Fender custom Vibrolux Reverb

[Leo_Gnardo]

Right..... but Fender did not make it easy. You have to pull the two Trem Pots off the front of the chassis and turn the little PCB over so you can get at that resistor.
It is not the end of the World. But for a fixed-bias amp, you would think Fender might have put the bias circuit somewhere that was more accessible.?
I was thinking to install the new power tubes and see where the bias is at. If they are not too hot or cold, it should be fine.
I was thinking that with -41 volts on the grids, i am probably not in danger of red-plating the new tubes. So i should have time to check without doing any damage to the new tubes.?

Haven't seen the schemo for this amp posted. Anyway, if Fender didn't include a bias adjustment pot, it's not too much trouble to add one. On their amps where the bias supply is on the vibrato control board, I swap the lower leg of the bias voltage divider for a fixed resistor plus trimpot series combination. If you feel like drilling a hole & installing a Fender style trimpot go ahead. Usually what I do is use a Bourns "blue cube" trimmer & mount it right on the PC board.

[trem]

Haven't seen the schemo for this amp posted. Anyway, if Fender didn't include a bias adjustment pot, it's not too much trouble to add one. On their amps where the bias supply is on the vibrato control board, I swap the lower leg of the bias voltage divider for a fixed resistor plus trimpot series combination. If you feel like drilling a hole & installing a Fender style trimpot go ahead. Usually what I do is use a Bourns "blue cube" trimmer & mount it right on the PC board.

Yeah...that would be a good idea. This is my friends amp (i am a drummer anyway, i do not play guitar) and he has a gig tonight. So i do not want to try something like that now.
But if he brings it back to me for a new RCA Jack, i will ask him about that.
Thanks For That Tip

If i can ask a "Theory Question" ....... The voltage divider is R58 and R59.
You two guys have said to change R59. Why is that a better option than R58.?
Would changing R58 cause a problem.?
Thank You

https://el34world.com/charts/Schemat...lux_manual.pdf

[Leo_Gnardo]

If i can ask a "Theory Question" ....... The voltage divider is R58 and R59.
You two guys have said to change R59. Why is that a better option than R58.?
Would changing R58 cause a problem.?
Thank You

https://el34world.com/charts/Schemat...lux_manual.pdf

While it's true that varying R58 would also adjust bias voltage, you'll find that most designs that include adjustment make the variable resistance appear in the leg of the voltage divider that goes to ground. In this case R59. Should there be a failure of the potentiometer, it will usually go open circuit, and if it's in the grounded leg that means bias voltage will fly up to some larger value. If the pot goes open circuit in the leg fed from the bias supply, bias goes to zero, poof go your output tubes, and maybe they take the output transformer with them.

A couple years ago I was tasked with repairing an old Marshall Major 200W that curiously enough had the bias pot in the supply leg. Dam' big pot it was too, about 2 inch diameter. Came in with a blown OT and wrecked output tubes, a helluva repair bill! I didn't hesitate to re engineer the bias adjustment scheme before handing the amp back to its owner. Why Marshall did that - who can guess? Wasn't a good move...

[trem]

While it's true that varying R58 would also adjust bias voltage, you'll find that most designs that include adjustment make the variable resistance appear in the leg of the voltage divider that goes to ground. In this case R59. Should there be a failure of the potentiometer, it will usually go open circuit, and if it's in the grounded leg that means bias voltage will fly up to some larger value. If the pot goes open circuit in the leg fed from the bias supply, bias goes to zero, poof go your output tubes, and maybe they take the output transformer with them.

A couple years ago I was tasked with repairing an old Marshall Major 200W that curiously enough had the bias pot in the supply leg. Dam' big pot it was too, about 2 inch diameter. Came in with a blown OT and wrecked output tubes, a helluva repair bill! I didn't hesitate to re engineer the bias adjustment scheme before handing the amp back to its owner. Why Marshall did that - who can guess? Wasn't a good move...

Ah...OK....right.
That is another thing i have asked before, and forgotten over the last few years.

Unfortunately, when i got Craig's amp back together, the Verb is not working (Again).
Maybe the loose RCA jack is shorted to its self.?
I have run out of time, i do not have the Jacks to replace it, and i will just have to give this back to him.
It looks like i will be asking about this Reverb again in a few weeks. He will have to bring it back to me later. This sucks
C'est La Vie

Thanks Again for all of the help

[vintagekiki]

I have run out of time, i do not have the Jacks to replace it, and i will just have to give this back to him.

Take the finished RCA/Cinch cable, cut it on half and replace (solder) directly from the PCB.

[trem]

Oh Boy ...... this was weird.
I got the new Jacks on, one of the old ones WAS Shorted.
So i tested it, out of the chassis with a spare tank...just because that was easier, and it was fine.
I put the chassis back in, and there was no Verb.

Common sense would dictate some type of problem with the tank.
So i pulled The ORIGINAL tank out of the cabinet, and out falls that foam...kind of 3x3 inches that frequently (always.?) gets shipped with a new tank.

So, i double checked with my friend.
He bought this amp used, a few years ago, from the original owner, and the Verb worked fine until just recently. Craig said the Reverb got noticeably "Weaker" and then did not seem to work at all.

I guess Fender never removed that foam.?
It probably wedged itself (just enough) in the tank...for a long time...and then eventually worked its way onto the springs.
With that foam gone, of course, the Reverb Tank sounded fine.

Some of you guys, that have worked on A LOT of amps might have seen this before, from a major company like Fender.?
They assemble A Zillion amplifiers each year. I do not suppose it is outrageous that they make a mistake like this every once in a while.
I still have a hard time believing the Reverb ever worked at all, but it did.!

So, it is fixed. My "reputation" is intact.
Thanks To All, for the help and suggestions