Blackfacing a silver face Pro Reverb's bias circuit...

[dehughes]

Hello all,

After some encouragement from you all, I finally figured out how to re-wire the "output tube matching" pot in my friend's late '70s Pro Reverb to a true bias pot....but....in placing the AB763-AA165 values into the bias (and PI) circuit I'm still getting anywhere from 500 to 509v on the 6L6 plates, and anywhere from 6 to 33ish milliamps of current through the 6L6s (transformer shunt method). Now, these voltages are about 50v too high for an AB763 Pro Reverb, yes? Granted, this amp has SS rectification (late '70s, I think...), but I should be able to get more than 34ma through the 6L6s with the AB763 bias circuit, right? I mean, I'm only getting abou -69ish vDC off the cap.... The tubes are brand new JJ 6L6s rated for about 34-35ma in your "typical" Pro Reverb.

At what should I look? Is there something that I should change?

The schematic I used for the bias circuit was essentially this one:

http://www.ampwares.com/ffg/schem/pro_r ... _schem.gif

Thanks!

[dragonbat13]

I may be wrong, but I don't believe that the Late Pro Reverb uses the AB763 circuit. I know that my 74 Twin doesn't use it. Check out The Fender Amp Field Guide and look at the Master Volume Pro Reverb Schematic. Also the amps from the late seventies used ultraliner transformers. That may be causing you problems also.

[dehughes]

You're probably right about the ultralinear PT...I hadn't thought about that. The amp is up and working and out of my hands now....biased up at about 500v on the plates and drawing about 33ma. It sounds good enough for being used as a Rhodes amp....but if it were mine I'd either sell it or replace both trannies and get it setup like a real AA165 or AB763. Cool amp, though...

Thanks for the reply!