Issues biasing a late 70's Twin

[markmalin]

Hey all,

I'm working on a late Silverface Twin Reverb, push-pull master-volume model.  Plate voltages look fine across the amp.  I wanted to set the bias on the power tubes.  I'm using the transformer shunt method.  I read half the current on one side, though.  The 2 tubes on one side read 1/2 the current of the 2 on the other side.  If I set the bias pot so I read 70 mA on one side (so 1/2 that meaning 35mA per tube), when I read the current on the other pair of tubes I'm seeing 35mA.  Swapped the tubes in pairs, left to right, and still see half on the same side of the transformer.  I did a search on the board, but wasn't finding anything...but feel free to point me to other posts.  Seems like I've seen this issue before.  Puzzled.  Could the output transformer be the issue?


Thanks.
humbly,
Mark

[HotBluePlates]

Is the bias pot still a bias balance arrangement? If so, you should adjust it to see equal currents for each side.

[markmalin]

Is the bias pot still a bias balance arrangement? If so, you should adjust it to see equal currents for each side.

It is.  My bad.  I haven't worked on many this new. So the stock pot is a "bias balance"?

[HotBluePlates]

To my knowledge (I'm lazy, though I should be looking at the schematic), by that date Fender had switched their amps over to using bias balance pots.

In the thread on the 70w Super Reverb, I talked myself into believing it is because Fender wanted the bias voltage left alone at the designed bias. In that particular amp, they'd want to do that because it can only make the claimed 70w if everything is "just right".

[markmalin]

To my knowledge (I'm lazy, though I should be looking at the schematic), by that date Fender had switched their amps over to using bias balance pots.

In the thread on the 70w Super Reverb, I talked myself into believing it is because Fender wanted the bias voltage left alone at the designed bias. In that particular amp, they'd want to do that because it can only make the claimed 70w if everything is "just right".

Interesting.  I remember reading about these.  It's been a long day...(100 degrees trying to diagnose an AC problem on my son's 95 BMW 5'series).  I'll revisit the issue tomorrow and see if I can balance it out.


The amp sounds "ok", but sure doesn't sound like a Twin Reverb should so I may replace the power tubes but wanted to check the bias first and was kind of puzzled by the readings. 


Thanks, HBP


Mark