Weird 100w from 1974

[cdemike]

Had this old amp that I used to gig with a long time ago and am getting it out of storage. For some reason I thought it was from 1976 but the serial number ends with "F," denoting 1974. I bought it before I knew much about amps, so I was pretty surprised when I opened it up in terms of what was still stock, what was modded, and how some of those mods got done. The first thing that jumped out was that there are pools of solder holding down nearly every ground, and that the grounds are distributed throughout the chassis. I'm not able to discern any specific thought process in terms of how the grounds are distributed outside of maybe just placing these soldered down grounding points with the lengths of wire already in the amp. I saw a number of other mods:
- Replaced OT: the original died during a power tube failure during a gig about 15 years ago. Sadly I didn't think to try to keep the original OT or else I'd have it rewound now. At the time I took it to a tech and had the old OT replaced with Metroamp 1202-132 style OT, which I know is very wrong for this era amp, but I thought it could use a mellower sounding OT at the time.
- V1 has been switched basically to Bass-spec with both cathodes sharing the normal channel's 820R/330uF setup
- Each of the 2x preamp B+ dropping resistors have been replaced with 2x 5K1 carbon comp resistors in series (4x total resistors)
- The 220k bias reference resistors appear to have been replaced (I suspect these may have died during the meltdown that toasted my old OT 15 years ago)
- The 1K screen grid resistors have also been replaced with sugar cube-style ceramic resistors (originals probably died along with the original OT)
- The polarity switch appears to have been converted into an ersatz terminal board (don't really care too much, but it's sloppy...)
- The impedance selector has been replaced with a 3-position sliding switch
- One of the speaker jacks was replaced with a non-matching, externally-threaded Cliff style jack with a plastic nut

There are some pretty interesting things in the amp as well:
- The signal path is surprisingly almost original, at least judging by the red warranty paint
- One area with red warranty paint (or at least a cover-up that has me fooled) is the presence circuit, which uses the late-style circuit where there's no DC across the presence pot. However, instead of the commonly-documented 680nF presence cap, my amp has 1uF (not a typo: see photo below showing 1uF presence cap)
- The amp appears to have been imported to the United States after having been originally shipped to Japan based on the inspection sticker

The amp appears to have gotten very hot at least a few times: the socket-mounted V2 plate resistor is clearly charred and appears to have had it the worst. However, the coupling caps coming out of the phase inverter, the 50pF cap between the PI plates, and and the 82K PI plate resistor each also show signs of heat damage. The PI couplers are worrying in terms of bias stability, but I haven't tested them for DC leakage yet.

After inspecting the amp, I think the bare minimum for service would be replacing all the electrolytic caps in the amp (including what appear to be factory original bias caps and Eerie cans!) and possibly replacing the V2 plate resistor. One of the screens node filter caps has begun bulging significantly, and that V2 resistor looks like it has survived quite a bit of abuse. The resistor surprisingly reads ~106k, which, while out of tolerance, probably wouldn't impact the sound that significantly vs a true 100k, which does give me some degree of pause in replacing it.

So I'm kind of in a weird spot because I'll clearly need to do some work if I want to get the amp playable again, and my level of confidence in the safety of this amp has dramatically fallen after checking inside and finding how weird and sloppy things like the grounding scheme were done. I've discovered in the 15 years that have passed between when I was last using the amp and now that I prefer bass-spec amps, and if I'll need to redo a ton I'm considering building a bass-spec turret board and swapping the board and pots (swapping both in order to keep the red warranty paint should I decide to sell it down the line to someone who'd want to convert it back to stock). On the other hand, the signal path is mostly original, including this weird presence circuit variant that I haven't seen documented before, and I feel kind of like a hack for considering replacing the pots and a fully functional board.

Posting to get advice: in amp with this level of butchery (especially WRT the chassis), am I desecrating this old amp if I went ahead and modded it? There's enough here to give me pause, but also enough butchery that I kind of am inclined to just do what I want with it. Maybe I should replace the bias and B+ filtering caps, see whether the pots are salvageable, and go from there? They're pretty crusty, but it's been in its head shell in a climate-controlled room this whole time so I don't see how corrosion from air exposure and a minimal amount of dust infiltration would make them unusable after cleaning them up. However, if a significant number of those pots are unusable that might alleviate some of my hang-ups about the more radical plan to replace the board and pots. I haven't taken a look at the underside of the board, but in the event that the traces have lifted I might desolder as much as possible (especially the coupling capacitors) and populate the original stuff onto a turret board.

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[Reeltarded]

Very cool!

I love the Operation style crime scene chalk for all the components on the early boards.