50W Marshall Homage Amp Build - Soundclip Added

[martin manning]

This is Jana’s circuit with a few small additional tweaks: Dual bright caps (0.001 and 270pF), “Plexi” mode switch (labeled “Drive”) has an additional level using an on-off-on switch (either a 100k or a 470k is paralleled with the 470k to ground at the input to the third gain stage), and the filament voltage is elevated about 70V to minimize stress on the CF filament insulation. I also added a Dumble-style PI trimmer. Transformers are Marstran 50W JMP, and the choke is a Magnetic components JCM800 (5Hy). Caps are Mallory 150’s and CDE silver mica, F&T’s for all the filters ($50 worth!), resistors are Dale RN65’s for the plate loads, and KOA Speer MF elsewhere except for those in the power supplies, which are Xicon MO. For now it has new EH tubes right across.

In the (not too good) pics you can see the layout is based on my non-traditional Express, which is in turn an interpretation of rj’s PCB Express. It’s a mirror image of a Marshall because this layout just didn’t flow as well otherwise, mainly because of the orientation of the tube pins. Same for the transformer locations. The Trainwreck placement really keeps the lead length to a minimum, and I have no induced hum even though there is only a quarter-inch gap between the PT and OT. The ground scheme is almost identical to what I used for my Express, and the amp is quiet and stable with everything maxed-out. The chassis is from rj, and the front and back plate were made by BNP Lasers. No power indicator installed yet- can’t decide on the color.

Even though there is only a bit more complexity in this circuit relative to the Express, it seemed about twice as difficult to construct. It felt like there was about twice as much wire in the tone stack, and I was worried about unwanted coupling. This is odd considdering I'd done something similar before, and recently, too. I like this construction style, but I don't ever want to have to get under the board.

How does it sound? Killer! Definitely has the Plexi-JCM800 vibe with some really nice rockin' distortion, which can be really dirty or cleaned-up, even more so with the mode switch. It has a pretty wide range of tones available, so it’s definitely not a one-trick pony. I had some fun looking at waveforms and playing with the PI trimmer this morning while checking it out. My son also played it for a bit and took a liking to it immediately- even put a smile on his face. More playing time is needed, but right now I see no reason to change anything.

So, thanks again Jana for the circuit mods and advice on this project!

[Reeltarded]

Woohoo!

First off, AWESOME. S'condly, your amp is empty! It's so clean it looks vacated! I am really blown away by that layout, not to mention your skills as always.

I would love to play that amp. This really cool basic layout is working for all the amps I like best. I need that!

Hats off to you, and RJ. This excites me!

[Jana]

Let me be the second to---WoooHooo!

That is a really nice and clean layout!

[Reeltarded]

Yeah Jana, look at him hiding the TS wires.. where is the ground bus? It looks transformerless from the inside. lmao

It just struck me! I need a stereo 50w power amp. Been thinking..

[M Fowler]

Outstanding work Martin.

Mark

[NickC]

Very, very impressive build!

[Cantplay]

Congratulations, really well done.

Now you need to make a proper cabinet for it.

John

[snuffy]

Unbelievable work. Outstanding!

[martin manning]

Thanks very much, all. Here's the layout drawing that shows how the grounds work. All the power supply nodes are closed at a local ground and then a lead is run to the chassis. The only exception is that the power tube cathodes and screen ground go directly to the chassis ground lug by the bias pot. The only difference from the way I did my Express is this time I grounded the bias supply at the reservoir instead of with the screens and power tube cathodes. This JMP 50 PT is massive- the amp does not look transformerless from the top! It's very off-balance, but I like the short leads better. There are quite a few wires under the board. When I powered it up for the first time I had no sound, and I was afraid that I might have an error under there. Turned out I just forgot to install the PI tail resistor (it was really late, er, early this morning!).

P.S. Did you Marshall mavens spot the Silver Jubilee knobs?

Edit: Layout updated

[xtian]

Want!!

[Bob S]

Martin - I'm highly impressed with the layout. Stellar work!
I believe that having the tube socket holes through the board is the best way to cut down on unwanted interaction.
The bar has been raised.
I just happen to have a set of Marshall 50 iron too...

[M Fowler]

The other thing to think about is one can also use TW PT and OT for a build like this as well.

Mark

[ampbldr2]

amazing build. How about a sound clip?

[epis]

Neat and beautiful, the chassis with personality

[Reeltarded]

Simple but so very radical. I have looked at what thismthing came from enough to understand like most of you fellas do with Fenders. I get this! Yikes! I bet this layout would do very well with an extra gain stage even in the same smallish space.

Excellent, Martin! I am always amazed, but hardly surprised. Good stuff!

I need a 50w head and a pair of 50w amps for a new rig. Let me know when you finish the power amp pair.

-coughs-

-looks busy-

Oh, I did spot the knobs, just not the question!