single ended tube amp help

[duffy878]

First off... the amp in question is a single ended kinda-champ style amp with a 12AX7 pre and 5881 power tube with SS rect. It has one input, a gain control, treble, bass, and a master volume.

The amp is passing signal and i can hear my guitar. tubes glowing. light on. nothing blowing up.

The issue is that if i turn the master up above around 4 i start getting insane distortion and a very high pitched sound (squealing i believe). this increases as i turn up the knob.

ive heard that this is usually attributed to reversing the leads on the OT, but would that be sensitive to the volume control or would that be constant?

any insight would help. thanks.

oh, heres some voltages if it helps.

12AX7
1 - 219
3 - 1.77
6 - 161
7 - 0.38
8 - 2.65

5881
3 - 359
4 - 365
8 - 29.5

[tubeswell]

Yes it sounds like you have +ve feedback going on, when what you want is -ve feedback. (Does the amp have an NFB lifting switch? - that is a quick way to confirm it)

And yes, a solution for +ve feedback is to swap the OT primaries around. (Not all OT come from the factory with the correctly-coloured wires in where you would expect them).

[Enzo]

And if your amp doesn not have negative feedback, then your amp is just unstable due to lead dress or some other layout issue. or possibly a lack of decoupling in the B+ supply.

[duffy878]

thanks for the help guys, ill try swapping the OT leads later.