I've used probably ten of them, and while the addition of a quarter-inch jack has been accomplished in a few different ways, they all had two things in common: Some of the best natural tube overdrive I've ever heard, and an incredible amount of noise (enough to make it unusable in many circumstances).
When I say they're really noisy, I don't just mean from a "recording snob" perspective... I mean they are really noisy. The noise can be divided into two main categories: mechanical and electronic.
The mechanical noise is pretty easy to address. I used infomercial FlexTape™ to make gaskets between the mating surfaces of the bent-steel cabinet, and also between the speaker and the cab. A bit more to the broadest surfaces of sheet metal damped some of the vibration. Sorted!
The electronic noise is primarily a prominent hum that gets worse as the volume control is increased. My first look was at the power supply filters--the schematic lists 10µF for all stages. But within my own example all stages are 20µF, which should be fine, and also past experience tells me that these caps are not the biggest issue.
Next I tried balanced heaters by adding two 100 ohm resistors as an artificial center tap. Maybe a tiny marginal improvement? But still way too noisy.
Finally I admitted what I long suspected to be true--the way the grounds are laid out in this amp is what makes it noisy, and I'd have to blow the whole thing apart and essentially rebuild it.
Long story slightly-shorter: dual star-grounds fixed the issue and now I can enjoy this amp with only normal amounts of tube amp noise (without losing anything I love about the amp).
On the schematic below, I've marked for deletion some parts that were not part of the actual amp (designed to send power to other Magnasync film equipment).
Connections that go to the first "input stage" star ground are marked with a "1." The second "power/output stage" star ground connections are marked with a "2"
Point "1" is on one end of the chassis, somewhat near the added quarter-inch jack (which I isolated from the chassis using a nylon shoulder washer or rubber grommet). Point "2" is a power transformer mounting bolt.
Before (note the ground bus between the two can caps, which contained everything from power supply filters to input stage ground reference):
After:
