Issue on AB763 Twin clone (tube current)

[72Blazer]

Amp was recently working fine today.  Bias was able to be set at ~44mA each tube.  The amp was sounding very nice and loud

Issue now:  very very low current flow on all 4 6L6 tubes and a loud hum.  I’ve got 1ohm shunts on each tube pin 8 to ground.  I’m talking the full range is 2 – 20 mA current flow.  using the adjustable bias pot.  I’ve got -44v.  The grid stoppers and plate resistors all check fine.  Another issue is the voltage reaching the tubes is only 335v.  That is down from 430v earlier today when the amp was stable/working.  I’ve got -69v coming from the bias pot to the main board.  I hope my rudimentary explanations make sense. 

[passaloutre]

Do you have another set of 6L6s?

Pull the power tubes, what’s the B+?

Insert one at a time, where does it go wrong?

[AlNewman]

Bad rectifier?  Filter cap short?
 

[72Blazer]

When I pulled all the power tubes, the B+ went back up to a healthy 440v.

I inserted the tubes one at a time until all 4 power tubes were back in and now the issues I was having are gone.  The amp sounds great again and the obnoxious hum is gone.

This really confuses me.  Do I have a bad power tube socket with an intermittent issue?  Dirty tube socket? 

Any thoughts on this strange behavior?

btw, Thanks!

[72Blazer]

I've isolated it down to a bad tube.  It was a burnt 1 Ohm resistor

[AlNewman]

What would make a 1ohm resistor burn?  it would take 500 mA to blow a 1/2 watt resistor.

[Ronquest]

What would make a 1ohm resistor burn?  it would take 500 mA to blow a 1/2 watt resistor.

I used to use Vishay (tan metal film) 1 watt for my 1 ohm resistors cathode to ground until they started looking over heated, so I now use 2 watt.   

So what was causing the 95V B+ drop ?

[72Blazer]

Fellas I don't have an answer for you other than me poking around inside the chassis.  Thanks again for your suggestions.  I've got a lot of DC on this fiber board and I'm moving on to trying to get some random noises taken care of.