Blackface Fender Vibrolux Reverb AA964 Bias Setting?

[ArnC.]

Anyone know the bias setting for this amp?
I finished recapping and a few resistors and just want to finish up.

Thank You!
ArnC.

[52 Bill]

How are you measuring the output tube idle current?

With any 2-6L6 amp power stage, I'd guess anywhere from 25-30mA per tube. The original schematic calls for -42 volts at the bias pot.

[pdf64]

See
Idle Current Biasing - Why 70 percent?
and
http://www.webphix.com/schematic%20h...a964_schem.pdf

It would be a bad idea to assume that the output tubes were suitably biased on the basis of setting the bias voltage to -42V; far better to monitor plate dissipation.

[mikeboone]

There are a few options... You can buy a bias meter from one of the many folks out there who sell them. You can install some 1 ohm resistors between your cathodes and ground and measure the voltage across those, the reading you get in DC mV will be the same as mA. I wouldnt mod a Blackface if its in really good condition though.

If the amp sounds like it should, is not having any issues, and that factory voltage is correct or close, you're not going to damage anything. Knowing the bias point is important though.

[Enzo]

Look on the schematic, it says -42v. If you set it to that, you won;t be wrong. People may convince you the tubes must be at whatever hotter setting, but the amps have worked for 50 years at that bias setting, so they will continue to do so. If you know what you are doing, you can calculate all manner of stuff and bias it however you want, but since you lack that experience, I'd suggest just going with the factory specification.

[pdf64]

Some notes of notes of caution for a novice proceding along that route
1/ the voltages are representitive and noted as being +/-20%; however a bias voltage of -34V, although within that spec, would almost certainly be so unsuitable as to damage the amp.
2/ the -42V bias voltage should be in proportion with the 410V B+, eg if the line supply is high the B+ will also increase, say 440V. In that case the bias voltage should also be proportionately higher, in this case to -45V.
3/ the voltages were representitive of operating characteristics of the tubes available at the time, eg RCA blackplates. Many current production 6L6GC seem to need more bias voltage for suitable operating point. You can see this by comparing the schematics of the original BF Fenders to their reissue counterparts, see Fender Amp Heaven Schematics - Tweed Blackface Brownface Silverface Bassman Champ Deluxe Princeton Super Vibroverb Reissue
In the light of the above, the attractions of setting bias by monitoring cathode idle current (based on plate dissipation) should be apparant.