67 Vibrolux Reverb - Distortion/no signal???

[billyboy]

I rebuilt the power supply in this amp 2-3 years back, all eletrolytics , and now it wants to distort when you plug in and play at what should be a clean level. It may put out a clean note or two and then it goes distorted and very quiet but still distorted, sometimes a high pitch whine/howl noise. I opened it up and cleaned grounds, ins and outs, checked voltages, cleaned pots, did the poke at components with dowel, and tested tubes(for what that's worth). There were a couple of not so smooth(turned a little stiffer) pots but seemed to work fine after a wee bit of cleaner but still slightly stiff. Put it back together and worked fine and left it on for a few hours and it still worked fine. Worked as it should the next morning when the guy came to pick it up. Two weeks later it's doing the same old same old. Is it possible that it's just a bad tube doing this? I forgot to do the bang on the tube test when it was here. Don't know where else to start. Could an intermitant power cap do this? Help!!

[pdf64]

I'd want to check voltages (plate grid and cathode for all tubes) with it in the fault mode. Replace screen grid resistors? Maybe a bad connection (tube socket retension?) or shorted signal or bypass cap screwing up bias. Worst case would be bad insulation in the OT - has it been run into above spec impedance speaker? Peter.

[billyboy]

Yes, the screen grid resisters were replaced. I also thought maybe there might be a bad cap in there throwing off the next stage. It's always been run in the stock speakers as far as I know and the guys had it for many years but who knows the previous history,, well, maybe it's a bad tube, I had put in some 5751's that I got from a shop closing it's doors.

Thanks for the input!!

[Enzo]

It could also be an intermittant resistor or a failed solder connection to some grid resistor. If the grid resistor of some 12AX7 stage is open, then that grid can accumulate charge and drift up to some DC voltage that upsets the amplification. One sign this is hte case would be that just connecting your meter to the grid will wake it up - your meter internal impedance will function as the grid return.

[billyboy]

I did do a pretty good check of the loose resister, components with a wooden dowel with signal applied and the voltages were where they should be but then the amp was working as it should at the time. I did find a sort of noisy preamp tube and replaced it but couldn't get it to react like he said it was doing. We decided to have him take it home and hope it was the tube. I guess if it reacts again it's got to be a bad cap or resister lurking in there somewhere.

Thanks for the tips guys.

[catstrat]

vibrolux reverb early distorting

This amp came to me in need of a filter cap service which i did along with replacing all bypass caps and bias voltage supply filter cap. It begins to distort at 3 on volume on either channel. I changed output tubes and pi tube and checked all voltages and all were good. Also checked for cracked solder joints and found none. Bias is at -48 volts at grids which should be OK and plate volts are 420 volts are so. It's a silverface with some sort of a pull switch on the vibrato channel volume. I can't seem to locate a schematic that matches. Any ideas?