1976 JMP restoration

[Colossal]

Gents,

Thanks for documenting your amps. Always nice to see these old Marshalls. How thick is the lamination stack on your output transformers? 1.5" or 2.0"?

Thanks.

Mine is 1.5"

Thanks Steve! Reason I ask is that some amps had Dagnall C1998 OTs with 1.5" stacks and some had the Drake 1202-132 2.0" stack. Roe can probably elaborate on the timeline for these transformers.

[Roe]

Gents,

Thanks for documenting your amps. Always nice to see these old Marshalls. How thick is the lamination stack on your output transformers? 1.5" or 2.0"?

Thanks.

Mine is 1.5"

Thanks Steve! Reason I ask is that some amps had Dagnall C1998 OTs with 1.5" stacks and some had the Drake 1202-132 2.0" stack. Roe can probably elaborate on the timeline for these transformers.

The 132 drake was used in 1967. The c1998 was introduced in late 67 and dominated until the early 1970s. At this point the insulation was changed

[Riffmonster]

Here are some more photos of the amp. I just removed the second MV and screwed in metal input jacks. I did not have the time to do any more work. Also, I gave it another thought and will just order 6 F&T 50+50uf caps and then adjust if I do not like the sound. I measured the laminated stack on the OT and got 5cm (2"). I just measured it with my phone, I should go back and measure it with a real scale to confirm this.

Have a great weekend!

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[Riffmonster]

Some more pictures.

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[_Steve]

Thanks for posting the pics!

Looks like you have the exact same OT as me, unfortunately with the exact same spot where the sticker used to be and has since fallen off lol. Will be interesting to see the exact measurement of the laminations. I think these are Dagnalls. I was just curious because they appear to be laydown transformers mounted with a bracket which I hadn't seen before.

Also I see your amp was built the day after mine

Personally I wouldn't change out the caps unless you know for sure they are fried. I think its really cool we have unusual configurations that mark a seminal point in time and change in Marshall's history. Thats just my 2c though

[_Steve]

Also I noticed you have something wired on your output jacks that I don't have on mine. It looks like a wirewound resistor?

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[Roe]

Could be the drake 1202-99 perhaps or 1209-3

[Riffmonster]

You are welcome Steve! I am sorry the sticker on the OT is not there anymore. I will keep you updated on the exact size of the laminated sheets!

And yes, I know the debate about changing anything on amps like this can be quite emotional. However, this is my main live amp (so I don't have to for collecting purpose) and I can not imagine that the 40+ years caps still work like should. Also the amp has already been modded with that MV, so changing the filter caps does not hurt. It will probably sound better afterwards. Also, the initial issue I had with this amp was nasty crackling noises when turning up the volume, so I hope this problem will be gone after changing the filter caps too. I probably get some new tubes too.

I am unsure if that resistor on the output jacks is original, but I suppose it is to protect the OT if the amp is played without a load.

[Riffmonster]

Steve, I had another look at our amps and noticed the 100nf coupling caps on the phase inverter. That is a rather uncommon value for that year, isn't it? Might that be related to the big filter cap values?

[Roe]

yes that's the superbass value. but this looks like an early 2203. the very first 2203s (and 2204s) had one gainstage less than the later models and some even use a 330k plate resistor on v2. cf https://www.marshallforum.com/threads/a ... 204.19786/

[Riffmonster]

That is very interesting, thanks for sharing! I will dive into that link later. I kinda fear that the amp will sound a bit flubby now when I put in the lower filter cap values (and not 200uf as it is now).

[_Steve]

Steve, I had another look at our amps and noticed the 100nf coupling caps on the phase inverter. That is a rather uncommon value for that year, isn't it? Might that be related to the big filter cap values?

Good spot I hadn't noticed that! The amp is so super tight though even with these.

Actually I just took another look and noticed our first stage is completely different. You have a big .68 cap (cathode bypass?) and I have nothing. If i get some time over then next few days im going to trace out the first couple of stages.

Its funny I didn't even know about the unusual cap values etc until I saw your post. To me this amp just sounded exactly as I remember my first ('78) 2203 more than 20 years ago so I didn't bother looking any closer at it

[Riffmonster]

Yes, true. Your first stage looks quite different. And if it sounds good, no need to change that stage!

New filter caps are in now, got about 448V on the EL34 plates. I could not play it yet and still have to set the bias, but it is looking good I think.

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[_Steve]

Nice work. Do let us know how it sounds!

[Riffmonster]

Nice work. Do let us know how it sounds!

Sorry Steve, it has been a while! I finally got some new tubes in and recorded a little bit with a load box today. I am quite happy how the amp sounds now, although I can imagine the tubes still need some time to break in.

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