tube amp oscillation

[W1RKW]

I have this old stereo tube amp (50 years old) that was my fathers and I figured on getting it to work since it was in very good cosmetic condition.  The problem with it was the filter caps were not up to the task so replaced them.  Amp works good for the most part however, when on the scope I noticed one channel oscillated at about 200khz when the treble control was turned to the max cut position, with a perfect sine wave too. Poking around and simply passing my hand passed one of the output tubes (6BM8) caused the oscillation to diminish or vanish altogether.  I narrowed it down to one output tube which happened to be the same type as the other 3 but a different brand, Sylvania vs. Mullard. The Sylvania tube was the oscillator.  Moving the tube from one channel to the other verified that it was indeed the tube.  Looking closely at the Sylvania and the Mullard there were subtle construction differences.  Would that be enough to cause this?  On the TV-7 tube checker the Sylvania checked good. It was slightly weaker than the other 3 Mullards but tested good none the less.  I guess maybe the Sylvania is a lesser quality tube.  not sure.  Just odd. I wouldn't think the circuit would care.

[kb3ouk]

Wonder if moving your hand over the tube might've had some kind of neutralizing effect while your hand was there?

[W2PFY]

I guess maybe the Sylvania is a lesser quality tube.  not sure.  Just odd. I wouldn't think the circuit would care.

I think it may have been the tube itself. Maybe you can find another Sylvania tube of the same vintage and plug it in and see if it does the same thing? Another thing to do is whack it with a screwdriver while it is in operation and see what the scope does. If the problem goes away, just might be some loose structures within the tube.

[kb3ouk]

That bit about whacking it with a screwdriver reminds me of a Sylvania 6K6GT I had in an old Crosley one time. After you turned it on and it got warmed up, if you tapped on the tube, it would immediately start glowing purple. Shut it down, turn it back on and it would be fine until you hit it again then the glowing would start over.

[W1RKW]

I guess maybe the Sylvania is a lesser quality tube.  not sure.  Just odd. I wouldn't think the circuit would care.

I think it may have been the tube itself. Maybe you can find another Sylvania tube of the same vintage and plug it in and see if it does the same thing? Another thing to do is whack it with a screwdriver while it is in operation and see what the scope does. If the problem goes away, just might be some loose structures within the tube.

Hi Terry,
Yeah, I suspect the Sylvania tube itself. Moving it from one channel to the next, the oscillation follows.  I just acquired 2 Mullards. I'll try them since the originals are Mullards as well.  Looking very closely at the Sylvania and the Mullard there are differences in the construction.   The plate structure in the Sylvania is slightly shorter too.

I guess this is one of those things like the HW101 I used to have. Certain brand and dated tubes behaved differently, some good, some not.