1979 Bandmaster Reverb question

[johnnyguitar]

My 1979 Bandmaster Reverb has 2 adjustments in the back one is hum balance and the other is output tubes matching...can someone explain these to me..and there is no internal pot to bias amp could one of these be this bias adjustment??..thanks in advance

[Pearly Gator]

I have a '79 Pro Reverb with the same controls. The hum balance is a divider circuit on the filament power supply. It's basically set to mid-point, and minimum audible hum. The balance pot balances mis-matched output tubes to conduct current equally. Like the hum pot, adjust for minimum audible hum and you'll be okay. If you have a signal generator and an oscilloscope, you can tweak to perfection.

My amp was running real cold and adjusting the bias circuit to increase output tube idle current made the amp open up sonically. This requires a resistor change.

Best regards,

Gator

[loverocker]

I think I remember reading that there the bias adjustment was one of the differences in the blackface-silverface development. If so, you can find details on changing it on any of the online Vibroclone guides (how to convert a sf BMR to a bf VibroVerb).

[Jef]

The BMR's with those controls are probably the late 70s ultralinear 70 watt versions. They are not easily blackfaced because of the different power supply (SS rectifier, higher voltages) and UL output section.

Those amps are pretty good amps in their own right. They're not going to sound as organic as a BF or early SF, but if you like a tight, responsive, loud and clean tube amp, they're the ticket.

Jef

[Blue Strat]

The balance pot can be rewired to be a "true bias" pot by a tech. This is highly recommended.

All you'll need then is a bias probe similar to a Bias Rite www.webervst.com and you'll be all set. No scopes or signal generators required.