Thomas Vox Repair Boards

[R.G.]

I have recently finished testing repair/replacement boards for the preamp of Thomas Vox "big head" amps: the Beatle, Guardsman, Buckingham, and Viscount.

These are the original circuits, on a same-size board that fits the original mountings, but with the wiring made sane by placing all the wire connections on the edge next to the controls so they can be serviced. The preliminary documentation is on the index page at geofex.com.

Installation consists of clipping off all the old wires at the controls / switches / etc. and removing the old board, then soldering in the new (short, direct, not-in-laced-bundles) wires to the controls.

This worked well enough that I'm putting the finishing touches on a similar board for the Cambridge/Berkeley2/Pacemaker/Pathfinder and Berkeley 3.

The idea here is to keep Thomas Vox amps out of the dumpster.

[M Fowler]

I'm never going inside another one of these amps.

[R.G.]

I'm never going inside another one of these amps.

That's the received wisdom of nearly everyone who has had to get one running after bending the board up to get at the solder side, breaking off another wire or two.

This repair is actually kind of cathartic after that experience. The repair process is:
(1) Open up the preamp chassis.
(2) Clip off all the wires at the non-PCB end.
(3) Toss the PCB.
(4) Solder the wires from the new board.

All wires run from the board to the control, and are in order to the control down the length of the PCB, and no wires have to cross. All pot hot/wiper/cold wires are in the same relative order, and the wire-to-control is (well, should be) made with about 3" of #24 stranded.

All components are through-hole, and bending the board up to get at the solder side is non-damaging to the wires. And the board works in the bent-up position.

I was taking revenge on whoever laid out the originals in that ugly, nasty way!

[Stevem]

Thanks for the info R G!
Hey, I have a 66 reverb twin and I want to change the outputs to silicon types, are there any value bias resistors you recommend starting off with?

[R.G.]

Thanks for the info R G!
Hey, I have a 66 reverb twin and I want to change the outputs to silicon types, are there any value bias resistors you recommend starting off with?

What model number?

Edit - I should have also noted:
There is a fair amount of info on repair/refit/refurbing Thomas Vox at my web site, geofex.com and the general process is outlined there. But if you'll tell me the model number, I'll run the calculations for you.

[martin manning]

So you are that R.G. ... Welcome to TAG; the level of expertise around here just jumped!

[Teleguy61]

Wow yes!
Welcome R.G.

[xtian]

No, kidding! Very pleased to have you here, RG!

[dcribbs1412]

Thanks RG
I was able to save a Buckingham/Head and Cab from the dumpster.
Although I would have liked to have kept it stock, I was overwhelmed by the solid-state PCB.
It now has a new lease on life with a vox style 18 watt tube circuit.
http://ampgarage.com/forum/viewtopic.ph ... ox&start=0
I will keep your replacement board in mind for the future.
+1 on what Martin said
Very much appreciate your time and work.

D

[Stevem]

Its the model 1081 R G!

[Blindog]

Welcome to the Garage RG!

Those boards are very cool, nice work!

Mark

[R.G.]

Its the model 1081 R G!

Change the 3.3 ohm bias resistors to 12 or 13 ohms nominal. This gets you to about the same "on-ness" that Thomas used for the outputs of the Berkeley 3 or Guardsman silicon outputs. Those are the closest standard values to the value I calculated, 12. 7 ohms.

There is a tradeoff here, between biasing too cold and getting crossover distortion, and biasing too hot and getting runaway temperatures. I'd go with 13 ohms, then run it and keep feeling of the heatsink plate. If it never gets too hot to keep the tip of your index finger on, you're fine. (most humans won't hold their fingertip on something over 130F, and that's safe for the transistors if they're mounted correctly)

[Stevem]

Thanks a bunch R G!
I will report back for Ya.
If you have any interest I own a 1966 Kustom 200 and have the schematic for it and these Amps came about in 1965 and use a PI / driver transformer like the SS Vox amps!
I have the resistance specs on the transformer also.
If you want yo see the schematic and some internal photos then pm me your email and I will fire them off to Ya,or if you go to the vintagekustom site you can find the schematic in the techical section, thanks again!

[R.G.]

Thanks, Steve. Yes, I'd like to see the info.

I've tracked down what I *think* is the right way to design that transformer and output stage, but it took me nearly 20 years of occasional when-I-thought-about-it looking. And I'm sure that by no means do I have the full story.

I have a recipe for a Vox driver transformer that I will wind up and try when I get enough time to set up a winder and do it right. Then I'll start finding my mistakes.

I'll PM you.

[rooster]

R.G., yes, welcome indeed! As to your offerings, I'm wondering if you could PM me your prices for the big box boards? And then I will also ask, are you anticipating that folks will pull the old parts and then stuff the new boards..., or are you thinking that folks will buy new parts and stuff the new boards with new parts? A combination of both new and old parts perhaps? Thanks.