1979 Silverface Twin Reverb UL 135w mystery pot inside amp

[sleepingAwake]

I'm helping my guitarist out with his Twin Reverb.
I do have some limited experience with fixing amps and managed to bring my 72 Bassman Ten back from the dead.

His complaint is that the reverb and vibrato do not work. I've already solved the reverb issue by replacing the rca cables.
The vibrato problem is related to the "roach" I believe. The neon light does change intensity and speed but switching the vibrato on and off has no effect on sound. I did try swapping out the vibrato tube.

My complaint is that his amp has more hum than normal and is just noisy in general.
There are a couple of wires and a ceramic cap that are microphonic. I reflowed a bunch of connections as I'm sure there are a few bad solder joints in here.
Also take note of the sloppy wiring and the excessive flux. Man they got sloppy in the later years. My Bassman was built with much more care.

Now to what I'm really confused about.
When I opened her up there was a pot added to the circuit that I believe is related to some sort of push pull volume removal.
I'm included a couple pics. What the heck is this thing doing?
Micophonic cap and wire circled in red. Pot connections circled in green. The pot seems to be connected to the reverb cicuit and obviously the push pull MV has been replaced with just a MV.

[Jazz P Bass]

I would think that the 'best' approach would be to draw the circuit.
I have attached the schematic.

[sleepingAwake]

I would think that the 'best' approach would be to draw the circuit.
I have attached the schematic.

Unfortunatly I'm not going to have time do that.
I have to give the amp back on Saturday as he needs it for practice.

I'll get it back again in a few weeks when he has the money to order the parts to replace the rest of the electrolytics and the LDR.
I at least got a chance to track down a couple noisy solder points and trim some excess wire out of the doghouse.
Whomever recapped the filters weaved some of the wires through the cap leads and generally left things messy.

[Tom Phillips]

...Whomever recapped the filters weaved some of the wires through the cap leads and generally left things messy.

The wiring in the 75W & 135W UL series is pretty messy right from the factory. I recently reworked a 1979 Pro Reverb. In addition to the factory mess there was a lot of damage caused by work over the years including many wires melted through to the conductor by careless soldering iron use. I cleaned everything up during the rework. I was surprised at how much the hum was reduced when I replaced the heater wiring with a nice tight twisted pair and redressed the routing. It wouldn't be economically practical to pay a shop to spend so much time on the amp. I just got into it and took it on as a project to learn how much I could improve the amp.
The attached photo shows all the leftover parts and wire that I pulled out of the amp.

[JoeM]

Tom, Do you have a photo of the inside after you re-did the wiring? I'd like to see what you did. I've got a SFSR that could probably benefit from some re-routing and re-working.
Thanks.

[Tom Phillips]

OK Joe. I don't mean to hijack sleepingAwake's thread but per your request and at the risk of stirring up a passel of controversy I've attached before and after amp porn photos. All the changes were done to make improvements that were verified by a combination of listening tests and measurements. As you will see, I don't necessarily populate the amp with special elite mojo parts. Anyway, the sound went from ice picky harsh to very nice classic BF tone and resulted in some very complimentary followup emails from the owner.
Regards,
Tom

[JoeM]

Nice job on the rework Tom!

[sleepingAwake]

No worries on the thread jack.

Nice work on the re-wire Tom.