Ampeg SB-12 Buzzy distortion/High Freq oscillation

[Westrek]

1973 Ampeg SB-12 bass amp. Preamp out into another amp sounds fine, but consistent high frequency oscillation (probably in the 100K area) at the OT transformer output. The amp is loud with raspy distortion. Plate, screen voltages are all in the ballgame, bias is -52 vdc with around 30ma current draw at the plates. Tubes have been swapped out. Several out of tolerance resistors replaced. It came to me from the customer with a Fender Showman OT transformer hanging by one bolt, so it was replaced with a Fliptop replacement. The NFB has been lifted with no effect except the oscillation level being slightly higher. It's a pc board amp and I've checked the traces and soldering. There are unshielded wires connecting the eq sub-board to the main pcb. I've replaced these with shielded cables with no improvement.

I've worked on hundreds of guitar amps and have never seen an amp that is this simple be so frustrating. I have seen other posts here and elsewhere describing similar issues with the SB-12, but never really saw a resolution to the problem.

Any troubleshooting suggestions would be great.

Thanks

Mike

[TigerAmps]

Just a long shot:

Since the OT was replaced, did you try to reverse the OT wires going to the plates of the tubes?
I once replaced the OT on a B25 and the wire colors were reversed; surprisingly, the 7199 tube had the oscillation symptoms.

I have a 67 SB-12 using the 7868 tubes (similar; but different) ...

Could you post pictures please?

[52 Bill]

Bad filter cap? If the caps are old, try paralleling a new one across the old ones, one at a time and see if it helps.

Worst case is add a few small value snubber caps and see if you can stop the oscillation. The worst case I've had to deal with was an Oliver amp that I had to cut the tone control traces from the pc board and run shielded cables elevated away from the rest of the circuitry.

[Westrek]

Thanks. Yes, the OT plate wiring is correct. One on the many avenues I've pursued. Here's a couple of pics.

[g1]

I see an insulating washer on the ext. spkr. jack, and I see some nuts and washers loose below. As well, you mentioned it's been worked on before you got it.
Is the ext.spkr jack completely isolated from chassis? It should have another shoulder type insulating washer on the inside.
Maybe not related to your problem, but worth checking. It's something that can cause odd problems and is often messed with because it seems unimportant.

[TigerAmps]

Not sure where you are ...

The next logical thing is to isolate the problem with the external amp jack.
Disconnect the signal going to the phase inverter and run it into another amp, is it distorted?
If you inject signal into the phase inverter, with the preamp disconnected, is it distorted?
In other words, is the distortion in the pre-amp or power amp?
Also, you can try removing the little tubes one at a time to see which one affects the problem.
My gut is that it is in the power section of the amp, but you still need to prove where the problem is in the amp.

Did you replace the 25/25 cap in the PI?

Also, pictures of the circuit boards?

[MarkusBass]

I'm not sure whether I can help but just few days ago I have very similar problem with Ampeg Portaflex. The reason for the problem was that the wires from the OT are very close to the input tube. I had to move one of the wires to the OT away by 1-2 cm. And this solved the problem. Since you replaced the output transformer, I suspect that you may have very similar problem. What happens when you remove the first tube and provide external signal to the power amp? Does the problem goes away?
Can you post a photo of the circuit board?

Mark

[Westrek]

Thanks. I checked the insulating washers and it is isolated from the chassis. From the chassis to the nut of the jack it measures 1.8 ohms which would the DCR of the OT secondary winding.

[Westrek]

Yes, it's in the power amp. The pre output sounds fine. I did replace the PI cap. I'll do another round of tests in a few hours and report back.

[TigerAmps]

Now - just to rule out some other stuff:

Do you have another known good output transformer?
If so, disconnect the one that is in there and wire in the known good one outside the amp (sit it on the table next to the chassis).
I would extent the primary wires to around 1 foot and connect the secondary to an output jack outside the amp (floating).
I like the cliff jacks or any other plastic jack. Tape everything up so it is insulated and does not short to anything (including unused secondary taps).

Now you have a transformer and output jack "isolated and outside the chassis".
This eliminated the output jack and the Fliptops transformer.

I this works, then the issue is either the the jack or the transformer ...

Also, I once had a problem where the "shorting" output jack was not opening and this caused me all sorts of pain.
I had this happen on an Ampeg B25 and a Peavey Butcher.

Also - Leave the feedback loop disconnected ...

[Westrek]

Thanks to all for the suggestions. Never could resolve the problem and returned the unit to the owner.

Thanks again!