It looks to me like the Cut Control is 250K.
The cross-line master appears to be 1M with a Push-Pull...On and Off switch incorporated.
Maybe your bad Power Tubes zapped it pretty bad when it took out the Grid and Screen Resistors.?
best

You did a nice job.
That has always been my gripe with "Cliff" jacks. They basically "tighten" from the back side via the washers.
I worked on a few Soldanos and I noticed they used a metal shim under the Cliff Jacks so that either just one or no washers were required.
But I prefer those to the Switchcraft jacks in most cases.
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Train Wreck Rocket is basically an AC30 Top Boost (Ceriatone Rockit).
 Some of the Matchless are close as well.
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OK..... that explains the visual quality of your posts.....
and your electronics knowledge.?

Well done/pictured/explained Wyatt......
BTW, if you do not mind me asking, what is your "day job"/back ground.?

Yeah, check Every Thing between the in jack and the tube.
Speaking of tubes.....it is possible to have 1/2 of an AX7 go bad. Did you try a simple tube replacement.?
Best

Thanks for the info.
Sounds like a Very Nice head.
Best of luck with it.....!!!!

What is a Mk II Brown exactly.?

Hey.....you did a nice job.
Hope it sounds as good as it looks.
Well Done

Doesn't the chassis kind of look older than the components and the circuit board.?
Like the demarcation around the PT cut-out.....and yeah, I would say that is definitely not the lead-dress work of Nik or his employees.
Best

Mr Dumble did a lot of work on Fenders. That "ground switch" is just a remnant of days gone by, when amps had 2 wire power cords.
I kind of wish Nik would get rid of that.....but he knows what he is doing better than I do.
It is probably not the lack of ground that is causing your noise troubles anyway. No doubt you plug into shared circuits with all kinds of crap living ing the AC. You need a dedicated circuit for audio, but that is wishful thinking.
Good Luck
Anyway......you guys can probably find a suitable ground to use in that building.
Best

Did this amp ever work correctly.?
Or is this a new build that has problems.?
Best

I might be going too far back....
But, I would lift the entire secondary of the Power Transformer, all you would have is mains power coming into the PT and that is it.
Does it still blow the fuse.? I  would bet not.
I would then connect the secondaries one at a time.
What do you have on that side.?
High Voltage
Heater
Rectifier Tap
Is that right.?
I would connect those (2 wires at a time) one at a time and see what snaps the fuse. Kind of thinking heater or rectifier taps.
I usually let Wyatt handle this stuff. He is WAY better/knowledgeable than I am. If he suggests different, I would surely defer to his recommendations.
Best

Not sure I would trust a "tube tester".
Pull the rectifer tube and the power tubes. Replace the fuse. Turn the amp on.
Does it blow the fuse now.?
If not, replace the tubes.....one at a time.....and see if any of those blow a fuse.
Best

Per usual, Wyatt has covered the bases quite well.
Yeah.....not sure you would really need/benefit with a Fx Loop on this amp. Not sure you would want "half-power" on this amp either.
And, as mentioned above, I have never heard that mode that sounded good to me. I think it is better achieved with less/different speakers, Master Volume, Attenuator, or just having a few different amps to operate in the power band(s) that you need.
Not much help, I know.
Best