Replaced V1 plate resistor, now I am getting squealing

[kingtf]

I am working on a Fender Blues Jr. It had some cracklng in the output. I tracked it down to a faulty 100K plate resistor (R3) on the V1 (12ax7). So, I just replaced it with a new one of equal value. And the crackling has disappeared. However, now I am getting a high pitch squeal that gets louder as the volume is turned up. It is barely noticeable at lower volumes, in fact I cannot hear it at low volumes myself (bad hearing, go figure) but others can.

If I remove the V1 tube (12ax7) the squealing is gone. So... what is causing this squealing? I am assuming something to do with the resistor, quality, type, value (although it's the same as it was originally). The amp sounds fine other than the squeal.

Any HELP!! would be greatly appreciated, thanks all.

[Jazz P Bass]

12AX7 squeal

Can you try another 12AX7 in position V1?
If you do not have a known good one, swap V1 with the phase inverter tube (last preamp tube before the power tubes)

[kingtf]

Thanks for the reply. I have tried swapping out tubes I have extras laying around. Same result, squeal... it did not do it before I replaced the resistor.

[Jazz P Bass]

Fender Blues Jr

Do any of the controls effect the squeal?

[kingtf]

The only thing that affects the squeal is the volume, the pitch is the same just louder or softer. At lower volumes it is barely audible to me.

[Jazz P Bass]

Fender Blues Jr

You are going to have to take some voltage readings.
Compare them to the schematic.
Post them here.
Specifically V1B pin 6,7,& 8.
V1A pin 1,2 & 3.
My thinking is the 100K plate resistor that was crackling, may have damaged C1 and (or) C2.
Also make sure test point 2 is sound. -14 Vdc.

[stokes]

A common source of oscillation in these amps comes from the PI.It is caused by the ribbon cable being bent in the wrong position.Try "stretching" the ribbon that goes from the tube to the circuit board.Stretch it so it forms a kind of right angle,vertical from the tube socket and then 90 degs to the board.It could be that when you replaced the resistor you moved this ribbon out of position.I know it sounds simple,but I read it somewhere,and actually used it in one case.Some clown thought it would be "neater" if he pushed these ribbons down closer to the chassis and freaked out when he turned it back on and it squeeled like a pig,hence the warning,"No user serviceable parts inside".

[52 Bill]

And don't forget to check the filter caps.

I've seen a bad cap for the preamp section cause this kind of oscillation.

[kingtf]

Jazz, thanks for the suggestion. I will check those pins and the test point and post back here soon.

[Enzo]

Watch the blue and brown wires from the output transformer as they connect to the tube socket board. MAke sure the wires are routed to be at right angles to the wide ribbon they cross. Well maybe only one crosses, but position matters.

[kingtf]

Yeah, I am aware of those wires crossing the ribbon cable on the phase inverter. It is actually the blue wire that crosses. It's as far away as I could get it, still not a lot of room between the two though. But that isn't generally a problem on the cream boards according to BillM.

Anyway, I need to take some voltage readings and check the C1 & C2 caps.