Thomas Vox Repair Boards Update

[R.G.]

When last I posted, I was testing a one-size-fits-all PCB for repairing all of the bigger Thomas Organ Vox amplifiers.

That board worked; I did a couple of forehead-slappers on it, but there were easy workarounds and patches, so the boards were usable. I'm patching it into a recipient Beatle amp now, at my usual (slow...) pace.

But I did discover a problem. The reverb tanks for the earlier amps are not the same as the later ones. After some thought, I reworked the repair board to work with the earlier, transformer-driven tanks, and also fitted the early distortion switching and bipolar preamplifiers. I just finished testing this version of the board, and it's working on the bench. There is a Royal Guardsman that's going to have this one implanted in the next few weeks, so I'll have a full listening test then.

It looks like this is going to work out - clip out the entire old PCB and all the wires, solder the new direct-to-control wiring with flexible hookup wire, and have a fully rebuilt Thomas Vox amp.

I'll post some pictures of the rebuild.

[drewl]

Oh no, I like being one of the only people brave (stupid) enough around here to work on these.
Maybe I can sabotage your work.

Just kidding, and keep up the great work.

Got a sweet AC50 in for return, sounds awesome but damn what a stupid layout.

[R.G.]

... or you can think about offering a complete rebuild service for these amps. All you need is to clip out the old board and solder the new one in. So if you had a source for replacement PCBs... hmmm "Honest drewl's Vox repair site"

[52 Bill]

So if you had a source for replacement PCBs... hmmm "Honest drewl's Vox repair site"

So R.G. are you planning on selling boards or publishing the art at your site?

Now if only we could find someone who could reverse engineer those pesky driver transformers in the power amps...

[R.G.]

The plan is to keep the amps out of landfills, so although I don't know exactly what will happen, I'm open to suggestions.

I think this will enable someone to rebuild their amp at under the cost of a recap - which most techs won't do anyway at any price. I'm still thinking about whether boards alone or populated boards are the best way to save the most amps. I know how and where to get contract assembly or boards, so it's conceivable that I could get assembled and tested units ready to install. That requires placing a large monetary bet on the number of boards built versus the number to pay back the up-front costs. It might be better to sell just boards, although that will lower the number of amps kept out of landfills, because many owners can't solder, or can't solder well.

Suggestions?

[R.G.]

Now if only we could find someone who could reverse engineer those pesky driver transformers in the power amps...

Ooops. Missed that one. Yeaaah.. if only. Give me a bit of time. Almost there.

[cjlectronics]

I just completed a repair of a Royal Guardsman. See this thread http://music-electronics-forum.com/t36954/

I took two Viscount cabinets along with various parts and made one good amp. What I have left over is a Viscount preamp and Viscount cabinet. Do I understand you correctly that you are going to have a Viscount amplifier or generic retrofit amplifier design/layout?

The Viscount amplifier design and layout looks simple but the transformer is an issue... correct?

CJ

[R.G.]

I'm actually attacking all of the Thomas Vox issues, although a lot of the fixes are not ready for prime time yet. Here's what I have in the works:

- A complete replacement board for the preamp guts, in two flavors, the V11*3 late series and the earlier V11*1 series. Between them, they are drop in replacements for the entire circuit guts of the preamp chassis, re-using the controls and switches and in the V11*1 series, the reverb transformer. The signal circuits are the same as the original, but with modern parts. Fits the same mounting brackets, but tosses out the incredibly failure prone wiring in favor of specific wires from board to each control. One of these two boards can rebuild any of the Beatle, Guardsman, Buckingham, or Viscount by leaving off the unneeded controls. I believe the early Westminister will work as well, but haven't dug into that much. Includes the V112/113/114/115 era early ones as well as the V112x/113x/114x/115x models.

- A replacement board for the power amp; I haven't put this on the net yet. It uses the original circuits and driver transformer, but puts the driver transistor and heat sink on the PCB. Since it's the original circuit, it ought to work, but I have to filter out any forehead slapping errors first. This may need additional work as there is more variation in the earlier power amps; or it may be that all power amps can be replaced by the latest and greatest. More work needed here.

- A replacement/rewind for the Beatle and Guardsman driver transformer. This would take a modern 24VA power transformer and rewind it to be a driver. This is in the middle of work. It requires that I get off my duff and finish my coil winder so I can wind the test units. It may be that a single driver can replace them all, or it may require two or three. More work needed here. There is an article on what and how to do this geofex.com.

- A replacement for the power switch, which is otherwise hideously expensive or simply unavailable. Uses a PCB with a $6.00 rotary switch and two PCB-relays to do the switching functions of the custom switch Vox used, and fits the same space. Ready for prime time, just not posted.

- Drawings of the sheet metal chassis. It is possible to bend up new sheet metal. Still in the drawing stages.

- Scans and repro panels for the control appearance plate. I have a silk screening shop quoting on doing repro panels now. This looks to be impossibly expensive for one bent panel as used in the originals, cheap and reasonable if this is made into two panes and the bend at the back of the base preamp sheet metal painted black.

All of this is stuff I'm doing for myself, but might be useful to others. Exactly how I make it available is still work to be done.

I got pushed into making the single-preamp-replacements by a plea from a fellow who has a Guardsman with a broken main PCB. Without something like this preamp board design, that amp's going to be parted out or tossed into the land fill. It accelerated the work on that part of the grand scheme.

I'm working on the issue 52 Bill brought up - boards? DIY layouts? Parts kits? Assembled units?

[52 Bill]

The plan is to keep the amps out of landfills...

I thought that the solution for this was to just send them all to you.

T.O. Vox fans are varied in that some just love the sound, some love the looks and some love them for their historical past. Sick people like me love all three aspects and keep rebuilding them as true labor of love more than anything else. I think that if there were more of these amps out there that were working 100% there would be a bigger fan base, but a lot of the comments you hear are based upon playing an amp that is in desperate need of a total recap.

Some collectors would balk at total strip out replacement of boards, while others would probably just be happy to have amps that worked correctly. Players would be happy to have an amp that didn't need to be serviced all the time.

And if your power amp boards got down to the lower levels of the Viscount/Buckingham, they could also be engineered to replace the Germanium outputs all in one shot.

At least once a year I'm faced with a solid state power amp repair that can go nowhere because of a dead or missing driver transformer. Vox, Kustom, Peavey and a number of others all used them at one time or another. My hope for your transformer research was that it could lead to simple formula and direction set that could be used to rebuild or replace any of them.

Now if you can come up with a solution for the particle board problem...

[loudthud]

I'm wondering about the features that were added on as the amps evolved. I think the first was the Fuzz, then the G tuner and the Repeat Percussion. Each change got a new model number. R.G., does your board support these features?

Later versions of the guitar amps had a JFET input stage, but the bass amps never made the switch and kept the original bipolar input. Do they sound any different?

[R.G.]

I thought that the solution for this was to just send them all to you.

That worked until the garage filled up...

Some collectors would balk at total strip out replacement of boards, while others would probably just be happy to have amps that worked correctly. Players would be happy to have an amp that didn't need to be serviced all the time.

I can't help the collectors. Well, I could. One stop on the long path was to reproduce the main PCB exactly. I can do that. But it does not address the real issue with the amps, that being the wiring harness. For people that want it to play correctly, I can fix that problem, as well as many of the reliability issues.

And if your power amp boards got down to the lower levels of the Viscount/Buckingham, they could also be engineered to replace the Germanium outputs all in one shot.

Yeah, that's not really a problem. It will take some analysis, but I think one or two boards fits all.

At least once a year I'm faced with a solid state power amp repair that can go nowhere because of a dead or missing driver transformer. Vox, Kustom, Peavey and a number of others all used them at one time or another. My hope for your transformer research was that it could lead to simple formula and direction set that could be used to rebuild or replace any of them.

Chances are bad for that. The problem is that most of them need a driver that converts the available driver voltage and current to the necessary output transistor base voltage and current. Those are different ratios and amounts of power. The overview is simple, and I can teach you how to do that, but the execution may need tweaking to get it right.

Now if you can come up with a solution for the particle board problem...

That would be to grind it up into a fine, fine power like its original state, compost the powder, and replace the original panels with plywood. Oddly enough there are several cabinet cutting diagrams for Vox stuff at geofex.

I'm wondering about the features that were added on as the amps evolved. I think the first was the Fuzz, then the G tuner and the Repeat Percussion. Each change got a new model number. R.G., does your board support these features?

Later versions of the guitar amps had a JFET input stage, but the bass amps never made the switch and kept the original bipolar input. Do they sound any different?

The model number chain was even more complex than that, but I spent quite some time looking at how to cover all the features. The short answer is yes, one of two boards cover the entire set of models. The prelim documentation on geofex now has a table of features per model, I believe.

First, only the V1143 Beatle had repeat percussion. The V1123 Buckingham and V1154 Viscount and V1133 Guardsman had all the V1143 Beatle features except perhaps the E tuner. I made one V11*3 board that supports all the circuits in the 1143 Beatle. They also work correctly if you don't hook up the controls, so if you don't plug in an E tuner or have a repeat percussion control and enabling footswitch, the preamp works for the lower 11*3 models, which differ only in the power amp and external cabinet.

The 11*3 models had JFET preamps as well as no-transformer reverbs. All the earlier models had bipolar preamps and transformer reverb. The second preamp board, the V11*1 series, has the fullest set of features of all the lower line, and can replace any one of them by ignoring any circuits not used in that model. The 11*1 series has the relay-switched Fuzz on the PCB, exactly the same signal circuits.

So in short, yes, I think the two boards between them support the entire chain of models by subtraction - there's a full-gallon set of the features, and you can choose to leave features unused in lower models. This is *exactly* what Thomas did, although they did it in the earlier models by adding baby PCBs for fuzz and reverb, and in the later models by leaving bare places on the same main PCB.

I don't know if the JFET and bipolar preamps sound different. But one of my early adopters/beta testers is rapidly filling up a JFET PCB to test it against an original bipolar-preamp Guardsman.

That leaves the question of whether these boards sound the same as the original circuits. They should - they're the same schematics. I tinkered in small ways with the power supply (regulated, not simple R-C droppers so it's stable) and some signal routing (to avoid signal wires off board), but they are the same circuits. Literally, it's a new PCB that does the same as the original, but with new parts, and wires that are designed to not break, and if they do to be replaceable without tech disasters.

[52 Bill]

Chances are bad for that. The problem is that most of them need a driver that converts the available driver voltage and current to the necessary output transistor base voltage and current. Those are different ratios and amounts of power. The overview is simple, and I can teach you how to do that, but the execution may need tweaking to get it right.

I've rewound small output transformers before and anything beyond a vintage Champ is pretty much not worth the time and effort. The fact that there are no replacements for these driver transformers is what makes it worth looking into finding a way to replace/repair the missing ones.

That leaves the question of whether these boards sound the same as the original circuits. They should - they're the same schematics. I tinkered in small ways with the power supply (regulated, not simple R-C droppers so it's stable) and some signal routing (to avoid signal wires off board), but they are the same circuits. Literally, it's a new PCB that does the same as the original, but with new parts, and wires that are designed to not break, and if they do to be replaceable without tech disasters.

They will probably sound different, but how will anybody be able to tell. For those of us that are old enough to have heard these amps when they were new, comparing the sound to what we remember will be difficult at best. And comparing them to an old one will only make your new one sound that much better.

[loudthud]

I had a Buckingham in 68, my first real guitar amp. I remember trying to play Hendrix that the amp had terrible blocking distortion if you turned the volume up too high. Was that the Load Limiter (or whatever they called it) or some other circuit shortcoming? Otherwise it was really cool sounding. I especially remember the sound you get as you twist the treble or bass controls. To me, that's was a guitar amp should sound like.

[R.G.]

I had a Buckingham in 68, my first real guitar amp. I remember trying to play Hendrix that the amp had terrible blocking distortion if you turned the volume up too high. Was that the Load Limiter (or whatever they called it) or some other circuit shortcoming?

Blocking distortion? Hmmm. I think that may have been a problem with the driver circuit before the driver transformer. The demands on that circuit to keep the driver transformer transforming are pretty tough. If it ran out of current, it could drift off and have to recover. Could have been high leakage in the germanium outputs, possibly. Other possibilities are a bum electro cap in the mixer/limiter.

Actually, there was a service addendum on the later amps to change the three input electros for the limiter from 10uF to 1uF. There's a possibility there, too. And there's always some random circuit fault.

The Load Limiter was actually an adjustable clipper. The adjustment was intended to make the Load Limiter clip the signal just a bit before the output transistors would clip. That way, they never saturated, and never had to come OUT of saturation, which was a bigger challenge for those early devices. I suppose if it was misadjusted and did not clip early enough, it could let the outputs get into unusual situations.

But it wasn't inherent in the circuit design.

Otherwise it was really cool sounding. I especially remember the sound you get as you twist the treble or bass controls. To me, that's was a guitar amp should sound like.

They were actually quite good at that. Elyea's Vox book cleared some of that up. Denny actually went to the Thomas plant to work with Thomas designers on the amps, including the sound. He had a hand in tweaking the sound, so it shouldn't be surprising that it's good.