Compensating a tube amp for oscillation

[rf7]

I've got a Cathode Bias EL34 push pull Conrad Velvet Hammer amp on the bench. It has 2 different secondary OT taps for the output. The 8 ohm tap works fine, but when I hook up the other tap, I get oscillation on the output as I turn up the volume pot. The oscillation goes away when the pot is turned past 50%. This is with no input.

The other tap is not labelled, but it puts out the most power into 4 ohms. It measures a higher impedance to ground then the 8 ohm tap, but puts out less power into 16 ohms. I am waiting to hear back from Conrad regarding what the desired impedance is for that tap.

Should I add a compensating cap to the amp to stop that oscillation? Or should I be looking elsewhere?

[olddawg]

Has the OT been replaced? Have you tried reversing the primary wires?

[rf7]

Has the OT been replaced?

No, it's the original one.

Have you tried reversing the primary wires?

Yes, and it's way worse.

So, it's been confirmed that the other tap is 4 ohms.

[Chuck H]

What about layout??? Does this other tap lead run a different route than the 8R tap? I think this other tap may be causing some greater EMF sensitivity somehow. Otherwise... Is the NFB connection at the output jack? If so then you may have a different NFB ratio with the other tap in use. Try moving the NFB connection to the 8R tap.

[rf7]

What about layout??? Does this other tap lead run a different route than the 8R tap?

No, they run the same route.

I think this other tap may be causing some greater EMF sensitivity somehow. Otherwise... Is the NFB connection at the output jack? If so then you may have a different NFB ratio with the other tap in use. Try moving the NFB connection to the 8R tap.

The NFB connection was on the 8R tap. I moved it to the 4R tap and the problem is gone. No more oscillation on either tap.

Thanks for your help Chuck!

[Chuck H]

I moved it to the 4R tap and the problem is gone. No more oscillation on either tap.

That's interesting. Because tha NFB circuit is actually voltage dependent even though it's tapped from a power oriented circuit. So by switching to the 4 ohm tap you've actually decreased the amount of NFB. Yet when the OT is reversed the oscillation is worse. So there must be some phase errors happening within the loop. If the amp sounds significantly different (i suppose BETTER would be alright), or if there's a "presence" control that seems a little lame now or any other issues that may be related to the change, then you could try to hunt down the phase errors and correct them. Otherwise cheers and well done.

[Steve Conner]

What frequency is the oscillation?

For ultrasonic oscillation (or high audio frequency if the OT is cheap and nasty) the classic "remedy" is a little ceramic cap of about 100-200pF connected between the two plates of the PI.

For low frequency oscillations (motorboating) you may need to fiddle with the values of coupling caps.

Reducing the amount of NFB always helps.

[rf7]

It was 1Mhz. I don't have the amp anymore, so I can't try the cap fix.