Trace Elliot V4 bass amp oscillation issue

[diydidi]

I have this amp for repair.
See attached schematic.
It has 4x KT88's as outputs.
Tubes test fine on my tester.
All filter capacitors test good.
Managed to get sound from it, but it has one last issue.
There is an oscillation when reaching about 90% of full output. It can also be heard when connecting a speaker cab and playing bass through amp.
It sounds like a scratchy distortion on the peaks.
I have nailed it to the power amp section by injection the signal at the power amp input.
Anyone have experience with these amps and their issues?
Any help will be appreciated.

[Jazz P Bass]

I would be looking at that odd duck C14.

[pdf64]

Just wondering, never seen inside one but are the 22 ohm g2 and 1k g1 grid stoppers mounted on the tube socket terminals?
Are the protection diodes strung across the OT primary all good?

[diydidi]

I will check on Monday!!

[diydidi]

Do you think I should try removing it?

[diydidi]

C14 wasn't installed on this particular amp, but adding it didn't make any difference

I would be looking at that odd duck C14.

[diydidi]

Did some further testing today.
I think I might have a faulty output transformer.
I can hear it ticking when the instability happens at higher volumes.
How do I test it?

[The Dude]

My bet is that, if it is the transformer, the problem wouldn't show up via any test methods, since it only happens at higher volumes. It could be arcing at higher signal levels. If you suspect the transformer, I'd recommend tacking in a different one to see how it works. Do you have another OT laying around that you could temporarily hook up?

[TimmyP1955]

Make sure that all of the grounds are good. This can be easy to overlook.

Just for grins, I'd take D11 and D12 out and see what happens.

[diydidi]

Thanx for all the help guys.
I managed to track down the issue. When I cup my hand over a specific output valve the oscillation disappears. So it seems I have a faulty KT88. Will have to replace all 4 with a matched quad.