Marshall Randy Rhoads oscillation

[lowell]

There's something wrong w/ this amps PI stage. Power amp is good. When the amp is turned on it may or may not oscillate. As soon as the Presence knob is turned it begins. The Presence does affect the pitch of the oscillation. It goes from a high squeal to a motorboating sound. I've replaced the PI tube, replaced the presence pot, measured all resistors and continuity between components, DC voltages checkout good, I've replaced C24 which is the filter cap for that stage, grounding the middle lug of the treble pot does not kill the oscillation, connecting both PI grids kills the oscillation, all 10k dropping resistors checkout fine. Disconnecting the negative feedback loop makes it worse.

[Enzo]

Power off.

Do you have 15k from pins 3 and 8 of the PI to ground? Do you have 1 meg from pins 3/8 to pins 2 and 7? Don't measure the resistors, measure pin to pin resistance.

Power on.

OK, what voltages are at the plates? The cathodes? What voltage is from cathode to grid on each side. Do not measure grid to ground.

[lowell]

Enzo thanks,
I should've been more descriptive in my post. I did do these measurements.

There is 15k from 3/8 to ground. There is 1 meg from both grids to cathode.

PI Voltages:

1: 231v
G/K: -1.1v (both grids)
6: 216v
3/8: 36v

Also of note is that measuring voltage on grid pin 7 kills the oscillation.

[Jazz P Bass]

Stupid question.
Did you try swapping the primary wires of the output transformer?
It sounds like positive feedback.

[lowell]

Does sound that way and I thought of that. In initial post I stated that I disconnected it and the problem was worse.

[52 Bill]

Does sound that way and I thought of that. In initial post I stated that I disconnected it and the problem was worse.

I think that Jazz was suggesting changing the phase of the primary winding by swapping the two leads.

[lowell]

Shouldn't matter if the NFB is disconnected though right?

[lowell]

JPB, Right you are. That fixed it. VERY odd that disconnecting the NFB loop made it worse. ?? After exhausting all my efforts I was thinking of your post and saw that the OT secondary wires on PIN 3 did indeed look as though they had been resoldered, badly. So I hoped that someone had rewired it for whatever reason and didn't know what they were doing.

[Jazz P Bass]

Glad that you got it, Lowell.
O/k, here's the scoop.
If the amplifier has a negative feedback loop, set the amp up, dummy load and monitor the output (DVM, scope)
While monitoring the output, if you disconnect the feedback circuit, the output should go up.
If it goes down, then the primary is reversed. (positive (regenerative) feedback)

[lowell]

I'm still confused as to WHY polarity in the OT matters when there's NO nfb.

[Jazz P Bass]

The polarity does matter.
I rebuilt a Fender Champ & the replacement output transformer winding colors where incorrect.
Squeeled like a pig.
Not being a transformer engineer, I would suggest reading this very informative article.
http://nptel.iitm.ac.in/courses/IIT-...I/pdfs/1_7.pdf