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.

Biasing A Mesa Triple Rectifier.

I recently got a Mesa Triple Rectifier head in for a retube and general checkover. A little clarification is in order if the gibberish I have seen in researching this issue is any index.

Bias voltage is a negative potential applied to the grid of a tube, in this case power tubes,

A "fixed bias" amplifier only means that there is an external bias supply for the power tubes, and this can be obtained by means of a dedicated winding on the power transformer suitably rectified and adjusted, or by taking a little bit from one side of the power transformer high voltage and doing the same thing here-which means you have high tension in places where maybe it ought not to be. But nevermind. Fixed bias can be and often is adjusted with a potentiometer or, as in Mesa's case, with a fixed resistance value.

The other way is by cathode bias which means that a suitable resistance is placed in the cathode string of the power tube or tubes and this creates the desired potential. About this we need say no more.

One thing to do here is to measure the bias current and plate voltages in both tube rectified and silicon rectified modes before changing out the tubes unless there is other damage that prevents this. It gives you a basis for comparison, and there is a pretty wide spread between the two sets of values. We do not want to exceed the rated value in the silicon rectified mode, in case someone wants to use that and flips the switch. The tube rectified position will just have to be what it is going to be, and it will be significantly lower.

So....what to do?

You can install an adjustment pot in place of the 82k bias resistor which has the green test lead attached to it in the upper image, adjust and be happy, but you will have to remove that resistor and unless you remove the circuit board to get access to the underside you can damage the traces on removal and not know it.

You can also, as I did, take a different approach. I took some test leads and paralleled the bias resistor. The other end of  the resistor is the black wire on the 6L6/EL34 switch. Then, selecting a suitable resistance between 20k and 100k you can look for your sweet spot.

In my case a 39k resistor in parallel produced about 30 ma in silicon rectifier mode-which is good-and about 10 ma in tube rectified mode, which is not so great but will have to do.

I got out my magnifier, trimmed the 39k resistor and turned the ends into hooks and soldered it in.

Close enough but as I mused over the subject I got to thinking about the rather wide spread in bias values between tube rectified and solid state rectified it occurred to me that I did have three nice new Shuguang 5AR4 rectifier tubes that were worth trying in place of the 5U4GB Sovtek/Mesa tubes installed.

Well. Wonder of wonders. The 5AR4 rectified plate voltage was close to solid state, and the bias spread was only about 7 ma. It seems that the idea was good but the execution of the original was flawed. Now, I could get both plate voltage and bias schemes within spitting distance, stay out of crossover distortion country, and have the best of both worlds. I highly recommended it to the owner.

Going North For A Traynor.

Here are a few snapshots of a recently acquired Traynor YBA-4 Bassmaster. At least I think they call it a Bassmaster. Never mind. It is what you see, and it originally belonged to the Mound View Independent School District. That mighta been a cool school to go to if they had stuff like this.

I'd been looking for one of these for a while (my love of fifteen inch speakers) when I located one on the Minneapolis Craigslist. A price was agreed to and I committed to picking it up. Serves me right, I shoulda known better because it was actually up north of Brainerd, which is a long way from Des Moines. But I'd already agreed to it and it seemed like it would be a good road trip. I flew solo, which left the right seat vacant-a good thing because it did rain some.

Brainerd is about 375 miles from here, a couple hundred miles south of the border and maybe 75 miles west of Duluth. I met the young man who owned it, cash was exchanged and home I went, flying blind through a heavy fog. The Garmin GPS did a splendid job because I'd have never found the place otherwise.

Once I got home all it needed was a single preamp and a single power tube. I decided to replace the electrolytics in it and install 6L6GCs instead of the usual EL34s because I just don't like the way the EL34s sound. They're too Marshall-y (if that's a word), and I've just never liked the Marshall grind.

It's a drop in fit, but it's a good idea to remove the 47k bias resistor and replace it with a 22k resistor and a 50k trip pot so you can adjust the bias a little to suit the different power tubes.

Now. Pay attention here.

On the other hand if your amp has 6L6GCs and you want EL34s for some unknown reason that totally escapes me, this CANNOT be done without some surgery so beware, and if you can't read a schematic or wield a soldering iron leave it to someone who can. Aside from that the EL34 draws a lot more heater current and that's not a great thing.

One thing that's sure to please is that the circuit is almost identical to a 5F6A Bassman or a JTM45. There are some minor component differences in value, but the major difference is that the rectifiers are solid state and the power transformer and diode bridge is full wave with no center tap. Traynor calls the middle and presence controls the low and high range expander but they're pretty much the same thing.

The goal here is to make it as much a 5F6A  as is possible. I think a choke instead of the first dropping resistor a sag resistor modification, and conforming the rest of the component values ought to do the job.

As it is it is incredibly loud and the work I've done so far has it sounding really clean and bright. It's got a solid bottom end that makes for some good rockabilly plonking. It might even be worth investing in a few pedals, too.

Defeating the Microprocessor: Going Forward By Going Backwards And Ditching The Moose.

It does seem as if every millenial smart phone addicted design engineer in the world just cannot live without incorporating microprocessors in places where they do not belong because there's no reason for it.

What does this have to do with guitar amps? Well, not much, but it is technical.

My first experience with this was the infamous jazz board failure in my Maytag refrigerator a while ago.

Or maybe this stuff is now filtering down into every area of life because it's cheaper to make? I dunno. Maybe the Nanny State can't settle al Qaeda's hash but they sure know how to make life difficult for the rest of us with this energy saving nonsense.

Fast forward to about eight months ago. I was awakened from my stupor by a loud bang! as the gearbox in our Amana washer gave up the ghost after one too many overloads. It couldn't be economically repaired.

So down we went to Sears Roebuck and selected an Energy Star rated super environmental water savin' washer. That was a bust. It took forever to wash a load of clothes, they came out dirty, and it cost a lot of money.

Three weeks later She Who Must Be Obeyed had had enough of that, so, being chumps, we went back and got one considerably less expensive, with a real impeller, and thought we had it made. Wrong.

It has a digital mechanism in place of the old standby clockwork switching apparatus, and when it was running it sounded like a moose in the throes of moose coitus or maybe the moose was gettin' it on with a bale of hay-who knows? It has been the source of endless rude jests around here, and if you have time on your hands you can have a lotta laffs with this. I wish I'd recorded it.

Last weekend it did what microprocessors are real good at-it crashed, and those three LEDs sneered balefully at me as the whole apparatus came to a grinding halt like a machine version of Jimmy Hoffa. Patience had worn thin, and I was tasked to go and find a Speed Queen washer with an old fashioned clockwork switching mechanism.

Luckily enough, they are still in production although time is no doubt limited and they'll soon be gone. It's not a lot different than the Speed Queen we bought in Ohio in 1983 that lasted until 2005 but was getting cranky in its dotage.

It came yesterday afternoon, and we ran a test load of six very large bath towels, It was nice and quiet, got through the process in a timely manner and without all the moaning and humping.

Perfect.

Ampeg Gemini 1 Full Wave Bridge

An Ampeg Gemini 1 showed up a little while ago with the owner complaining that it tripped the ground fault interrupter in his residence and had voltage on the faceplate. After removing the so called death cap-a likely suspect-and isolating and checking the power transformer for leakage, the only thing left that had a path to ground was the full wave, no center tap diode bridge.

Although the diodes checked ok with a voltmeter, 1N4007 replacement diodes are inexpensive, and the originals were probably 50 years old. Not willing to bet the ranch for 15 cents worth of diodes I replaced them. Looking at the original bridge, it had a quartet of old school diodes identified by yellow and red paint dots, not standard markings, some of which were worn off.

The above drawing with the lines properly oriented will build you a proper bridge. You can even take a chunk of large sized heat shrink tubing and use it as an insulator with a red marking for polarity.