Amp porn - safe for work

[gui_tarzan]

Here's what's coming to my bench as soon as I finish repairing a JCM800.

It's what looks to be a 100% original JTM45, somewhere around 1965, s/n 5003. There are ZERO date codes on the transformers, caps, tubes, nothing. The chassis looks to have had hand-cut holes for the transformers. Possibly made by JM himself?

Whoever built it, it's damned cool. The nameplate is silver with red lettering, the front faceplate is white with engraved numbers and letters showing the black inside laminate, the rear plate is cream and is also engraved. The original 220v transformer is still in it and I have the 110 to 220 transformer and cable that came with it. Original Mullard ECC83 pre- and GZ34 rectifier tubes, Marconi power KT66 tubes. Original owner. Just. Plain. Awesome.

So I have a couple of questions; would it affect anything if I replace the 32/16 can with a 32/32 can? On this particular amp are there any suggestions for replacing the resistors with CC (if they're drifted too far) or should I use MF? The shop that sent it to me didn't mention anything about keeping it original, I think they just want it to work as good as possible.

[martin manning]

So I have a couple of questions; would it affect anything if I replace the 32/16 can with a 32/32 can? On this particular amp are there any suggestions for replacing the resistors with CC (if they're drifted too far) or should I use MF? The shop that sent it to me didn't mention anything about keeping it original, I think they just want it to work as good as possible.

As it is now, is it only the 32uF section that is used in the original 16-32 can? If so then replacing it with a 32-32 and only using one section like the original should be fine. What are the values in the other can, also 16-32?

Seems like its value might be higher with the original type of resistors, but I don't know.

Really cool to see that amp!

[Blackburn]

Wow! That thing is sweet! Gotta love the crooked circuit board. Sure looks early. Check out that presence pot!

[gui_tarzan]

Both the 32 and 16 are being used, the can is grounded so the ground wire isn't hooked up. It even says "can negative" on the casing.

[Blackburn]

Double trying to edit..

[martin manning]

So what's the power supply look like? Should be 32u, choke, 32u, 8k2, 16u, 10k, 16u or something like that? Is the reservoir 32u+16u now?

[gui_tarzan]

It's got a can on top that's a single 32/500 and the can inside is dual 32/16/450. Then there are two 8/450 and a 16/450 on the board (the big grey ones). There is a 20h choke on top as well, that's the smallest of the three transformers (obviously).

[gui_tarzan]

I haven't had a chance to chase down the circuit comparing it to the schematic yet, I just took pics and glanced at it briefly.

[martin manning]

Okay, on the board the 8's are the bias supply, and the 16 is the last filter. The 32 mounted on top is likely the reservoir (good because it can have its own ground), and then the 32-16 inside takes care of the screen and PI nodes. If that's the case a 32-32 (increasing the PI node to 32u) is probably fine.

[gui_tarzan]

Thanks. That's what I thought but this being such a fine piece of history I wanted to be sure.

I posted on another forum where one of the Marshall employees from the '60s is a member and they said it was a 1964 due to the coloring of the nameplate.

[martin manning]

Wow, very nice to find a living link!

[LeftyStrat]

That's really awesome, such a great piece of history.

[roberto]

Fantastic amp!
Never seen anything like that before!

[billyz]

I would not be changing resistors just because they have drifted. They would have to be open or noisy. Probably change the power supply caps and bias supply caps. Might be all it needs to be reliable and sounding good. This is amp is quite valuable if authentic, I would be for preservation of as many original parts as possible.

[jelle]

Very nice! I would personally even consider retiring it.