Super Reverb Bias Gross Imbalance

[John Andrews]

Hey folks,

I built a 50w Plexi kit ~6 years ago, and am still delighted. I gig with it religiously and STILL get tons of compliments on my tone. And, the reason I'm popping back on this forum is that I really appreciated the expertise and willingness to help. I have a fairly extensive electronics background, but am stumped here.

I'm working over a '70s Super Reverb, and did the ol' 1-ohm resistor trick in line with the cathodes of the output tubes (6L6s of course). When I went to set the bias, I have a gross mismatch between the cathodes, like one tube is showing half the current of the other one. I've checked the components ad nauseum, and swapped tubes several times. I've even reinstalled the 1-ohm resistors thinking I may have a bad resistor or bad solder joint, to no avail. The only thing that makes sense right now, painfully, is the output transformer, which I REALLY hope isn't the case.

Anyone out there have any ideas how to troubleshoot this further? Thanks a bunch, in advance.

[Carbia]

Change the two phase inverter caps.

If one has leaks the bias voltage in that side goes wrong.

[John Andrews]

Agreed - just placed them on order this morning. And, the way these old 70s amps bias, it's more of a balancing game with the adjustment. The schematic shows both sides connected across the bias pot, and if I get one side to optimum cathode current (~30mA), the other side is way out of whack. Thanks for the confirmation!

[Carbia]

Ah, ok, balance bias circuit.

you have two options.

Balance bias current in both tubes, and then, if it's too much, replace the 15k resistor from the wiper of bias pot to ground with higher value. If it's too low, do the opposite.

Change the bias balance circuit for a regular bias circuit. Use the AB763 circuit and copy.

[John Andrews]

Makes sense. After doing more research I tend to agree it's best to have the AB763-type bias circuit. That will be my next mod on this amp. I'll get the parts ordered up. In the meantime, I can see if I can get the bias where I need to be by replacing the resistor to get things a bit closer. Thanks again - and THIS is why I'm still a member of this forum, because of help like yours. You rock, my friend..

[Carbia]

Good luck
When I take silverface amps to service I always change the phase inverter and bias circuit to blackface specs. It's easy and improves the amp.