Justinis- The feedback resistor ratio that determines overall feedback is the 100 ohm R6 and the 2.7 k R10. Decreasing R6 or increasing R10 will reduce feedback. The 220 pF C4 is the compensation capacitor. Ideally, you would like to not need C4, but it is unlikely that you will be able to take it all the way to zero with anything but very low feedback. But the real thing I would like to get rid of is the R14 C 5 network. All of this stuff is the way it is because the designer is trying to get the best possible damping factor and lowest measured distortion into a resistive load. This is the old specmanship game, but will not lead to the best sound with most speakers, and may oscillate with some. It is also one of the reasons that we were told never to run a tube amp without a load. Without a load these designs frequently oscillate and arc over the tube sockets and catch fire, and even destroy the output transformer. Fully testing the amp with reactive, especially purely capacitive loads ( you can use 1 kHz square wave), will reveal problems. If I were to probe your amplifier's performance and optimize I would start by removing the R14C5 diddle and C4, and increase R10 to a high value such as 50k. Then measure the square wave at a couple of frequencies. You will likely find the bandwidth somewhat disappointing, but you should be able to put any capacitor as a load and get stability. The damping will not be very good either, but now you will want to begin to reduce R10 in steps to see where the amp misbehaves, like excessive ringing that doesn't damp out or downright oscillates with capacitive loading. Also try with resistive loading to see where things are bandwidth wise. Once you find the limit on where you can go with R10, raise it back up, say by 50 % to make a safe margin. Now this gives what may be an acceptable performance in terms of damping and bandwidth, but it may fall short. If it falls short, then you can start reducing R10 again, but at the same time injecting some C4 compensation back into the design, always testing with those capacitor loads at 1 kHz and bandwidth performance with a resistive load at 10 kHz. You want to determine the output impedance for damping as well--you would like to be under four ohms on the eight-ohm tap. This measurement can be done at 60 Hz by injecting the line frequency though a suitable, say 10-ohm resistor, from a 6-volt filament transformer and measuring the voltage on each side of the resistor to determine the current through the resistor and then applying Ohm's law to determine the impedance of the amplifier output port.
Anyway, I would be happy to have you over with your amplifier to show you the way I do these measurements and optimizing. Other issues may come up when feedback is reduced, like increases in hum and noise that was canceled better with the high feedback.
Good luck, David
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