Blackstar HT100 Barbecue: Replacing The Tube Board.

I'd worked on this Blackstar HT100 head a while ago and it has served its owner well for the last few years. I'm not entirely sure how the story goes but it is said that a beer got spilled on/in it but who knows?

Either way it had a major cookout which lunched the tube board. Lucky for me it is a separate piece of hardware AND there was available a schematic for the tube board.

On the other hand, the tube board is made of unobtainium. My first thought had been to get a new tube board but that was out of the question.

Looking at the diagram it seemed that I could hand wire it ala Marshall and then I'd figure out how to connect it to the main board. So I installed four ceramic sockets and started in on the wiring and resistors and hookup wire.

When I got to connecting the tube board to the main board, removing the molex connectors was going to be problematic because they're glued to the board. I decided to make loops in the ends of the hookup wires, and lucky me, there was enough separation so that I wasn't going to have problems with bridging.

I finished the wiring today and it powered up the first time. So tonight I'll let it have a slow simmer and see how it goes.

Bored With Boards: Repurposing an Ampeg SJ12T, Part 4.

As I've been thinking about this project some more-well, let me back up.

I've suddenly got a lot more time on my hands due to the ending of my teaching gig.

It happened in a way that was proof of the existence of chaos theory if you needed it. There is a meeting somewhere, people sitting at a conference table, and with the scratch of a pen, everyone goes home.

Meanwhile a couple of years down the road and a hundred miles away, people lose their jobs.

But nevermind.  Let me continue-this line of thought leads only to frustration and less money in my bank account.

I sourced out some 50A bridge rectifiers, the kind that are potted in a metal case-it seemed the answer to my problem about building a hybrid circuit board of 6G3 derivation, and also made more room under the hood, thanks to a thread posted by J.R. Frondelli of DBM Audio in the Big Apple concerning the age old question of what to do with a torched Blues Junior.

Being in an optimistic mood, I then decided that I would out source a circuit board for a 6G3 so as to save myself some work.

What I got was a board that was warped and off dimension, and half the eyelets were not installed. The ordinary dimensions of the 6G3 board are approximately 2-3/4 x 10-1/2 and the backing board is the same size. On the other hand, here is what the board should look like, populated. It took me a bit of time to make up a drilling mask, drill the board including larger pass through holes, clean up the burrs with a razor blade, stuff it with eyelets, tape them down and clinch them. Then, clean off the tape residue and stuff the board with components and solder them in.

 It should look like this:

 The red marks were for orientation of the components. The lower shot is the comparison.

Now. How did I do it? It's posted elsewhere on this blog but it consists of obtaining 3 inch vulcanized fiberboard, making a drilling mask from a photograph or screen shot of an existing board, sizing it with a freeware program called Irfanview, printing out a drilling mask on 8-1/2 x 17 inch paper, trimming the board to size, taping and drilling the 1/8 holes. Then you can trim the burrs , install the eyelets and clinch them and stuff the board.

To complete a board like this one, stuffed with the components of your choice once you have paid the one time cost of the tooling and eyelets should cost you less than $20 in parts.

And you'll have exactly what you want, when you want, with no waiting and no shipping or paypal fees. The folks on fleabay will charge you upwards of a C-note plus shipping for something you can knock out all day long for pennies on the dollar.

Haste, in my case, surely did make waste, but as always it's a teaching moment. I'm making all my own boards from now on, thank you.

Thanks again to Jeff Gehring for being so generous in sharing his knowledge.

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.

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.