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Old 4th October 2009, 10:01 PM   #11
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So, something like this?
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Old 5th October 2009, 02:27 AM   #12
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Yep.
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Old 5th October 2009, 03:39 AM   #13
ddd is offline ddd  Bulgaria
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Originally Posted by dsavitsk View Post
So, something like this?
This way is allmost impossible practically. The set of R-s are wery hard to select due to numerous contradictory requirements. If you stick on this way, take the signal from the plate (trough a C) for far easier matching (high-Ohm Rfb2).
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Old 5th October 2009, 06:11 AM   #14
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Originally Posted by Miles Prower View Post
Yep.
OK, then the $100 question -- any hints on how to calculate these feedback resistors.

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Originally Posted by ddd View Post
This way is allmost impossible practically. The set of R-s are wery hard to select due to numerous contradictory requirements. If you stick on this way, take the signal from the plate (trough a C) for far easier matching (high-Ohm Rfb2).
I attached a possible schematic below to see if this is what you mean. Any C and R suggestions there?

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You can ground the cathode resistor through the second coil or part of it. Add a cathode capacitor to increase the gain of the stage as well.
Mind the direction of the coil.
Also on the schematic. Is this what you have in mind?
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Old 5th October 2009, 06:14 AM   #15
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Originally Posted by Gordy View Post
I think that I have seen that done, but I can not remember where.

If you have not purchased the transformer yet then you could specify one specifically for cathode winding feedback. Tim de Paravicini did that in his EAR HP4 headphone amp.
I made a SE EL84 amp a while back grounding the cathode through the secondary. My EL84 SE The problem in doing that way makes it basically a unity gain stage that will require a driver.
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Old 5th October 2009, 08:08 AM   #16
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That may well have been where I saw it. I also just remembered that Jan E has an amp with transformer cathode feedback on his site: http://www.veiset.net/PX25/index.html . Not now applicable to dsavitsk, but interesting anyway.


Dsavitsk, I think the feedback equations are available in the Designing Single-Stage Inverting Feedback Amplifiers section of http://www.aikenamps.com/


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Old 5th October 2009, 10:41 AM   #17
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Originally Posted by dsavitsk View Post
1.I attached a possible schematic below to see if this is what you mean. Any C and R suggestions there?
2.Also on the schematic. Is this what you have in mind?
1.Not easy task! Ex. Rfb2=1M, Rfb1=22k, C=100n. The feedback will depend on the position of the pot.
2.Yes.
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Old 5th October 2009, 10:51 AM   #18
ddd is offline ddd  Bulgaria
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The problem in doing that way makes it basically a unity gain stage that will require a driver.
Right. It's 100% feedback. You need a coil's tap to lower it.
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Old 5th October 2009, 07:33 PM   #19
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1.Not easy task! Ex. Rfb2=1M, Rfb1=22k, C=100n. The feedback will depend on the position of the pot.
That's probably not good ...

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Right. It's 100% feedback. You need a coil's tap to lower it.
100% feedback is fine as phones rarely need any more than a buffer. The problem, however, is that the secondary winding DCR is ~6 ohms. With 20mA of current through the tube, that puts 120mV of DC on the headphones, which is way too much. What if only the Rk bypass cap goes to the secondary with the resistor going straight to ground?
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Last edited by dsavitsk; 5th October 2009 at 07:38 PM.
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Old 6th October 2009, 07:31 AM   #20
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Originally Posted by dsavitsk View Post
That's probably not good ...



100% feedback is fine as phones rarely need any more than a buffer. The problem, however, is that the secondary winding DCR is ~6 ohms. With 20mA of current through the tube, that puts 120mV of DC on the headphones, which is way too much. What if only the Rk bypass cap goes to the secondary with the resistor going straight to ground?
Why not triodestrapping w/o NFB instead? YouŽll still get 1W instead of pentode 2,5W with Ia=40mA.
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