Marshall 4212 to SDM 2558 - thanks SDM!!

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Marshall 4212 to SDM 2558 - thanks SDM!!

Postby Joe L » Mon Sep 06, 2010 11:57 am

I've been building amps since a Mesa Mark III build in '86 after a dissapointing mail order purchase of a 4212 combo. I played with a guy a year or two later that had a Jubilee and we loved it so I took pics and drew up a schematic with the intentions of building one. I've built about 15 amps over the years but since working with a couple of other friends cloning the SLO in 1999 life seemed to take over.

A couple of weeks ago the juices started flowing again and I eyed that 4212 sitting in the closet. I had built the first clone prototype P2P SLO board in it and didn't need it anymore since I had finished my SLO 100 watt head with a prototype 6SX chassis we had Weber make for us. He still sells it in fact.

As far as I know this was the first SLO clone - late '99.


But I digress...

Then I found Steve's 2550 layouts :D Let's see here, correct transformers, more than enough controls and correct labeling! I'd even have a couple of free preamp sockets and the Clean channel controls to play with - maybe a 2204 or 1987 preamp switchable with the 2550.

In my haste and excitement I almost removed the board with a propane torch but took my time and added enough turrents to have 3 continuous rows to the left of the PI and bias section which remained the same. This would make it easier to convert from an SLO to 2550 layout and allow any mods I would want to add.

Using SDM's layout as a general guide, it was a piece of cake to bolt in a short block 2550 preamp with switching. I did use ceramic caps in most of the same places as the original instead of film/foil like Steve so I would have a close to the original tone and make adjustments as needed.

OMG! After doing my usual blueprinting, scope testing and voltage readings with power tubes removed I biased the pair of Seimens up to a toasty 37mA and slowly turned up the controls to max in clean mode and heard nothing. WTF! Then I put my ear close the speaker and heard the slightest hiss :shock: Could it be that quiet? Yep.

Plugged in the living room beater Strat and jammed for the next three hours with the biggest smile on my face. I know your newest girlfriend is always the best ever but it was THE TONE that I remembered from 20 years ago. That's what I wanted and that's what I got.

Thanks Steve! You saved me a ton of work and time with that layout.

..Joe L

PS. Have you ever identified a source for those funky cheap 100v axial ceramic caps that look like resistors? They sure worked hard at saving a few bucks.
Joe L
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Joined: Sun Aug 15, 2010 1:28 pm

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