Thursday, January 22, 2015

Repurposing an Ampeg SJ12T, Part 3

.If you've read this far you have a pretty good idea of this project. A brief review, then.

I acquired an Ampeg SJ12T a while ago, it is one of the pre-Loud Technologies Diamond Blue Series amps that SLM turned out in the mid 1990s, In theory it was a good bet-a single 12" Eminence speaker, 50w from 2 6L6 or EL34 power tubes, vibrato, and reverb.

That's where the joy ended. The reverb quit and then shortly after the vibrato left town. Nothing could be done to restore it so it sat.

A few weeks ago I decided that it could be repurposed, but into what?

After some study of schematics of likely suspects and examination of the available real estate , it seemed that a Fender 6G3 Deluxe, suitably modified for the power supply and preamp architecture would fill the bill.

There are. of course, two channels on the 6G3 and only two inputs and five total chassis holes for pots on the SJ12T. I did have a couple of dual pots of the right value for volume and tone so that takes care of the two channels pretty nicely. A couple of vibrato pots were added, and I have a single pot hole left which was formerly occupied by the reverb control, and I think I will add a presence control there.

I already installed a proper pilot light assembly and fuse holder.

I started cooking up a 6G3-ish circuit board that will accommodate the diode bridge in place of the rectifier tube as the SJ12T used a full wave, no center tap design. and at this time I hope to continue with the original SLM power transformer which is the same as that installed in the VC50 Crate and presumably the SJ12R as well. Is it up to the job? Only time will tell,.

Choice of tubes was relatively simple, as I have plenty of new old stock Soviet 6SL7s and found a pair of new original type 5881s in my tube overstock carton. That and the Classictone AB165 Bassman output transformer should take care of the power management chores.

Power supply filtration chores will be handled by the JJ multi cap can, and the output transformer lead through provided a nice pilot hole for my Harbor Freight knockout punch

I've got some more things to add to the board and then start thinking about stitching the wiring together. Stay tuned.

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