Peavey Classic 50 feedback issue

[cablercdd1]

I'm working on a Peavey Classic 50 with 4 10" speakers. When the presence control is turned fully counter clockwise even with no input it starts to feedback at 6.2 khz with harmonics at 12.4 and 18.6 khz. I resoldered tube sockets, checked component values in the presence circuit and couldn't find anything wrong. It seems to be coming from the phase inverter. If I change the 12AX7 to a lower power tube like a 12AU7 it doesn't feed back. I've tried several 12AX7's and they all do it. Anyone ever run into this?
Paul

[cablercdd1]

I've attached the schematic that I'm using.

[Enzo]

Any chance the OT primary wire connector is reversed?

[cablercdd1]

Enzo, you're amazing. That's what it was. I would have never thought of that.
Thanks

[nosaj]

Now that you've heard that sound itll likely be the first thing you check in the future.
nosaj

[cablercdd1]

You've got that right!

[Chuck H]

I take this

When the presence control is turned fully counter clockwise even with no input it starts to feedback at 6.2 khz with harmonics at 12.4 and 18.6 khz.

to mean that the amp was actually stable with conservative settings(?). Which has happened to me also. Usually when you have OT leads inverse with amps that use NFB the problem is constant, instant and obvious. But there ARE times when it's more subtle and the amp just sounds "wrong" but you can't put your finger on it. Until something like instability shows itself. Even then, when it's only with something like high presence or treble settings there are other things to suspect than the OT leads.

Enzo is a Peavey guy that serviced many of those amps I'm sure. I'm still impressed that he was able to make this diagnosis with no reference by the OP that the OT leads had ever been disconnected. I might have suspected a microphonic tube, a mistake in lead dress for a repair or a bad ground.

[g1]

I take this
to mean that the amp was actually stable with conservative settings(?).

No, he mentioned turning presence off (counter clockwise) was when it happened.

[cablercdd1]

The customer complaint was it didn't sound right. It did seem something was off about it when I played though it. I marked the connectors before I took it apart to resolder the tube sockets and I put it back the same way. The presence control got turned to 0 after I put it back together which is why squealing showed up then I guess. Someone must have reversed the transformer plug before me.

[Enzo]

I do think it is unfortunate they didn't use a connector that polarizes. The very fact you can connect it backwards pretty much guarantees someone will. It is so simple to use, say, two males and a female pin in the connector making it impossible to install wrong.

[nosaj]

You know on just about all the 70s peavey stuff I've worked on plugs were all pretty much polarized using male or female pins.
Seems odd they would have stopped.
nosaj

[Enzo]

I don't know that they stopped so much, as they just didn't on these newer models output transformer connectors. Other connections in the same amps are keyed. Beats me.

[Dave H]

...he mentioned turning presence off (counter clockwise) was when it happened.

When the OT is reversed turning the presence CCW increases positive feedback at higher frequencies causing oscillation.

[Chuck H]

No, he mentioned turning presence off (counter clockwise) was when it happened.

Ah. Right. Makes perfect sense now.

EDIT: Enzo is still pretty cool though

[Enzo]

It is a Zen thing...I AM the amp.