Tech: Biasing valve amps

Subject: Tech: Biasing valve amps
From: Bonzo3legs
Date: 04 Apr 21 - 08:17 AM

I have a 1980s Carvin X60 valve combo amp, which is loud - very loud for its specified 60 watts. Probably about time for a service and possible replacement of the filter capacitors and rebiasing to preserve performance for DIY noise drowning duties!!!

Subject: RE: Tech: Biasing valve amps
From: Bonzo3legs
Date: 04 Apr 21 - 08:18 AM

Ah, my question is are there any sound characteristics which indicate that rebiasing is necessary?

Subject: RE: Tech: Biasing valve amps
From: BobL
Date: 05 Apr 21 - 02:55 AM

What's "rebiasing"? I've been an electronics hobbyist since the days when all amps were valve driven, and I don't recall the term. Is it setting the DC bias on a push-pull output pair in order to minimise crossover distortion? In which case I'd have thought you'd only need to readjust it after replacing the output or driver valves.

Subject: RE: Tech: Biasing valve amps
From: Bonzo3legs
Date: 05 Apr 21 - 05:31 AM

I've since found out there is more than 350 volts flying around inside my amp, so I think I'll leave it a to a guitar amp technician!!

Subject: RE: Tech: Biasing valve amps
From: GUEST,Phil d'Conch
Date: 05 Apr 21 - 05:50 AM

Better safe than fried. Did you ever get those noisy controls fixed?

A Pictorial Guide To Biasing Carvin Tube Amplifiers

WARNING: During the course of biasing, you will be exposing yourself to dangerous voltages up to 400+/- volts D.C. If you feel at anytime that you are not sure of what you will be doing, don’t do it. It is recommend that you practice each step several times with the amp’s power cable unplugged so if you make a mistake, you will not pay for it with a damaged amplifier or a damaged body.

Subject: RE: Tech: Biasing valve amps
From: Bonzo3legs
Date: 05 Apr 21 - 05:59 AM

Yes I managed to fix all but one - the reverb level which I replaced, I never use the reverb anyway.

Subject: RE: Tech: Biasing valve amps
From: BobL
Date: 06 Apr 21 - 03:15 AM

Thanks Phil, from your link it appears I guessed right.

With valve equipment one recommendation when working on live circuits was to keep, if possible, the other hand in your pocket. I've had the odd belt in my time, and shed no tears at consigning those days to history.

Subject: RE: Tech: Biasing valve amps
From: ripov
Date: 06 Apr 21 - 06:00 AM

can you still buy those 9.0v grid-bias batteries? As BobL says one hand in pocket with all things elactric and TEST BEFORE TOUCH!