SF twin reverb 135 watts needs refurbishing.

[Tom Phillips]

...Some diagrams have the middle post connected to ground and a wire from 1 post to V2p9 and the remaining post connected to V2p4/5. This makes sense to me but before I do it I want to ask if this is right.

Yes, That is correct. One explanation may be that the pot went bad and someone disconnected it and then used the grounded lug of the pot as a tie point for added fixed virtual heater ground resistors. Note the smudge on the chassis under the pot. That what it looks like when the pot is blasted with high voltage such as when a tube shorts its plate or screen to a heater pin.

Before you re-wire the pot you should check its operation with an ohmmeter.

[rockman627]

Thanks. The red and black wires were tucked near the scorch marks and I thought they were the cause.
ps that pot measures open. Good call.

[Tom Phillips]

Thanks. The red and black wires were tucked near the scorch marks and I thought they were the cause.

Replacement hum balance pots are readily available. See https://www.cedist.com/products/pote...fender-control for one source. Often the balance pot properly adjusted will achieve a lower noise floor than using the fixed resistors. However, sometimes the fixed resistors do just as good a job. Given a choice, I would use the pot and re-check the adjustment whenever tubes are changed. Remember that you can use one or the other but make sure that both are not installed at the same time. Some added parts could be hiding in that Silverface rates nest.

[rockman627]

Geez, my mild emphysema got really bad suddenly and I was out another week. Fortunately my boss is pretty cool. Anyway I'm waiting for the hum balance pot to come in. I was wondering, when biasing the tubes, should you set the output tubes balance pot to the middle before starting? And when you hear hum, should you go for the hum balance first or output tube balance?

[Tom Phillips]

...I was wondering, when biasing the tubes, should you set the output tubes balance pot to the middle before starting?...

That can't hurt. However, the best practice is to measure the actual bias voltage reaching pin 5 (the grid) of each output tube socket without any of the power tubes installed. You should find around -50V ±20% at each pin with respect to chassis ground. Set the bias balance pot to make them all equal to start.

...And when you hear hum, should you go for the hum balance first or output tube balance?

Output tube balance (Bias setting) first. Then hum balance.

[rockman627]

Well all I did was change the filter caps (and the corresponding resistors) and replace the humdinger pot. I got the tubes biased and balanced and our guitar guy says it sounds good. I was thinking of replacing the output tube sockets and their carbon comp resistors but everything looked good and the resistors measured correctly. Since I was advised to not replace things not broken I left it at that. I guess we can consider this closed and once again I thank everyone who helped.