Plexi-800 amp to pair with my mhartman based SSS hybrid

[Mike Johnston]

Thanks to information on the Amp Garage forum I have constructed the Steel String Singer hybrid posted by mhartman. The SSS hybrid is a great amp and got me thinking that having a Marshall Plexi or 800 clone to match up with it for overdrive would make about as good a rig as one could have, at least for my tastes.

The attached schematics reflect my view of what the perfect match for the mhartman SSS hybrid would be. My thanks to 10thTx for his help in preparing the schematics as I am incapable of work of anywhere near that quality. The technical acumen I possess is evidenced by the cut and paste layout that I prepared and have also attached.

This amp is based on a design by sluckey posted on the Hoffman forum. It combines Plexi/800 preamps in one amp that can be switched between or used with individual jacks. The preamps share a common tone stack, PI and power section.

I took what sluckey did and added some additional features to it. This includes adding a second tone stack so each preamp has its own tone controls. My rational for this , right or wrong, was that with an A/B/Y switch it would give me two amps to choose from. For my purposes, I didn't think a shared tone stack would optimize the best of both amps.

Because of my experience on the Amp Garage site I couldn't see how anyone could make an amp without a Dumble based overdrive circuit and an onboard Dumbleator. I also thought a LarMar based PPIMV would be a good addition so I threw it in too.

The overdrive circuitry is a direct copy from the Dumble 124 amp. The 124 amp has 100K plate resistors as do the Marshall's so I thought it would be a good starting point. Does anyone have any experience with putting an overdrive circuit in a Plexi or 800 based amp? If I can correct any misconceptions on my part during the design phase of the project it will make for a better amp and save time and money. Any comments here would be greatly appreciated.

The FX circuitry is a direct copy of the Dumbleator three knob active effect circuitry. I took it from a post by Funalicousgroove titled D-lator Tutorial step by step as I do it. I am running it off the amps power transformer. Again, if you see anything I could do better please advise.

The PPIMV circuitry is from the LarMar circuitry that is on many different sites. It is a proven circuit that seems to be liked by some and not by others that acts as a master volume. I will leave a turret in the PI section so I can add two 220K resistors if I don't like it and want to remove it from the amp.

Thanks
Mike

[Mike Johnston]

Attached is a photo of my mhartman based SSS hybrid.

Thanks
Mike

[dreric]

WOW! gut shots? Way to make a splash!

[dobbhill]

Many years ago, I built an ODS into an old Super Reverb chassis. I put a second preamp in because I had room: JTM45 style preamp. I could select either preamp, which then fed the OD section. I liked it, but the tonal difference between the preamps was significant.
My suggestion would be to try it for yourself.
D

[Mike Johnston]

Many years ago, I built an ODS into an old Super Reverb chassis. I put a second preamp in because I had room: JTM45 style preamp. I could select either preamp, which then fed the OD section. I liked it, but the tonal difference between the preamps was significant.
My suggestion would be to try it for yourself.
D

Thank you for sharing your experience. Doesn't sound like you had a bad experience with your build.

I am waiting for my metal man to finish my chassis and another shop is finishing my faceplate and back plates. I use the faceplate and back plates to drill the holes. Just put a + sign in the middle of the spot for pots, jacks, switches, etc. Use a center punch and then drill to the appropriate size. Much easier to line up the holes that way.

Thanks
Mike

[M Fowler]

I like to do the same thing, get the plates, mark, and then drill away. I get the plates made with pilot holes and drill them to size later.

[mhartman]

Great looking build Mike! I'm glad to hear that the SSS layout was helpful for you. If you get a chance, post a gut shot and/or clip!

[Mike Johnston]

Great looking build Mike! I'm glad to hear that the SSS layout was helpful for you. If you get a chance, post a gut shot and/or clip!

Much thanks to you for sharing your work. If I can figure out how to use the microphone on my laptop I will get a clip out. Basically, it sounds a lot like yours did, not too surprisingly. I will try to get a gut shot posted during the next few days too.

Thanks
Mike

[10thTx]

For anyone who would like an editable SCH version of this. B+ rail and PT is on the second page of the file.

* Note a correction on the DPDT bypass wiring on the FX. The posted GIF
file has an error.

With respect, 10thtx