AB763 low bias voltage

[Cathode_ray]

I have an AB763 BandMaster. Just replaced all 6 B+ diodes and first 2 filter caps(70 @ 350v) and will replace the 20's shortly.
Seem to  be getting low bias voltage though. From tranny reads approx. 50v, after diode only reads about
-22v. 470 resistor reads correct out of circuit. Is this correct and if so how do you get -48v at the output tubes? 
Confused...

[sluckey]

-22 is too low. Most likely suspects are the cap and diode. Change the bias cap first. If it still doesn't increase, change the diode. Not much else to go wrong.

[eleventeen]

1: Most likely, your bias electrolytic cap is bad. I like to replace these with caps with a lot higher voltage rating: a 25uf / 50 volt cap has essentially no safety margin running at ~~50 volts. Install w/correct polarity = POSitive end is grounded, unlike every other cap in the amp.

2: Make sure there isn't something exceptionally goofy w/the tremolo circuit, which can suck down (actually, UP, towards ground, since we are talking about negative volts) the raw (right on the anode of the bias diode) voltage. A very, very slim possibility. The trem circuit is isolated by a 2.2 M resistor so it shouldn't have any effect on the raw bias voltage. Could check this by shorting/unshorting the tremolo pedal RCA jack.  

3: I have seen the current limiting resistor, usually a 470 ohm, 1 watt, cooked up in value. Again, rare. The bias circuit usually handles very, very low current but if a 6L6 tube shorted out in the past, this resistor could have shifted in value.

4: Comparing a Deluxe Rev to a Bandmaster bias circuit, the final resistor in the voltage divider comprised of the 470 ohm > the 10K pot > the 15K resistor, on a Deluxe, that resistor is a 22K on the Deluxe, but only 15K on the Sandblaster. That (the higher res value) would have the effect of shifting downwards (eg; [edit] more negative, more distant from ground) the range covered by the bias pot. If all else fails, I would change that resistor to a higher value: But shouldn't have to. 

[Cathode_ray]

Electrolytic in bias circuit solved bias problems. Guess the cap provides voltage on half the rectifier swing - from22v to 50v now. Thanks!!! Not only solved problem but learned something. Now if I just don't forget it...
Have new filter caps on the way so that so hope no new issues develop.

[HotBluePlates]

Electrolytic in bias circuit solved bias problems. Guess the cap provides voltage on half the rectifier swing - from22v to 50v now.

All caps leak a little d.c., and electrolytic caps leak more than film caps (like you'd use for coupling). As they fail, they leak more and more.

More d.c. leakage = more current pulled through the diode and series resistor, which causes more voltage to be dropped across the series resistor, and lowered voltage at the cap. It's a common failure mode.