updated January 18, 2015

Mark Huss's Plexi 6V6

I've known about Mark's little Plexi for several years and always thought that I might build it some day. I came close in 2007 but finally decided to go with the November amp from ax84.com. Mark's Plexi 6V6 and the November are basically identical Plexi preamps but one drives 6V6s and the other drives EL84s.

Late last October I had run out of warm weather projects. The turnip patch was coming up and I began itching to build the "just one more" amp. I wasn't sure what I wanted to do but I knew I'd like to build something with a Hammond AO-43 organ amp. So I began shopping eBay. By mid December I had aquired two AO-43s. Now what to do with them. Since the AO-43s use EL84s it was just natural to lean toward another EL84 based amp. But what?

About this same time Mark's Plexi 6V6 was being discussed at Hoffman's forum. The discussion began with the idea of modifying Hoffman's Plexi board to accomodate the Plexi 6V6. Easy enough. Hoffman's circuit would only require small tweaks to work with Mark's circuit. Then the discussion turned to "Can we put this in a 12 x 6.5 x 2.5 box?". At that point I knew what I was gonna do with one of those AO-43s.

The circuit is simple enough. The real challenge would be making it all fit in that small box! And can I make a small board with a logical layout too? So, I began by taking measurements of the AO-43 iron. (Very nice iron BTW.) Then I began shuffling the major pieces around in Visio. I quickly realized that the tubes would be close together and the board should not exceed 7.5" in length. Visio page two became the board layout. It actually went faster than I had originally thought.

Everything came together quite nicely. The fit was tight, but logical and even resembles a typical Marshal plan. The little amp is very quiet at idle but can really roar! It can easily play any venue where you might use a Deluxe Reverb. The 6V6s have a fuller bottom end sound than my November EL84 Plexi. And it has plenty of Plexi grind although that grind does not come on as early as the November. Still, the master allows nice preamp grind at low sound levels.

I made a couple changes to Mark's schematic. I dropped the Fat switch, increased the NFB, included a 2204 style master volume, and installed a tube socket for the rectifier. That allowed me to experiment with different rectifiers including a SS plug to see what effect the different B+ voltages would have on the amp. The GZ34 produced 400V and is my choice for this amp.

I'm really pleased with the way this project turned out. There is nothing I would change if I were to build another. Here are a few pics of the project. Enjoy...

The board is drilled, staked, and laced. A full sized drill guide in included in the documentation file. Can you spot the error?


This pcb stand is really handy for loading the board.


Everything is dry fit and the PT is wired.


Pot buss, shielded cable, and filaments are all wired up. It's too cold in the shop to do any more for a couple days.


All done! It's a little warmer today and the amp is completely wired. (Click the pic for a closeup.)


Top view


Front view


Rear view


I just had to throw this pic in the mix. Maybe I'll get inspired to put some Celestions in this cab!

Here's the documentation... 6v6plexi.pdf (1.2MB)

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