagogical exercises. |
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| SY |
| Frank, something along those lines was very much one of the options I had in mind. The gain would need to be knocked down a bit, but hey, more degeneration never hurt anyone. |
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| G |
| quote: | Originally posted by SY
Frank, something along those lines was very much one of the options I had in mind. The gain would need to be knocked down a bit, but hey, more degeneration never hurt anyone. |
The first one that Frank posted is about what I had in mind so let's start there. No need for the input cap though. |
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| SY |
Well, I'd probably keep the input cap just because I don't trust the offset of my sources. Heck, my soundcard has a nice 60 mV of DC riding on one channel.
But... I'd have the input cap feeding the top of a potentiometer (you DO want to control volume, don't you?), something in the 100K range, then take the wiper tap to the tube grid through a 1K resistor (for RF suppression). If it were commercial product, I'd also tie a safety resistor between the first stage grid and ground (maybe a meg or so), just in case the wiper of the pot goes out. You'll also want to tie a large (>1M) resistor from the input side of the cap to ground just to prevent bangs and pops when things are (accidently, of course!) hot-plugged.
Finally, the output should be shorted to ground during warmup and particularly on shut down. You can use an NC relay or (horrors!) a bipolar transistor coupled to a timer circuit to do this task. The bipolar has the advantage of speed, and when it's cut off, the resistance from collector to emitter is really, really high and totally swamped by the shunt impedances across it.
Again, all this is optional and probably not going to make an audible difference, but it cleans things up a bit. You can indeed go DC in, there's a nice simplicity to that, but you'll have to cross your fingers that nothing feeding it is going to have an output offset ever.
The advantage of the input cap where Frank has drawn it is that you can play a few degeneration tricks with that first stage to knock down the gain without worrying about what the DC level on the grid is. |
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| G |
| quote: | Originally posted by SY
Well, I'd probably keep the input cap just because I don't trust the offset of my sources. Heck, my soundcard has a nice 60 mV of DC riding on one channel. |
No offense but I would prefer to live dangerously. One cap in the circuit is enough for my taste.
| quote: | Originally posted by SY
But... I'd have the input cap feeding the top of a potentiometer (you DO want to control volume, don't you?), something in the 100K range, then take the wiper tap to the tube grid through a 1K resistor (for RF suppression). If it were commercial product, I'd also tie a safety resistor between the first stage grid and ground (maybe a meg or so), just in case the wiper of the pot goes out. You'll also want to tie a large (>1M) resistor from the input side of the cap to ground just to prevent bangs and pops when things are (accidently, of course!) hot-plugged. |
I plan on using a 100K 24 step ladder attenuator at the input. I will definately use a suppression resistor to the grid.
| quote: | Originally posted by SY
Finally, the output should be shorted to ground during warmup and particularly on shut down. You can use an NC relay or (horrors!) a bipolar transistor coupled to a timer circuit to do this task. The bipolar has the advantage of speed, and when it's cut off, the resistance from collector to emitter is really, really high and totally swamped by the shunt impedances across it. |
I'm not following why this is needed. I understand doing this on SS outputs but I've never seen it done on a tube preamp. At least not on a DIY one. I guess if I use a SS rectifier this is a necessary precaution?
| quote: | Originally posted by SY
Again, all this is optional and probably not going to make an audible difference, but it cleans things up a bit. You can indeed go DC in, there's a nice simplicity to that, but you'll have to cross your fingers that nothing feeding it is going to have an output offset ever. |
All of my sources (all two of them) use caps on the outputs so I think I will chance it.
| quote: | Originally posted by SY
The advantage of the input cap where Frank has drawn it is that you can play a few degeneration tricks with that first stage to knock down the gain without worrying about what the DC level on the grid is. |
I don't have a problem with local feedback. I just don't know how to implement it properly yet. I had planned on not using a bypass cap on the gain stage triode's cathode to hold the gain down but I'm not sure how effective that will be nor how it will sound. I just want to have a good sounding tube linestage that is capable of driving a 20K - 50K load with grace. I was considering buying a Foreplay and customizing it a little during the build but after researching it I decided that I don't like the operating points they use so now I'm back to square one and more frustrated than ever. |
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| SY |
Well, in Frank's circuit, the cathode resistor is indeed unbypassed. That gives some local degeneration. And you've got plenty more, in fact 100%, in the cathode follower. You can alter the degeneration in the first stage if you use an input cap, but if you don't, you'll just have to live with more gain. I haven't run the calculations, but this circuit as it is drawn looks like the gain will be about x10 (20 dB).
As far as the power-on/power-off mute, it's a really good idea even if you're running into a tube power amp. For one thing, it is just a matter of time until the turn-on or turn-off sequence is done wrong. Especially after a nice evening of wine. There are no doubt quite a few examples of commercial tube preamps that do this, but one I know offhand (because I've got one) is the classic Berning TF10. And Berning has the schematics posted on his web site, so you can swipe it easily. |
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| G |
| quote: | Originally posted by SY
Well, in Frank's circuit, the cathode resistor is indeed unbypassed. That gives some local degeneration. And you've got plenty more, in fact 100%, in the cathode follower. You can alter the degeneration in the first stage if you use an input cap, but if you don't, you'll just have to live with more gain. I haven't run the calculations, but this circuit as it is drawn looks like the gain will be about x10 (20 dB).
As far as the power-on/power-off mute, it's a really good idea even if you're running into a tube power amp. For one thing, it is just a matter of time until the turn-on or turn-off sequence is done wrong. Especially after a nice evening of wine. There are no doubt quite a few examples of commercial tube preamps that do this, but one I know offhand (because I've got one) is the classic Berning TF10. And Berning has the schematics posted on his web site, so you can swipe it easily. |
I will have to explore my options on the muting. I may just use a time delay off relay as the signal will not be passing through the contacts. Or maybe a vacuum tube delay relay.
I noticed the symmetry in Franks design. In that I mean he has dropped the same voltage across the anode resistor on the gain stage as he has the cathode resistor on the output stage. Doesn't that mean that the grid of the second triode is at the same potential (in relation to ground) as the cathode? Does it not matter because the tubes are direct coupled? What controls the current through the second triode? Since it seems to be at 0 volts potential. |
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| SY |
The cathode of the CF will be slightly more positive than the grid, just enough so to set the current to the (approximate) ratio between the plate voltage of the preceding stage and the cathode resistor of the CF. That's feedback in action for you!
Look at the feedback this way: if the cathode voltage with respect to the grid of the CF is more positive than required, the tube will conduct less. That reduces the drop across the cathode resistor, thus bringing the cathode less positive wrt to the grid. Feedback. |
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| G |
| quote: | Originally posted by SY
The cathode of the CF will be slightly more positive than the grid, just enough so to set the current to the (approximate) ratio between the plate voltage of the preceding stage and the cathode resistor of the CF. That's feedback in action for you!
Look at the feedback this way: if the cathode voltage with respect to the grid of the CF is more positive than required, the tube will conduct less. That reduces the drop across the cathode resistor, thus bringing the cathode less positive wrt to the grid. Feedback. |
Let me run this by you. Using Kirchoff's law there are three elements in the first stage that have a voltage drop across them. The anode resistor, the triode and the cathode resistor. The cathode resistor of the first stage would seem to be the key to me. Let's say there are two volts dropped across it. That would in turn mean that there are two less volts at the grid of the second triode than is being dropped across the cathode resistor of the second triode. That would mean that the cathode resistor of the first triode sets the current through the second tube as well so long as both tubes are fed from the same value of B+. Is that correct? I'm not vey good at AC analysis but I will never understand how this circuit works if I can't get a handle on the DC analysis. |
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| fdegrove |
Hi,
It has to...
Admittedly I look at it in a different way than you do but the net result is still the same...
Looking at the circuit, the cathode resistor of the first triode is causing a voltage drop X across itself, this will inevitably reappear in the next stage as the cathode resistor of that triode has the exact same value as the plate resistor of the first triode causing a voltage difference equal to X.
So if we measure, say a 100V, on the plate of the first triode and direct couple that plate to the grid of the next triode we'll measure 100V + X on the cathode of the second triode, ergo the grid of the seconde triode will be negative by the amount equal to the difference between the two, namely X.
If we'd want a different bias for the second triode we'd have to change the value of it's cathode resistor but by doing so we'd also lose much of the inherent AC stability of the original circuit.
Why?
To borrow John Broskie's analogy of the see-saw, the use of equal plate and cathode resistors for first and second stage respectively forces equilibrium on our see-saw: whichever side is pulling or pushing, the other is going to try to regain equilibrium using the same amount of force.
Changing either side will break that equilibrium and our see-saw will struggle.
It's a little oversimplified but I hope it helps nonetheless.
Cheers, ;) |
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| G |
| quote: | Originally posted by fdegrove
Hi,
It has to...
Admittedly I look at it in a different way than you do but the net result is still the same...
Looking at the circuit, the cathode resistor of the first triode is causing a voltage drop X across itself, this will inevitably reappear in the next stage as the cathode resistor of that triode has the exact same value as the plate resistor of the first triode causing a voltage difference equal to X.
So if we measure, say a 100V, on the plate of the first triode and direct couple that plate to the grid of the next triode we'll measure 100V + X on the cathode of the second triode, ergo the grid of the seconde triode will be negative by the amount equal to the difference between the two, namely X.
If we'd want a different bias for the second triode we'd have to change the value of it's cathode resistor but by doing so we'd also lose much of the inherent AC stability of the original circuit.
Why?
To borrow John Broskie's analogy of the see-saw, the use of equal plate and cathode resistors for first and second stage respectively forces equilibrium on our see-saw: whichever side is pulling or pushing, the other is going to try to regain equilibrium using the same amount of force.
Changing either side will break that equilibrium and our see-saw will struggle.
It's a little oversimplified but I hope it helps nonetheless.
Cheers, ;) |
Hi Frank. I guess the design below won't work very well then. I was trying to keep the voltage down so I can use my stash of Elna Cerafine caps. Nice big cans with low ESR. I think this would work alright into a 20K or so load. 6SN7s are supposed to sound nice. I could raise the B+ to 250 I think and run both stages at 5mA instead. My caps are rated for 350v. What is your opinion. Granted it is a simple circuit but I think it has potential. |
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| SY |
| quote: | | Let me run this by you. Using Kirchoff's law there are three elements in the first stage that have a voltage drop across them. The anode resistor, the triode and the cathode resistor. The cathode resistor of the first stage would seem to be the key to me. |
Keep it simple. The CF draws essentially no current from the first stage. So for the purpose of DC analysis of the first stage, you can ignore the CF completely. Using load lines or CAD or whatever, you can determine what the plate voltage of that stage is going to be. It will be determined by the cathode resistor, the plate resistor, and the B+ rail. They're all part of the equation. But the point to take away is that you can calculate what you need about this stage in isolation. Let's call the voltage at the plate of this stage Vp. Doesn't matter what it is, we can go calculate it.
Now, let's move to the CF. Clearly, the voltage at the grid will be Vp because it's direct coupled from the previous stage. So with that as a given, we can look at this stage in isolation. The voltage at the cathode will be Vp plus a couple of volts, which are needed for biasing. But here's the trick: the required bias voltage in this circuit is very low compared to the drop across the CF's cathode resistor! So we can, to a very good approximation, say that the cathode voltage is also Vp. Sure, sure, it's actually going to be Vp plus something like two volts. But Vp is going to be something like 60-70 volts, so saying that 62 = 60 is not too bad an approximation.
Now... with this in hand, we can either calculate the value of the CF's cathode resistor from the desired current in that stage OR we can figure out the current in that stage from a given cathode resistor. Let's say, for argument, that Vp is 60 V. So the voltage at the cathode of the CF will be slightly higher than 60, but not significantly so. Then for the 20K value of CF cathode resistor, the current will be about 60/20 or 3 ma. You don't have to worry about whether that CF is going to need to sit 2 volts above Vp or 3 volts above Vp or whatever- the current will take care of itself. The variation in required biasing voltage will cause the current to be not exactly 3 ma, but who cares if it's 2.94 ma instead? Not me.
Is that a little clearer? |
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| fdegrove |
Hi,
| quote: | | Granted it is a simple circuit but I think it has potential. |
Sure, but you know me, right?
BTW, if the figures on the diagram are correct (I didn't cross check the math) then the CF is biased at 0V...
Not good but it's most likely the drawing that's not quite right.
As for the 6SN7s, yes most of them sound fine but I would only use them if I had plenty of them...
There are still sooo many sleepers out there that are just as good and dieing to be revived.
Cheers, ;) |
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| SY |
| quote: | | BTW, if the figures on the diagram are correct (I didn't cross check the math) then the CF is biased at 0V... |
I didn't do the math either, but didn't need to. On inspection, one can see that the voltage values are not correct. See my above post on biasing. The CF will be running at approximately 5 ma current, so the cathode will have to sit a few volts (4 or 5 maybe) above the grid. |
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| G |
| quote: | Originally posted by fdegrove
As for the 6SN7s, yes most of them sound fine but I would only use them if I had plenty of them...
There are still sooo many sleepers out there that are just as good and dieing to be revived.
Cheers, ;) |
You mean like a 6C8G?;)
I can change the cathode resistor on the CF to 22K. But that is where I'm getting lost. Does the resistor set the current through the tube in the CF? If so then a 22K resistor would make the cathode 10v more positive than the grid. |
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| Sch3mat1c |
Dude, graphing a CF isn't the simplest thing because of the NFB element. But it's not necessary anyway, at least until you get to the nitty-gritty. Just set it at a convienient voltage (determined by the preceeding stage as mentioned) and current (determined by the voltage Vp SY was using, and Rk of the CF). It's a simple, stable circuit element that can be broken down with Ohm's law. As SY also said, no one cares about the 2V grid to cathode.
Tim |
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| G |
| quote: | Originally posted by Sch3mat1c
Dude, graphing a CF isn't the simplest thing because of the NFB element. But it's not necessary anyway, at least until you get to the nitty-gritty. Just set it at a convienient voltage (determined by the preceeding stage as mentioned) and current (determined by the voltage Vp SY was using, and Rk of the CF). It's a simple, stable circuit element that can be broken down with Ohm's law. As SY also said, no one cares about the 2V grid to cathode.
Tim |
The problem I'm running into is finding the right resistor values. I may just put a 2 watt 50K carbon pot in // with a 50K resistor on the CF cathode and adjust the bias that way. The 20K resistor is only dissipating .5 watts so that should work fine. Dude.;) |
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| SY |
| Which resistor values are you trying to calculate? The cathode resistor for the CF? If so, I've outlined how in pretty complete detail. The biasing takes care of itself! |
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| G |
| quote: | Originally posted by SY
Keep it simple. The CF draws essentially no current from the first stage. So for the purpose of DC analysis of the first stage, you can ignore the CF completely. Using load lines or CAD or whatever, you can determine what the plate voltage of that stage is going to be. It will be determined by the cathode resistor, the plate resistor, and the B+ rail. They're all part of the equation. But the point to take away is that you can calculate what you need about this stage in isolation. Let's call the voltage at the plate of this stage Vp. Doesn't matter what it is, we can go calculate it.
Now, let's move to the CF. Clearly, the voltage at the grid will be Vp because it's direct coupled from the previous stage. So with that as a given, we can look at this stage in isolation. The voltage at the cathode will be Vp plus a couple of volts, which are needed for biasing. But here's the trick: the required bias voltage in this circuit is very low compared to the drop across the CF's cathode resistor! So we can, to a very good approximation, say that the cathode voltage is also Vp. Sure, sure, it's actually going to be Vp plus something like two volts. But Vp is going to be something like 60-70 volts, so saying that 62 = 60 is not too bad an approximation.
Now... with this in hand, we can either calculate the value of the CF's cathode resistor from the desired current in that stage OR we can figure out the current in that stage from a given cathode resistor. Let's say, for argument, that Vp is 60 V. So the voltage at the cathode of the CF will be slightly higher than 60, but not significantly so. Then for the 20K value of CF cathode resistor, the current will be about 60/20 or 3 ma. You don't have to worry about whether that CF is going to need to sit 2 volts above Vp or 3 volts above Vp or whatever- the current will take care of itself. The variation in required biasing voltage will cause the current to be not exactly 3 ma, but who cares if it's 2.94 ma instead? Not me.
Is that a little clearer? |
Hi Sy. You are saying the cathode voltage figure that I put in the schematic is wrong and that it will be different because of feedback. That means I can build it as shown but the voltages will not be what I calculated them to be, but the circuit will work as drawn. Correct? If I set the cathode resistor to drop 100v then the cathode voltage will actually be about 102v because of the feedback. That's what I'm getting from this post. Thanks for replying. |
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| SY |
| If you're talking about the 6SN7 preamp schematic, yes, that's it exactly. It will work fine, the cathode will take care of itself by biasing up a couple of volts. |
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| SY |
| BTW, though the 6SN7 circuit you drew will work, it's not optimal. The first stage will be running too little current to be linear and the output Z will be higher than your stated target. Frank's design will be better in these respects. |
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