67 JMP 50W Put Original Transformer Back In?

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67 JMP 50W Put Original Transformer Back In?

Postby mike1001 » Sun Apr 26, 2020 12:00 am

Hi everyone.
I have few questions and this looks like a place to post them.
I have a vintage Marshall head, from looking at the "Historical Data" page I think it is a 67/68 JMP 50W, it looks almost exactly like the one on that list http://marstran.com/6768%2010862.htm
I would like to put back to stock.
When I got the amp a long time ago it was running EL-34s. The amp came with all of the parts that had ever been taken out (including old tubes for some reason) and it included the original transformer
that was in it. I am pretty sure it is the original transformer, the red paint that is around it (and the power transformer) is also on the amp chassis where it used to be mounted. I asked if the transformer was shot and was told it was fine but originally the amp was running 6L6 tubes and the transformer was swapped to accommodate the EL-34's. I have no idea what the
current transformer is that is in it.

A couple questions.
1) I don't see any reference to one of these amps ever running 6L6 tubes, can that be what was originally in it (is it possible he confused KT88's for 6L6's)?
2) Is there a way I can figure out the specs on that transformer to figure out which tubes were in it?
3) I have not seen pictures of other amps from around this period with that red paint around the transformers, any ideas why that is there.

Here is some pics of the OT and the amp it is out of. The power transformer looks like it has never been touched.

Thanks in advance for any help :)

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Re: 67 JMP 50W Put Original Transformer Back In?

Postby Brian Wallace » Sat May 09, 2020 5:26 pm

Hi Mike,

I don't think that OT is original to the amp. Your amp would have a Drake OT in it. The OT in it now looks like a Drake or Drake style OT.

A couple questions.
1) I don't see any reference to one of these amps ever running 6L6 tubes, can that be what was originally in it (is it possible he confused KT88's for 6L6's)?
They didn't use 6L6's in your amp.
2) Is there a way I can figure out the specs on that transformer to figure out which tubes were in it?
I looked through a few online catalogs and couldn't find this particular OT. There is a way to check what the primary Z would be given a given output load but it requires equipment you may or may not have.
3) I have not seen pictures of other amps from around this period with that red paint around the transformers, any ideas why that is there.
The paint was more than likely put on the transformers to cover rust that had formed on them over the years.
Remember....You can fix it with a little extra energy and a positive attitude. ;)
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