I have a wierd Biasing problem that's really got me confused

[Gravity]

The amp is a Carvin X-100B and it has 6l6GC (V4, V5, V6, V7)tubes.

 

There's an aftermarket bias pot that appears to be properly installed

 

Idealy, V4 and V7 (the outside tubes) should work as a pair and V5 and V6 should as well (inside pair)

 

But anyway, V4 and V5 are drawing about 2 times as much current as V6 and V7.

 

I checked all 4 tubes in another amp (using a weber bias-right meter) and they are not mismatched.

 

I also tried substituting different PI tubes but that did not help either.

 

I then just reflowed the old solder on the pin connections of the PI.

 

The amp plays and all, but not as well as it should. When V4 and V5 are biased @ 40mA, V6 and V7 will be @ 20mA, so everything is completely lobsided.

 

Any ideas of what might be the culprit in a scenario like this?

 

 

here's a link to the schematic

 

 

X100b Schematic

[Prog]

Where is the the "bias pot" located based on the schematic, and what ohm value is it? As the schematic looks, all 4 tubes should be running identically.

[Gravity]

It's at R127. I'm not sure of its value.

[edster]

If you haven't already, you need to check your neg bias voltage, screen grid voltage and plate voltage on each tube. Pins 5, 4 and 3 respectively.

Post your results here.

 

Also, make sure the bias pot is not set up as a balance pot.

[Gravity]

cool

 

 

I've got a few things to play with then

 

 

Right now my band is doing work in a recording studio and the other guitarist is using this head. Ironically, it sounds pretty good. The only thing that seems to be suffering is the clean channel which we don't use anyway.

 

I'll get back with my results when I get another chance to tear it apart.

 

 

Thanks