Just a quick impulsive question:
I've had very good results using local "Schade" (plate to plate) feedback in (SE and PP) amps with pentode drivers and pentode output tubes.
I ordered some output pentodes with very nice triode connection curves.
Now I'm wondering if triode connected outputs would work as well as pentode connection in this configuration.
Of course I will try it, but they aren't even paid yet, so that's for later. Meanwhile someone might have experience or an opinion.
Cheers!
I've had very good results using local "Schade" (plate to plate) feedback in (SE and PP) amps with pentode drivers and pentode output tubes.
I ordered some output pentodes with very nice triode connection curves.
Now I'm wondering if triode connected outputs would work as well as pentode connection in this configuration.
Of course I will try it, but they aren't even paid yet, so that's for later. Meanwhile someone might have experience or an opinion.
Cheers!
I thought the reason to use Shaded feedback was so the response of the output Pentode would be Triode-like. If you are Triode connecting the output Pentode, it seems there is no need for the Shade feedback any more. Maybe I'm missing something.
Should work, resulting in triodes with very low mu and Rp. I seem to recall that Broskie has written something about 300B and Schade?
Yes, it's used that way, but there's no rule that says it can't be used as feedback for its own sake. My experience has been that the circuits I've listenend to that use it have sounded better to my ear than the circuits I've listened to that used feedback taken from the output transformer secondary .
One example, Michael Koster's Meteor used it to lower the effective Rp of triodes like the 35TG.
I would try this using a higher value resistor than you would normally use with a pentode, like start at 2X the value, maybe 560K+
The advantage of Shade is that you can ser the rp and gain of the stage, plus you can swing down to pentode-like anode voltages (below 50V) chewing out more watts from the same amp in triode configuration.
The open loop gain (g1 grid, to plate) of a Pentode or a Beam Power tube is relatively high.
Applying a fair amount of Schade negative feedback is easy.
The open loop gain (g1 grid, to plate) of a Triode Wired Pentode or a Triode wired Beam Power tube is relatively low.
Applying a fair amount of Schade negative feedback is hard to do.
(Trying to set the amount of Schade negative feedback to be more than the open loop gain . . . is like a dog chasing its tail).
If the open loop gain is too small, versus the Schade negative feedback gain setting, then you might as well take out the schade negative feedback, and only use the Triode Wired local negative feedback.
In addition, whenever you use Schade negative feedback, you need to consider the effect of putting a lower impedance load on the driver tube . . .
What it does to the driver tube distortion (can make it good, or can make it bad).
Have fun!
Applying a fair amount of Schade negative feedback is easy.
The open loop gain (g1 grid, to plate) of a Triode Wired Pentode or a Triode wired Beam Power tube is relatively low.
Applying a fair amount of Schade negative feedback is hard to do.
(Trying to set the amount of Schade negative feedback to be more than the open loop gain . . . is like a dog chasing its tail).
If the open loop gain is too small, versus the Schade negative feedback gain setting, then you might as well take out the schade negative feedback, and only use the Triode Wired local negative feedback.
In addition, whenever you use Schade negative feedback, you need to consider the effect of putting a lower impedance load on the driver tube . . .
What it does to the driver tube distortion (can make it good, or can make it bad).
Have fun!
This 'Schade' negative feedback thing is basically making the output stage an 'anode follower' (plate to grid feedback on the output tube).
Shunt feedback series applied reduces the input impedance of the stage, so the driver stage has to be able to drive the heavier load without running out of steam.
Some vintage designs took feedback from the output tube plate back to the driver tube cathode. The big RCA 50-Watt Amplifier design used plate to cathode feedback nested inside of 'Schade' feedback and a global NFB loop. Look at R22 and R23 in the schematic below, and then R21 and R24. R29 and R6 form the more usual global NFB loop around the OPT. Loopy!
Some push-pull triode amplifiers used only negative feedback from the output tubes' plates to the driver tubes' cathodes. I can't seem to find an example, though. (Sorry.)
I was struggling with these questions a few years ago, and folks here on diyAudio helped out with some answers. This might be useful (I hope):
https://www.diyaudio.com/community/...late-cathode-local-nfb-is-it-the-same.309129/
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Shunt feedback series applied reduces the input impedance of the stage, so the driver stage has to be able to drive the heavier load without running out of steam.
Some vintage designs took feedback from the output tube plate back to the driver tube cathode. The big RCA 50-Watt Amplifier design used plate to cathode feedback nested inside of 'Schade' feedback and a global NFB loop. Look at R22 and R23 in the schematic below, and then R21 and R24. R29 and R6 form the more usual global NFB loop around the OPT. Loopy!
Some push-pull triode amplifiers used only negative feedback from the output tubes' plates to the driver tubes' cathodes. I can't seem to find an example, though. (Sorry.)
I was struggling with these questions a few years ago, and folks here on diyAudio helped out with some answers. This might be useful (I hope):
https://www.diyaudio.com/community/...late-cathode-local-nfb-is-it-the-same.309129/
--
Interesting.
The first schematic's negative feedback is reduced by about 3 dB at 0.64Hz.
At frequencies below that, the feedback reduces further to be 0dB at DC.
The second schematic is similar, with the negative feedback reduced by about 3 dB at 1.6Hz;
and with similar effects all the way to DC as the first amplifier.
Not that these numbers have any real importance, there were not many music signals at that low of a frequency, not to mention the output transformer was already restricting the plate swing at such low frequencies.
Were my above calculations correct?
Thanks!
The first schematic's negative feedback is reduced by about 3 dB at 0.64Hz.
At frequencies below that, the feedback reduces further to be 0dB at DC.
The second schematic is similar, with the negative feedback reduced by about 3 dB at 1.6Hz;
and with similar effects all the way to DC as the first amplifier.
Not that these numbers have any real importance, there were not many music signals at that low of a frequency, not to mention the output transformer was already restricting the plate swing at such low frequencies.
Were my above calculations correct?
Thanks!
Lately I played more with the Plate to Cathode feedback indeed, like the Telefunken V69
http://www.hifi-forum.de/bild/telefunken-v69_646189.html
I remember member Shoog commenting that the plate to cathode simulated better than plate to plate. I do currently have an amplifier on the bench, using EF184 and GU50, all in pentode, where I want to compare plate to plate with plate to cathode.
And indeed, using triodes at the output is mentioned by Broskie, and that it will indeed further reduce its plate impedance. I built a GM70 amplifier that was not doing great on my friend's speakers, so I included some Plate to Cathode feedback in it, to further reduce the GM70s impedance and improve the damping factor, which worked out positively on his set of speakers.
What sort of amps did you build Miniwatt? Can you share some schematics?
http://www.hifi-forum.de/bild/telefunken-v69_646189.html
I remember member Shoog commenting that the plate to cathode simulated better than plate to plate. I do currently have an amplifier on the bench, using EF184 and GU50, all in pentode, where I want to compare plate to plate with plate to cathode.
And indeed, using triodes at the output is mentioned by Broskie, and that it will indeed further reduce its plate impedance. I built a GM70 amplifier that was not doing great on my friend's speakers, so I included some Plate to Cathode feedback in it, to further reduce the GM70s impedance and improve the damping factor, which worked out positively on his set of speakers.
What sort of amps did you build Miniwatt? Can you share some schematics?
I like the idea of negative feedback around the output stage that doesn't include the output transformer, since between the various parts of the amplifier (tubes, resistors, capacitors, etc.) the output transformer is likely to have the narrowest bandwidth and least good phase performance. If you have NFB wrapped around two gain stages with a similar rolloff, that will almost guarantee that some frequency will have a 360 degree phase shift and will oscillate. The less good the output transformer used, the more likely this problem will occur in a multi-stage amplifier. My thinking is that local feedback (plate-grid around the output tube) or plate-cathode feedback (output tube plate to driver tube cathode) will be easier to implement if using a less than spectacular OPT. You won't get the extended bandwidth from having the OPT inside the NFB loop, but if you can live with restricted bandwidth then you may get a better sounding tube amp in the end. Since I can't hear beyond 13kHz anymore, I suppose I won't mind if the final amp droops -1dB at 20kHz. Hopefully that translates into smoother-sounding midrange frequencies and reasonably well damped mid-bass, which is what I value most.
Anyway, that's what I was thinking. I still need to kang something together. I might try something along the lines of that Telefunken V69 amp, but using more normally available tubes like 6AC7 and EL84, 6V6 or 6L6. Something like that...
Anyway, that's what I was thinking. I still need to kang something together. I might try something along the lines of that Telefunken V69 amp, but using more normally available tubes like 6AC7 and EL84, 6V6 or 6L6. Something like that...
The Western Electric 131A uses feedback from plates to cathodes but minus the blocking caps, similar to the Telefunken V69. Tubes are quite common, essentially two 6J7s driving two 6F6s or 6V6s in push-pull. The 171C output transformer has 10K primary. And it has the best sound to my ears from the bunch. If I build it, I would use 1:1 bridging input transformers with 10k to 20K primary without center tap instead of step-up transformer. You can form a virtual center tap or ground reference with two matching resistors.
Maybe something like that is what I should build out of the Dyna ST35 transformer set I have sitting in a box in the closet. That's a 7k ohm primary, I think. Only has 8 ohm and 16 ohm secondary taps, which presents a problem with most modern speakers. I have a couple of Edcor 10k:10k input transformers to throw at it too.
For driver tubes, I wonder how 6CF6A would work out with a circuit like this. I have a sleeve of those.
In octals, I don't have any 6J7, but I have a few 6SJ7 (metal octal). I also have a bunch of 6AC7 (higher gm than 6SJ7). I got those cheap.
For output tubes I'd be tempted to use 6P15P or 6P43P-E, since I have a few sleeves of those. Also got those pretty cheap.
For driver tubes, I wonder how 6CF6A would work out with a circuit like this. I have a sleeve of those.
In octals, I don't have any 6J7, but I have a few 6SJ7 (metal octal). I also have a bunch of 6AC7 (higher gm than 6SJ7). I got those cheap.
For output tubes I'd be tempted to use 6P15P or 6P43P-E, since I have a few sleeves of those. Also got those pretty cheap.
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I slowly collect parts for WE 124 ( 6J7 inputs & 6L6 Outputs for 12W per channel )- what interests me is this : I had never done calculation for power transformer for class A/B amp.
The amp works with 403Vdc / 100mA after rectification for one channel ( this is seen on schematic, steady state conditions ) so this is 403Vdc / 200mA for two channels.
If I use graetz insted of rectifier tube and calculate 305 Vac x 1,3 i get 403 Vdc but what amount of current should secondary have .... probably 500mA but don't know for sure since this would be my first power amp build. Can have custom transformer built - toroid - just need the info on current or calculus for A/B class amps.
Can You indicate what amount of current is sufficient for the amp to work when power goes up from steady state ?
Ultra Linear at 20% to 23% is much closer to pentode/beam power mode than it is to triode wired mode.
Ultra Linear at higher percentages, such as 50% or more are closer to triode wired mode than to pentode/beam power mode.
There are Ultra Linear amplifier circuits that have up to 75%.
But in reality, each tube type has the lowest distortion at some particular percentage, different than some other tube types.
Tradeoffs
Ultra Linear at higher percentages, such as 50% or more are closer to triode wired mode than to pentode/beam power mode.
There are Ultra Linear amplifier circuits that have up to 75%.
But in reality, each tube type has the lowest distortion at some particular percentage, different than some other tube types.
Tradeoffs