Asking For Opinions From Little Evidence: JTM Super PA

[TrueFifth]

Mains hum may represent failing filter capacitors
Hiss does not, that is typically noisy resistors early in preamp (pots are resistors too) 1M ground resistors, 68k grid stoppers.
Earth (ground loops) also cause hum, poor heater wire routing and transformer field coupling cause hum, but not hiss.
Hiss is normal as you turn the volume up on these I am afraid
Stop the paranoia and play it so you cannot hear the hiss!

Thanks!!!
Mains hum is present (unfortunately), and the hiss like you say!

What about the very clearly present compression effect on the clean tone, even on low volume, I guess I can worry about that when it's combined with mains hum? (but alas, it makes the amp so incredible and lively to play)

And that last (possible very dumb question) if the compression effect is caused by old capacitors, could you retain this effect by installing new (smaller?) caps that can't deliver as much power as caps according to specs would do?

 

[TrueFifth]

Marshall 100w P.A. amps are the oddballs when it come to changes/transitions. This amp looks like something built around April 1969 based on what I see. I've seen other Super P.A. amps from 1969 with "black flag" logo front panels and the rest of the amps features being in-line with 1969.

Tazin, it seems you are very accurate again:

Got a final word from the seller, he has been informed of the date 31 july 69 for the mentioned Super PA, I guess it comes from the checklist sticker in the chassis.

That makes showed plexi plated Super PA a month later than my own metal plated Super PA, which is from June 69.

So this seems to be an example of a very late plexi-plated amp, made after metal front plates was introduced.