Fixing Thomas Vox amps: power switches and driver trannies

[R.G.]

I finally got off my duff and uploaded a couple of things that have been in the work queue for a while. There are two new articles at geofex; one on how to fix the blasted Thomas Vox power/standby/off switch when the wafers break, and one on building a replacement driver transformer.
New Page 1

[52 Bill]

Thanks for these updates. I've always felt that the bulk of the broken wafer switches were caused by frustrated repair people or owners that yank on the wires when working on these amps.

Do you think that the wafer switch contacts were really rated to handle 10 amps of speaker power? They always looked like the generic contacts that most of these rotary switches had. I've only had to repair two of these in my time, but I would replace the broken parts with new from a large selection of wafer sections collected through the years from surplus sources.

I like the relay idea and will try it when I get a chance to build one for fun.

As for the driver transformer, did you get to actually wind one and test it? Edcor sells EI-75 laminations and open bobbins (no winding center wall) cheaply. But they sell in bulk quantities, 5 bobbins in a pack and a 6 inch stack of laminations. I guess that if you have the TOV sickness you'll find use for all of the parts. And it will save the time of taking apart a donor transformer, not one of my favorite jobs.

[R.G.]

Thanks for these updates. I've always felt that the bulk of the broken wafer switches were caused by frustrated repair people or owners that yank on the wires when working on these amps.

It's probably a combination of effects. The old phenolic does get fragile. I suspect that one thing that may have contributed was that the crimping forces when the wafers were assembled may have induced starter cracks that thermal cycles made worse. After that, a much smaller force would make it break at stress concentrations. And you are very right - the wires are attached so that a good yank will damage the switch. Perhaps a good preventative would be to strain relieve the wires to the switch post with some cable ties.

Do you think that the wafer switch contacts were really rated to handle 10 amps of speaker power? They always looked like the generic contacts that most of these rotary switches had. I've only had to repair two of these in my time, but I would replace the broken parts with new from a large selection of wafer sections collected through the years from surplus sources.

Depends. This kind of setup is heavily self cleaning by wiping. They'd probably be OK as long as they didn't do many cycles of opening currents that really were high. If the amp was quiet when it was switched, the currents would be low. But you're familiar with my twin obsessions of engineering and OVER-engineering. Anything worth doing is worth overdoing if you don't have a production budget to mak.

I like the relay idea and will try it when I get a chance to build one for fun.

You probably have the tech background to see how to mod the PCB layout to make it work with the existing DC lights. I actually did a board with that, but decided not to publish it because of the obvious AC wiring hazards. I can almost trust someone who's skilled to do this one right. Wiring the DC power supply and indicators there, not so much.

As for the driver transformer, did you get to actually wind one and test it? Edcor sells EI-75 laminations and open bobbins (no winding center wall) cheaply. But they sell in bulk quantities, 5 bobbins in a pack and a 6 inch stack of laminations. I guess that if you have the TOV sickness you'll find use for all of the parts. And it will save the time of taking apart a donor transformer, not one of my favorite jobs.

It's in progress - slowly, like all the stuff that's dependent on how much bench time I get. The photos are of the disassembly of the donor for the one I'll wind.

Actually, there is a third article coming. I'm also documenting a coil winder setup that will be useful for these kinds of rebuilds as well as winding pickups.

[loudthud]

An On-On-On toggle switch will accomplish the function the Off-Standby-On rotary, except for the functioning of the lights. This switch link: https://d1sjrnpi226dnf.cloudfront.ne...pdf?1382030369 will also do the job, it's just an On-On-On striped down to the essentials.

[R.G.]

Yeah, there are ways to replace the whole switch, but getting a look-alike replacement rotary that the original knob fits on and that looks right from the outside, and manipulating the lights and speaker outputs gets tricky.

One of the big draws (to me, at least) is the way the amps look, and it was worth it to me to spend more effort to make a replacement look and work alike. It was a challenge.

[R.G.]

Just to put this in perspective and show you a little of how crazy I really am, I've worked out a set of drawings and machining steps to cut Vox knob look-alikes on my Prazi lathe.

[52 Bill]

Just to put this in perspective and show you a little of how crazy I really am, I've worked out a set of drawings and machining steps to cut Vox knob look-alikes on my Prazi lathe.

I've said this before and I'll probably say it again, but you are one sick puppy! I know it takes one to know one.

When I was around 11 years old a friend and I were learning to play guitar. His older brother had a band that went out and bought all TO VOX amps. I was hooked. I must have spent hours just looking at those amps. The grill cloth, the bulldog labels on the speakers, the gold trimmed logo plates, and those knobs. I don't think I played through the amp for more than 20 minutes, but I must have spent 20 hours looking at it. I too will always have a deep and devoted love for these amps.

[R.G.]

I've said this before and I'll probably say it again, but you are one sick puppy! I know it takes one to know one.

When I was around 11 years old a friend and I were learning to play guitar. His older brother had a band that went out and bought all TO VOX amps. I was hooked. I must have spent hours just looking at those amps. The grill cloth, the bulldog labels on the speakers, the gold trimmed logo plates, and those knobs. I don't think I played through the amp for more than 20 minutes, but I must have spent 20 hours looking at it. I too will always have a deep and devoted love for these amps.

We all remember the ... um, attributes, I guess ... of our first girl- or boy-friend.

To further emphasize how sick this sickness is, I also did drawings of the power chassis sheet metal.

I'm very near to being able to build a Beatle from scratch. Actually, I think I'm there, lacking only the time to spend doing it, and the skills to make the grille cloth from scratch. One thing I have in mind a is a working 1/3 scale Beatle model. At one point in my checkered past I was struck by the fact that 4" speakers are exactly 1/3 of the size of 12" speakers, and 1/4" rod or tubing is 1/3 of the 3/4" diameter of the Vox trolley tubing... and my mind was off to the races.

It gets sicker. I was exposed to counted cross-stitching in the past (it's a long story) and I can, in fact, make fake Vox grille cloth on 1/3 scale pretty easily.

I'm doomed, aren't I?

[52 Bill]

I'm doomed, aren't I?

I think it's time for an intervention.

[olddawg]

It amazes me how times and tastes change. I have always enjoyed a good AC30 or AC15. But ..., back in the day we used to buy a complete Vox Super Beatle (Head, Cab, and Frame) for $75 in pristine condition. We would pull the speakers out for wedge monitors and put the rest in the dumpster. You couldn't give away an old SS Vox amp for decades. Now I see them going for ridiculous prices. I guess pedals are better nowadays and people want a clean platform that looks cool.

[drewl]

FYI, I've found those wafer type switches in old test equipment and even more commonly found in router/modem computer switch boxes.
Shhhh..... Don't tell anyone.

[R.G.]

OH, yeah. I'm very familiar with the type. But each switch is usually semicustom. The contacts for theThomas Vox switch are a bit unusual in that there is only one wafer, which does the indicators and speaker switching, and there's an add-on rotary AC switch that goes to on with the first position after "off" and stays there for the next position.

If it was just wafers, we'd have been done with this a long time ago.

[52 Bill]

It amazes me how times and tastes change. I have always enjoyed a good AC30 or AC15. But ..., back in the day we used to buy a complete Vox Super Beatle (Head, Cab, and Frame) for $75 in pristine condition. We would pull the speakers out for wedge monitors and put the rest in the dumpster. You couldn't give away an old SS Vox amp for decades. Now I see them going for ridiculous prices. I guess pedals are better nowadays and people want a clean platform that looks cool.

Aspen Pittman writes about buying up Beatle cabinets to strip them of their Celestions.

I never found any $75 bargains. Maybe that's a West coast thing.