Taking the feedback after the output coupling cap means greater phase shift and frequency selectivity in the feedback circuit, which may or may not be a problem depending on the output transformer. And bypassing the cathode of V2 increases the open-loop gain by about 6dB. I knew both these things in theory--which is why I accounted for 'em in the circuit--but after seeing your message, I went to the bench and confirmed both experimentally just to be sure.
My trevails with this circuit serve as a reminder that SPICE can sometimes mislead you. Using 12AV7 models that have proven very accurate in other circuits, the simulation said that this circuit would do +22dBM before crapping out. On the bench, I discovered that it can, but not with a 12AV7 as the output tube! The old-school (non-SPICE) calculations I did intially turned out to be closer to the truth--but SPICE was less tedious than dealing with the loadlines in-depth.. and it told me something closer to what I wanted to hear, so I listened :roll:.
But all is not lost. It actually does reasonably well with a 12BH7, especially if I increase the negative feedback by 6dB by strapping another 100K across the 100K feedback resistor that's already there. Max output has gone down by 0.5dB due to the shunting effect of the (now-reduced) feedback resistor. But the near-halving of THD across the normal operating range seems worth the slight loss at the max end.
12AU7 does just a little bit worse than BH7 at +4 and +14 but craps out about 2dB sooner. And AV7 is pretty much the same as AU7 at +4 and +14 but runs out of steam at a mere +18.5dBM output. So either the AU7 or the AV7 could be used as an emergency sub, but BH7 is definitely the output tube to use in this circuit. A 12AV7 definitely works great for V1, however.
As I mentioned earlier, V2 plate and cathode resistors have been changed to 12K and 270 ohms, respectively. I'm thinking it'd be cool to add a switch to change between 100K and 50K (adding another 100K in parallel) to give "high gain" and "low gain" modes for the output stage. The cathode bypass cap could be switched in and out to do the same for the input stage, although I have yet to check if there'll be any problems there with increased hum from an unbypassed cathode. (I'm using AC heaters).
Well, I have paying work that demands my attention so I'm gonna have to put this on the back burner for quite a while.
PS: Bo, you had a good point about grid stoppers on V2. Adding 1K there definitely helped to get rid of some "squeeging" I noticed during testing, when running the output stage open-loop with signal input directly to its grid.
wink
