by EWBrown » Tue Jun 21, 2011 6:38 pm
If you want to inject the NFB at the SRPP cathode, one method would be to replace R7 with an 820 ohm resistor, bypassed, at the cathode, and in series with an unbypassed 180 ohm resistor to ground, and then inject the NFB at the junction of the 180 and 820 ohm resistors. That preserves the original 1K total R7 value, and allows injection of an NFB signal.
NFB could also be injected at the right hand grid of the LTPI, replace the "short" to ground with a 100 ohm, or less, resistor, and feed it there, though this method could get a bit "squirrely". It does work well with simple PP amps, in which the audio input signal is fed into the grid of the left hand side LTPI triode, and there is no preceding VA gain stage.
FWIW, as an alternative to the fixed valye 20 mA CCS device, you could use an IXCP 10M45S, which is adjustable, and then tweak up the LTPI "tail" current to its optimum value, by changing the current set resistor value, or use a 500 0hm or less) trimpot, to trim it in.
Just a thought, I'm sure that there may be other, and better methods...
/ed B
Real Radios Glow in the Dark