Crazy Loud Fender Vibrolux Reverb

[earache]

Hello All

I'm looking at a friend's Fender Vibrolux Reverb "Custom", one of the relatively recent reissue series.

This thing is LOUD and it seems like some gain is dialed up internally somewhere. Even with both volumes on "zero" it is still producing a relatively loud "hiss" like many an amp will do when it is turned up full!
A strum of the strings of a guitar plugged into the amp when it is on "zero" produces quite audible sound. I inadvertently bumped the amp and the reverb crash was startlingly loud.

What gives? What could make this amp seem to be all juiced up? It gets stupid loud at "2" on the dial.

Thanks in advance - earache

[The Dude]

Are the tubes redplating? Check bias.

[Chuck H]

A dummy load and a signal trace using a scope would probably reveal a bad ground somewhere in the early preamp. But that's just a guess.

[vintagekiki]

Check (clean or replace) volume pots that do not start from "zero".
For some reason they don't start with "zero"

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Custom-Vibrolux-Reverb-Reissue-Schematic.pdf

[pdf64]

It’s as its name implies, customised for higher gain, with no master vol or negative feedback. But looks like a regular old BF, so a wolf in sheep’s clothing kinda thing. As such it’s strictly a stage amp, without an attenuator etc there’s far too much unconstrainable gain for use in the domestic or other quieter settings.

[earache]

Thanks everyone, for the thoughts (and the schematic, vintagekiki!)

Gonna investigate the bias and also will perhaps look into adding a negative feedback loop.

Letcha know how it turns ouit.

[vintagekiki]

Do not change the factory concept of the amp. Adding a negative feedback loop will not achieve anything.
You will remove the consequence (relatively loud "hiss" when volumes on "zero"), the cause will still exist.
Do your best and find the cause. Check or replace R10, R21, R30 if don't work from "zero"
If you bothered that the amp being loud at "2", try to reduce the overall volume by reducing the power amp (V6...V8) excitation by reducing the R22 (22k) to a lower value (10k)

[g1]

In case you haven't seen it, there is another thread going about noise in this 'custom' series of amps:
https://music-electronics-forum.com/...sue#post925513

[earache]

Thanks G1 for the related link!