The Evil Pro

I've been jonesing for a certain Fender Pro that was made for a couple of years. This model is an AB763 circuit without reverb, and a 15 inch speaker and I guess it is pretty rare as I never saw one in the fleisch as the Germans say.

For a while I was thinking seriously of taking my Bantam and revising the circuit to add vibrato. But then a guy calls me and I was on my way to Boone, Iowa to look at a drip edge 1968 Dual Showman.

The Showman was all there but it was in rough shape, I bought it on the spot, head and loaded cabinet for $325, and I knew right away what I was gonna do with it.

The JBLs were both defunct so I'm gonna have to do something with it.

Some opinion sampling later and a few emails and I settled on J.D. Newell to build me a custom cabinet that would accept the Dual Showman chassis and a 15 inch speaker. They seemed to be the only people who knew exactly what I wanted and how to make it. Money was exchanged, a delivery date was quoted, and I proceeded to overhaul the chassis and spiff it up with a new blackface panel. The Fender logo I had already.

The cabinet arrived right on time and it is a stunning piece of work. A number of my friends have seen it and exclaimed "That's better than anything Fender built."

For the time being I installed a 4 ohm CTS speaker but I have a higher power Oxford that I may use.

How's it sound? Loud as hell, clean and bright, and with the channels bridged it gets even better and you can soften the vibrato by adjusting the balance between the channels. After about five minutes one of my brand new Ruby 6L6GCMSTR power tubes grenaded, so I slammed a Sylvania in the vacant slot and carried on. I've got a quad of Sylvanias I matched up, and there are plenty of RCA blackplates in the stash box. Preamp chores are being handled by Uncle Boris over at Sovtek.

It's the Pro that Fender never built, and I've named it the Evil Pro.

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.

Repurposing an Ampeg SJ12T, part 2

The next project has been constructing a circuit board for the repurposed SJ12T, loosely based on a 6G3 Deluxe.

I had a bunch of circuit boards populated with parts from the Judybox Amps garage sale that was had down in Austin a few years ago, along with some other odds and ends that came from the sale.

Part of the repurposing was figuring out the power supply.

The original Ampeg power transformer delivered about 400v of B+, and that was based on a full wave configuration with no center tap, all solid state rectification. The transformer itself is a St. Louis Music item that I've seen in a couple of Crate VC50 amps, which was punishing 4 EL84s in that application. It looks about the same size as a Deluxe Reverb power transformer, but with the full wave setup and no rectifier winding. It does have a bias winding, and what looks like a 12v center tapped supply which gives filament voltage as well as +/- 7 volts for some sort of solid state dog waste.

Aligned with that task was starting a circuit board. I cleaned off all the crap from

one of the Judybox boards, and installed eyelets along each side. Loading the components-all the tone, volume, phase inversion material from the 6G3 circuit plus the full wave bridge and bias supply pretty much used up all the real estate and identified some center holes that need eyelets plus a couple that need drilling. Power supply capacitor tasks will be taken care of by a JJ 40-20-20-20 can that I have not yet installed. The capacitance is a bit higher than the original 6G3 but not radically so.

What to do with the remaining hole in the panel? A presence control can be added.

The last issue will be figuring out how well or not well the octal preamp tubes will work with this circuit and the changes I've made to it. Some study of a few older Ampegs that use octal preamps like the M12 and M15 may give some clues to the voltages that will be needed.

After the octal sockets were installed I ditched the blue neon pilot light for a regular Fender style pilot light and wired up the sockets. That's where I stopped.

More to come as I do some more work on it later on in the week.

Repurposing an Ampeg SJ12T combo Part 1

A while ago I acquired an Ampeg (SLM) SJ12T combo because it sounded interesting. In a word, it turned out to be a complete dud. The reverb and vibrato went out to lunch and they haven't been seen since then, and no amount of endearment could get them to come back and behave.

I mean, the idea sounded good-an all American made tube amp, 50w, single Eminence speaker, reverb and vibrato. Who wouldn't have loved it? Of course, as I mentioned the reverb and vibrato were dodgy, they went out for lunch and never came back despite all my efforts, not to mention intermittent crackling and pops, and there was absolutely no way this amp could deliver 50w-not now, not ever.

So back in behind the water softener it went, to do its term of incarceration for felonious misconduct while I figured out what on earth I was going to do with the thing.

I pretty soon came up with the idea of repurposing it, and it's turned out into an all out project, which means that this is going to be in several installments. There are a number of reworks that are needed so I've had to figure these out as I went along.

After a lot of study I decided to ditch the cheesy reverb and build something close to a 6G3 Deluxe with octal preamp tubes and vibrato. There was room to broach another socket hole so in it went.

Next, I had to figure out how to get dual volume and tone controls. That was easier than I thought as I had two stacked dual 1 meg audio taper pots. Between that and a couple of switches I could have normal and bright channels and bridge them as well with the existing holes, opened up a little.

Considering that the output transformer seemed stingy on the iron for anything close to 50w, I decided a ClassicTone Bassman output transformer I had was just the thing, and it could be located more or less in the same neighborhood as the old one which can find a home in some other project.