Fender Super Reverb Vibrato Fades

[65basser]

I'm working on a 70's Super Reverb(non-master) that has strange vibrato problem. When you turn the speed knob towards the slowest or fastest settings, the vibrato fades away. If you turn the speed knob back towards the center position and sometimes play with the intensity knob, it will come back. The ckt has not been modified but it does have a very high B+ voltage of 505v.
Any advice is appreciated!

[Enzo]

I would first suspect those 30-40 year old caps in the circuit. And it is possible the trem tube is weak. But my money is still on the caps.

[65basser]

The caps are new and I've swapped out the tube with no difference. Seems to be especially touch on the low end of the speed range.

[Enzo]

OK, basics behind us, start at the start. How do ALL the voltages compare to the schematic? And it matters wherther the trem is running or not

The trem is fading away. Scope the trem oscillator signal at the plate, does THAT fade away, or does the LFO continue and just the audio function faded away?

The caps are new, how about the resistors? Are they more or less close to value? And by caps, we are talking about the three feedback caps and the cathode bypass on the oscillator tube?

And just so it doesn;t come up later, put your meter probe on some B+ eyelet in the trem circuit, now pick it up and stick it back down on the black eyelet board itself, next to but not touching the eyelet. ANy DC there? We COULD have the old partly conductive film formed on the eyelet board.

[65basser]

Got it! The cathode bypass cap on the oscillator side was missing. One 25uf cap later and it's up and running although I took the long way to get there. The electrolytics were replace recently and someone apparently forgot one. Strange thing is the trem tube is a sovtek and when I subbed a JJ it wouldn't work at all. I tried several different JJs and none would oscillate for more than a second or two. Seems like JJs don't like the high B+ in this amp or maybe I just got a bad batch? Thanks for your help Enzo.

[Tom Phillips]

... Strange thing is the trem tube is a sovtek and when I subbed a JJ it wouldn't work at all. I tried several different JJs and none would oscillate for more than a second or two. Seems like JJs don't like the high B+ in this amp or maybe I just got a bad batch?...

More likely is that you had a circuit that was just barely functioning. Changing any existing part, especially a tube given that each brand has their own unique characteristics, could through the circuit over the line of not working at all.
Cheers,
Tom

[Enzo]

I agree, a circuit that almost wants to work, and some tubes have just enough extra gain at these freqs that it allows some function that other tubes won;t. WIth the proper circuit, I bet all the tubes work.