Question re Allen Wright PP-1cs

[zintolo]

Thanks, I like alot the idea, but I see a drawback: when I balance the PI via the 100 Ohm pot, I'll change the amount of feedback on the two sides.

 

[U130421]

As long as the sum of the cathode resistors and the sum of the feedbacks remain constant, the input receiving triode gets the same NFB as the secondary remains floating. What bothers me the speaker wire can as antenna inject noise to the input.

 

[zintolo]

I will try a stereo shielded cable. I will try it on an amp I have with a cascoded PI with LSK170 on bottom and 12ax7 on top.

 

[U130421]

A good way to ground and ac gain balance is to use an adjust on the secondary of 500 ohms, it will ground by 125 ohms and offer you overall ac balance gain adjust.

 

[zintolo]

Thank you!
I have to say that I've applied (on LTSpice only by now) this differential feedback from secondary of the OPT to the sources of the U440 of this other amp I have (improvements on 12AX7 12AT7 EL34 schematic?) it seems it works better in that case than in this one. I will update that thread too for comparison.

 

[zintolo]

I will do some tests on the current amp I have before working on this one.
Which OPT you'd suggest?
Hammond 1650N ( https://www.hammfg.com/files/parts/pdf/1650N.pdf )
Toroidy 6k6 ( TTG-EL34PP - Tube output transformer [6,6kOhm] 2xEL34 / 2x6L6 Push-pull or similar - Shop Toroidy.pl )