Fender Pro Reverb - Hum

[Shawnobi]

Hi,
Been trying to suss the hum issue on my 79 Pro Reverb (UL).

It's a 60 cycle hum not affected by volume so I started switching tubes. When I got to the power tubes I switched them out together. When switching on the hum was louder then dissapeared then came back louder than with the previous tubes. One of the tubes started to look like it was red plating so I shut it down.

Replaced the old tubes and it was back to how it was. Less hum, tubes looking fine.

Can anybody help explain what is going on here? Why one tube suddenly was drawing more current? And any pointers on the hum? Could this just be a bias issue? I know the UL system was different and some people mod the bias circuit.

Thanks for all thoughts.

[Jazz P Bass]

There was a 45 watt & a 70 watt Pro Reverb Ultralinear amp.

Regardless, have you tried adjusting the 'hum balance' control?
There is also a 'tube matching' control.

As to the tube that redplated, I think it may have bad.

[Shawnobi]

It's the later 70 watt pro reverb. The OH NO NOT AN ULTRALINEAR one

Hum Balance control has no effect whatsoever. Tube matching pot must be inside as there is no other external pot.
Red plated tube seems fine in my other amp it came from :/

[Jazz P Bass]

There are any number of things that can make the output section 'hum'.

#1 is the grid. If there is any Vac on it, well......

So check the bias supply for unwanted ripple.

A leaky coupling cap from the PI is another culprit.

[TimmyP1955]

Make sure that the hum balance is actually functional. A failure is unlikely, but ya never know.

[Jazz P Bass]

It's pretty simple of a circuit.
If you want to see that it is functioning, put your meter lead on one of the outside lugs of the pot, the other lead to chassis ground.
Reading Volts ac, you should see 1/2 of the heater voltage, changing as you turn the pot.

cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.zip

[g1]

The matching/balance pot may be accessible from the bottom side of the chassis.

[Chuck H]

The red plating on the substitute tube could be indicative of a bias problem. Maybe the tubes you have in there are tolerating it better, but still hum. Have you checked the grid bias for the power tubes? Balance not withstanding, what is the bias voltage? Better if you can provide the bias current.

[The Dude]

Agreed. And I'll add......bias current for each tube would be most helpful. You could have a socket issue, tube issue, socket solder issue, etc. Bias may be fine on one tube, but not the other.

[Tom Phillips]

Power amp section hum can be due to imbalance in two sides of the power amp. It can also be caused by excess ripple in the bias supply. Check both of those parameters.

[Shawnobi]

Thanks folks. Plenty to look into in the meantime. Will post back when I have some more info.

[Shawnobi]

Pulled the chassis last night to tweak the tube matching as I was sure the pot was inside. Turns out it was accesible fromt eh outside and I'd mistaken it for a jack socket....
But a quick tweak and the hum is gone. Fantastic news, so thanks for all the advice. I was never quite sure about the biasing on these UL amps so am quite relieved it was a simple fix this time around.

[Randall]

Sounds like maybe a dirty bias matching pot. I'd squirt it with a dab of cleaner and work and to and fro a bit and then adjust.