Fender Ultralinear Twin Reverb master volume

[Randall]

I have another 135 watt Twin Reverb from the same customer (he is happy with the new volume pot). This one was red plating and blowing fuses, turned out to be one side was getting no bias voltage, and the other side was getting less than half of what it should be, I traced it down to the output tube balance pot, one side was 28 ohms to ground, even when fully disconnected. I opened it up and somehow it was misaligned and shorted internally. I was able to get it back together correctly, and now it works. I now have to balance the two sides, and am still thinking about hw to do that. Seems odd they provide that pot with no practical way to adjust it.

Anyway, the fellow wants to pull the master volume circuit and push/pull switch. He even has an aftermarket faceplate he wants to change out. Has anyone done this? What would be the preferred way to do it? And what does the push/pull switch actually do? I see one half is shorting a 1M resistor to ground, is this to bleed off plate signal from both channels?

And I see the other half switching the green wire of the reverb tranny to the ground bus. Why?

Fender-Twin-Reverb-SF-135-Schematic.pdf

[66 Kicks]

I hate that schematic.

[Randall]

I think I figured it out. Comparing to a AB769 drawing, I simply cut out the switch wires. Then I cut out the pot wires and ran a jumper from the 0.01 PI cap to the junction of the two 220K mixing resistors. Does that sound right? It seems to sound OK. One thing I did notice is the AB769 has the PI cap as .001, and this has a .01.

[66 Kicks]

Here are maps, in case you get lost.

[g1]

I now have to balance the two sides, and am still thinking about hw to do that. Seems odd they provide that pot with no practical way to adjust it.

...

And I see the other half switching the green wire of the reverb tranny to the ground bus. Why?

The switch does not ground the green wire, there is a 1K between. It sends some of the hot reverb drive signal to the mixer stage for extra 'dirt'.

The Fender instructions for adjusting the 'matching' pot were very simple: volumes down, adjust 'matching' pot for minimum hum.
Then turn up volumes and adjust 'hum balance' pot for minimum hum.

[pdf64]

... One thing I did notice is the AB769 has the PI cap as .001, and this has a .01.

The LTP grid leaks are down to 330k from 1M. And smaller caps feeding the power tube grids, 0.022uF down from 0.1uF, and smaller grid leaks, 47k down from 220k.
The benefit is that when overdriven it gives faster recovery from bias shift; guys complain about Fenders being flubby at high volume.
Those late 70s amps seem more bassy because the bass control taper changed from 10% in previous models to 30%, ie to the same pot spec as the treble control. Like Enzo advises, it needs to be adjusted so it sounds right, rather than to familiar, typical settings. Though the steep taper does make that something of a hair trigger.

[nickb]

I now have to balance the two sides, and am still thinking about hw to do that. Seems odd they provide that pot with no practical way to adjust it.

Assuming the tubes are a reasonable match to start with, just adjust the balance for minimum hum with the master at zero.