presence squeal in express (was: low end oscillation)

[austinb]

Some preamp tubes are more prone to oscillate without a grid stopper than others. Try different tubes in V1 if you choose not to use the grid stopper.

[Structo]

I'm having the same problem on an amp that I just built.
It's a mongrel 6V6 amp but if I max the volume and the presence I get the squeal too.

The NFB wire going to the presence pot could well be the problem if it's too close to the power tube grid wires, especially running parallel to them.

It looks pretty close in the photos of the Roe's amp too.

No it's routed around the inside of the chassis away from the power tubes.

[Roe]

problem disappeared after I reversed the wires on the presence pot and moved the two wires to pin 5 on the el34s a little away from the power supply wire underneat the board