Vibroverb 63 Reissue Bias Pot Mod

[jmaf]

IS not B+ constant??? Coming right off the PT...

B+ changes due to the power supply's own resistance. When the power tubes pull more current, +B falls.

[Dave Curtis, dB AudioTech]

This is right up LV's alley. Give him a call. I doubt that the Tubestore or Tube Depot would do it (I could be wrong, though).

[Newguy]

This is right up LV's alley. Give him a call. I doubt that the Tubestore or Tube Depot would do it (I could be wrong, though).

Dave...LV is only JJ tubes right?

Any way, where is guy located and any one have phone number? Or current web site?

Thanks

[nevetslab]

Tweak to Vibrato PCB's R59 to adjust bias on Vibroverb amps

I know this is an old thread, but in going thru the Vibroverb amps in our rental inventory, I had to pull the chassis to deal with some noisy pots, and then have a look at some other issues on the '63 Reissue Vibroverb on the bench. No Bias Pot, as I found out. Stumbled onto this thread while looking for a ''63 Reissue Vibroverb schematic, not realizing I kinda had one in my Vibrolux Reverb folder...the Custom Vibrolux Reverb schematic shown there IS the same PCB with a layout diagram.

Sure 'nuff...there's the rest of the bias circuit on the little break-away Vibrato board. I didn't have the foot pedal plugged in...normally the vibrato doesn't run without it, but on this amp, it WAS running, modulating the bias circuit that I was trying to read.. Killed the vibrato, then, seeing the plate current was lower than I wanted (20-22mA), I tweaked R59, sitting on the back edge of the PCB closest to the pilot lamp....trimmed it down with a 47k to yield 28-30mA plate current (slight imbalance between the pair of 5881's)...about 13W dissipation per tube.

Yeah, more tedious than having a bias pot for sure, but at least it's not difficult to get at...unless I'd have to increase, not decrease the value of R59 (load resistor of the bias circuit). Got three more of these to look at.

Schematic for the Custom Vibrolux Reverb amp looks to be the same as the '63 Reissue Vibroverb in the bias circuit area. It IS NOT the same on the front end...as the Vibrolux Reverb's V1B & V2B plate circuit shares the same plate resistor, and that amp is super-noisy. This '63 Reissue Vibroverb isn't wired that way. Here's the schematic & layout for dealing with the bias circuit.

Custom_Vibrolux_Reverb_schematic.pdf

[drewl]

So why not replace R59 with like a 10k resistor and 20k PC mount pot in series so you have a good range of bias adjustment?

I do this all the time with Blues Jr's and just finished doing it to a Pro Jr as they run the output tubes way too hot on these.

[nevetslab]

Not having the PC mount pots on hand to do it at the moment. I've some more to go thru, and parts to order, so I'll be doing just that soon. I gotta add some glide feet onto that amp I just serviced, and can make that mod then, when the parts come in. And, have some Blues Jr's ad Pro Jr's yet to look at, so thanks for the tip!

[Alex R]

for what its worth I generally find with all these bias-modulating trem amps that the bias has to be set really cold to quieten the trem.

In general I feel it is a mistake to see bias adjustment as a way of 'sweetening the tone'. That's not what it is.