Bright Cap's screeching on JTM45/AA864 build

[glassesnvests]

So I've finally got the '45/Bassman marriage working, for the most part:
The final issue I've been having is that the Bright caps, on both channels, cause a lot of high frequency screeching. Does this mean that I have other oscillation elsewhere in the amp that I need to trace down?
A schem attached... Thanks guys!

[sluckey]

Does this mean that I have other oscillation elsewhere in the amp that I need to trace down?

That's possible. Disconnect the NFB and see if the oscillation goes away.

[glassesnvests]

Alright, so I cut the NFB, and no dice.


I now have access to an O-scope and function generator, is there a way to utilize this to check for oscillations? The screeching is pretty bad... Swapping caps and messing with values didn't help at all.
Thanks,
GnV

[HotBluePlates]

If it's screeching on its own, you need only connect the o'scope to any tube grid and look for the noise. Start at the phase inverter grid and work your way backwards.

Ideally, you'd see a point where you hear screech but see nothing on the scope; you would know the oscillation is happening at a later point in the amp.

You could do something similar by unsoldering one leg of a coupling cap to break the circuit. Start somewhere near the midpoint of the amp circuit; if screech stops, the source is early in the circuit. Resolder the previous cap, unsolder a cap at the midpoint between where you just tried and the input jack. Keep dividing the suspect circuit in half until you find where it's happening.

99.9% of oscillation (that's not due to reversed-polarity negative feedback) is a wiring issue or high-level wiring passing near a low level input.