JMP 50 replica

[wombat]

Just finished my latest build.

I was aiming for early 1968 lead specs so I've gone with:

* Split cathode with 320uF/820R for V1a and .68uF/820R for V1b
* 50+50uF Mains and 32+32uF Screens
* 47k NFB to the 8 ohm tap
* .0022uF bright channel coupling cap
* diode rectification

It's got a 3H choke and transformers are Drake replicas by M e r r e n

The overdrive on this thing is surreal!

[harleytech]

Nice , clean build !

[Freebird]

Congrats! Very nice clean build.
Daniel

[Tazin]

Nice!!!

[RockinRocket]

Great build! We don't see them built like this very often. I bet it sounds as good as it looks!

[danman]

Sweeeeet!!!

[wombat]

Thanks for the comments

It's my new favourite toy at the moment.

I definitely recommend putting that 4700pF bright cap on a push-pull pot. It's almost like having two amps in one.

At low volumes and with the bright cap off, the clean tones are really nice and not overly bright. But at higher volumes and the bright cap engaged, the overdriven tones are so fat and huge that I can't stop grinning.

With the bright cap on, I usually have the volume around 4 - 5 and at that point I'm thinking to myself "why would anyone need more gain that this in a plexi type amp?". With the cap off, it's a different animal altogether - it's got more of an earthy growl to it and can take more application of the volume knob.

[revolver1]

That is beautiful!

Hats off, very nice work.

[bill bokey]

Very nice build !

What board material did you use ?

[wombat]

Thanks!

I used Carp Tufnol (resin impregnated fibre) for the board. The lighting in the photos may not do it justice, however, the particular sheet of Tufnol that I cut the board from is the EXACT same colour as some pieces of original Radiospares board that I have from the 60s. Score!

The downside though is that every time I've ordered Tufnol it's been a bit of a lottery as to what shade of "brown" I got it in.

I've read that some people have had issues with electrical leakage with Tufnol boards. FWIW, I haven't experienced any electrical leakage on mine (but I do live in a fairly hot dry climate and the amp does not get exposed to much humidity).

[bill bokey]

Would you mind sharing where you got it from ?

[wombat]

I'm happy to share - that's what this forum is all about.

I've gotten boards from a number of different sources, but I'm 99.9% sure that I got the one I used in this build from here:

https://www.hobbyplastic.co.uk/index.ph ... jl7ip5uk06

There are a number of places that sell it in the U.K. and of course you can also get it on ebay.

When I saw the colour of that board, I immediately put in another order from the same supplier ...but unfortunately, the second order was a quite different shade of brown.

If you hunt around you might get lucky.

Most early Marshalls used paper phenolic boards (ie paxolin), however, I have seen some original amps as early as 1969 that used fibre-resin boards so from that perspective, Tufnol may be a fairly authentic alternative as long as you are willing to cut and drill your own boards.

[RockinRocket]

Its a great looking board and a great find. I believe the cotton fabric stuff didn't start showing up in marshalls till the metal panel era.

Testing the mustards potentiometer side will tell you if the board is leaking.Even a small amount of mA is nough to upset the grid of the next stage. Mustards almost never leak.

The whale tufnol is crap.. wonder if the carp is any good wombat?

[bill bokey]

Thanks wombat !

[wombat]

The whale tufnol is crap.. wonder if the carp is any good wombat?

Whale grade Tufnol is only recommended as electrical insulation in low voltage environments.

Carp grade has better electrical insulation properties due to it's finer weave (not as good as paper phenolic though). To my eyes it's finer than the weave Marshall used in their own fabric laminate boards back in the day. In practice I've been running this amp for 5 months with no issues whatsoever.

That being said, if you are concerned about moisture absorption then you should probably use something else.