Negative feedback loop squealing?

[martin manning]

The V1B triode on this amp has a switchable cathode bypass cap with values of .68uF or 10uF, as well as no bypass. The negative feedback switch has an affect when the cathode bypass switch is either unbypassed or on the .68uF position but no effect at all on the 10uF postition. What is the reason for this?

With 10uF you have bypassed the cathode resistor over most of the audio range, so the feedback signal is mostly shunted to ground. As the cap gets smaller, more of the audio spectrum is un-bypassed, and you hear more effect.

On another note I inserted the NFB signal where the cathode resistor and bypass cap of the 6V6 meet the tube socket. I noticed however that on the blackface Champ schematic the negative feedback is inserted on the ground side of the same two components. What difference does this make?

You don't mean 6V6; 12AX7, right? Blackface Champs (AA764) have the NFB fed into the second triode's cathode like the 5F1, but the cathode resistor is divided into two parts, a 1k5 and a 47Ω. The 1k5 is partially bypassed with a 2u cap to increase gain at low-mid frequencies and above, but the 47Ω is un-bypassed, which allows the NFB signal to appear on the cathode at all frequencies.

[Yoda]

Excellent and informative reply, thank you!
And yes I certainly meant the cathode of the 12AX7 As the NFB injection point; not 6V6 that was a typo.