I'm finally getting some of the Thomas Organ Vox technical stuff into print. I need one proofreader to be ruthless about how confusing my writing really is...   

This first booklet is on how to fix your Thomas Vox amp's footpedal or make a new one from scratch.

I vastly prefer someone who has built amps or is at least a part time tech.

In the future this series will flesh out into a series of technical booklets, one per amp, to go with the full-replacement PCBs for the preamp sections of these amps.

Anybody out there that's (1) technically informed (2) interested in Thomas Organ Vox and (3) ruthless in a constructive way?

Quote from: ilyaa on June 07, 2014, 10:57:54 AM
1) does it matter where the switch is? because of the way things were set up in there, it was easier to wire the switch on the neutral side of the transformer primary, not the hot side. that doesnt really matter, right?

Only if you don't mind being a little dead.
The main power switch **must** interrupt the Line, not Neutral side of the incoming power line. So must the fuse.

The incoming power mains Line must go to (1) the fuse holder terminal on the end away from the panel, so that pulling the fuse holder out clears the Line connection first and you can't get shocked from an intact fuse (2) from the middle ring on the fuse holder to the mains power switch so the power switch opening makes the entire chassis not live (4) from the switched side of the mains switch to the power transformer primary. The neutral can go directly to the power transformer primary, but it's better safety practice to use a double pole mains power switch and break both Line and Neutral in case you have a backwards-wired wall socket.

The safety ground must go to a bolt/stud on the metal chassis that is not used for other purposes than grounding. The stud/bolt should have (1) a toothed lock washer (2) the ring terminal you have so thoughtfully crimped onto the ground wire, (3) another toothed lock washer and (4) the nut holding this down. The ground wire should be long enough that a pull strong enough to tear the strain relief (and you must use a strain relief on the line cord) out of the chassis will tear apart the line and neutral wires before the ground is broken.

Quote
2) the amp has a ground switch that was wired to the death caps. can someone give me a run down of why this switch is there and whether or not, with a probably wired 3 prong cord, it could serve any useful purpose? i know plenty of 3 pronged gear still has one for eliminating ground loops and that kind of thing. i disabled it completely, though, because im not sure how i would wire it in a useful way. insight?

The death cap switch is useless for AC power line functions. It was used to "ground" one side or the other of the incoming power line to the metal chassis for RF shielding purposes. Hum and RF were least when the mains line that was really connected to earth ground was connected through the cap to the chassis. But flipping the switch the wrong way let AC from the line mains wire leak directly onto the chassis, and if it leaked too much, shock - or kill - you. It is useless in a properly set up three wire system. Using it to do an ill-conceived "ground lift" is positively dangerous.

Quoteand a side note: when i was using the amp before, with the two pronger, the chassis was hot! meaning, it was shocking me when i touched the amp! and even with the ground switch flipped and the amp not shocking, there was still a differential somewhere because then the mic hooked up to my PA would shock me if I held the bass. thank goodness the new wiring setup fixed that.

And that's what three-wire is for. 

Standout amps from high school days?


Sure. Thomas Vox Super Beatle. Kustom 200 tuck and roll. Acoustic 150 with the six tens cabinet.

Quote from: joecool85 on September 02, 2010, 09:07:15 AM

Quote from: J M Fahey on September 01, 2010, 06:55:27 AM
... in the 40's and 50's it was the opposite.
Fender amplifiers often used 20uF filters !!!
Single ended ones from the 30's only 10uF !!!

Wow have times changed!  I used two 10,000uF caps on my LM3886 project  :lmao:

Times have changed, and capacitors are much smaller now due to improved materials. However, this is kind of misleading. The important thing to look at in thinking about capacitors is energy stored per unit volume.

The energy stored in a capacitor is E = (1/2)*C*V². The power supplies in the 50s were commonly about 400-500V. So for a 20uF cap at 450V, E = 0.5*(20E-6)*(450^²) = 2.025 Joule.

For a 10,000uF cap at 30V, the energy E = 0.5*(10,000E-6)*(30^²) = 4.5 Joule. The energy in the two is only different by 2:1.

The 10,000/35V rated cap will be smaller than the 50s 20uF/500V capacitor, but only by about a factor of two. So in 50 years, we have made capacitors able to store about four times more energy per unit volume.

The measuring stick that the pros use for caps is the product of capacitance and voltage per unit volume. In the definition of capacitance, C = Q/V or the capacitance equals the total charge stored (in Coulombs) divided by the voltage it's stored at. Big capacitance stores a lot of charge at low voltages. Small capacitance has to be packed very tightly by voltage pressure on the electrons to get the same charge; literally, the voltage is cramming them more forcefully into a smaller "bucket". So the charge stored (Q) is the product of C and V, and the "CV product" is the measure of how much storage you get. CV product per volume is the measure of how much charge you can store in what number of cubic centimeters.

Quote from: gbono on June 10, 2010, 02:39:01 PM
Okay, you have the GERBER files, what about the components? Are you asking someone to purchase materials? No problem with an NDA or whatever. IMHO patents are a waste of resources unless you have the $$ to defend them.

Oh, I didn't want someone to build a set for me. I was hoping that someone needed a set for themselves and would be interested in building them, and perhaps debugging if needed with my assistance to get them working. I just want to know they work, and don't have the time at the moment to make them.

There is nothing patentable in this setup, given that the base circuits were released publicly over 40 years ago, and patents run 20 years, if they were even patentable then. And yes, as a general rule, you do not really own *anything* you can't defend.

QuoteI worked for a company that had the laywers and $$ to get what ever they wanted and unless you were able to spend serious money to defend yourself in court you lost and they won - they won a lot.

I was in a similar situation. I just don't want to enable yet another startup company wanting to sell my work. That's happened too many times already.

Where do you live?

Quote from: gbono on June 07, 2010, 03:54:13 PM
Do you have GERBER files for these boards? I might be interested in building and testing your boards or you - do you have all the components or a usable BOM?

Yes, I have - well, OK, can generate instantly - gerber and BOMs. Release of Gerber would require nondisclosure and non-compete agreements enforceable in the jurisdiction where you live, though. I've enabled entirely too many commercial enterprises already to do more of them.

What may not have been obvious in what I said here is that I'd like for someone with some prototyping skills to try these layouts out. As a practical matter, it could be some months before I get to it. What's involved is making the PCBs from either toner transfer or photo positive processes, then populating and testing them. Any interested parties?

Quote from: J M Fahey on June 04, 2010, 05:26:28 AM
I understand that you have drawn them, but not physically made them yet?

That's correct. I did them from a schematic capture in a PCB design suite, so if I drew the schematic correctly, the PCBs are electrically correct. Frankly, physically making them is the most time consuming but least technically challenging part of the process. I probably average a PCB a week, and most of those go into volume production one way or another. Not having physically made them is the smallest portion of the worries about whether they'll work or not; although I would snicker at that statement coming from a beginner, too. 

QuoteWill you upgrade an already existing one or build a new one from scratch?

Well, the whole point of the exercise is to replace the old existing ones. They were good enough for their time, but are a disaster by today's standards. Upgrading existing ones is an exercise in frustration because of the huge amount of point to point wiring inside the old Thomas Vox amps.

Quote from: teemuk on June 04, 2010, 07:00:38 AM
Interesting. As I'm also in the process of designing a "modular" amplifier that spreads many of its functions to different PC boards I really started to worry how well such a scattered setup will actually work because there will be ample amount of common wiring passing from one PC board to another. Since Mr. Murphy will surely have his fingers in the outcome I'm quite worried that the whole thing could end up being pestered by some very nasty ground loops.

It does take some clear thinking about what signal is on what wire.  As a practical matter, I have a fair amount of experience with this kind of thing. The partitioning of the amp onto local functionally oriented boards is almost custom made for star grounding - and star power distribution as well. Both of these are some prescriptive measures for eliminating ground and power loops. And with the vast majority of wires being short ones, the chances of success are much improved. The nice thing about isolating the signal routing to a few places, perhaps one or two from each board, is that you can use shielded wire and have it mean something.

But no, you can't wire it in a random way and have it come out well.

I'll get a lot of flack for this in some circles, but some of the best SS amps ever made were the Thomas Organ Vox amps. The "big head" set, Viscount, Buckingham, Royal Guardsman and Beatle amps shared the same internal circuits, and had a lot of advanced features for the time, as well as a really decent sound. I view them as midway between what the world thinks of SS amps in general and highly regarded tube amps; better, certainly, than some tube amps.

But I'm a Vox nut.

I recently finished PCB layouts for the Vox line. These replacements are the same circuit, but cut up into bite sized pieces to minimize the internal wiring. For instance: for the brilliant channel, the PCB is about 1.5" by 3.5", and is designed to mount on the back of the volume, bass, and treble controls with adhesive standoffs. All the wires to the three pots, the two jacks, and the switch are local, about 2-3" long and right off the PCB. The only wires outside the "local group" are the power, ground, output signal and remote switch line for the MRB. Ditto the normal and bass channels. The other PCBs to finish this out are the tremolo PCB, Reverb PCB, and mixer/limiter. All are designed to be physically placed right on top of where all their controls are to minimize wire lengths, the bane of the TV amps. I also redid the power amp board, fits the same position as the original, but is a fresh layout and puts the driver transistor on a heat sink on the PCB.

I mention this because someone needs to make a test set of boards to verify them...

Unfortunately, that may not be a simple problem.

I'm guessing that the procedure went like this:
- turn up the amp medium loud
- turn on the signal generator
- wobble around the signal generator until the glass seemed to be vibrating
- then turn up the amp as loud as it would go and wait for the glass to shatter
- when that didn't happen, turn up the software tone generator to as loud as it would go and wait for the glass to shatter.

That's also a medium-good procedure to kill small amps, btw.

There's no good way to guess at the cause from this amount of information. I find it odd that they are all just humming loudly, same symptom, neither of them dead silent or with popped fuses.

Take your voltmeter and measure for DC voltage on the signal output of the amps. I'm guessing that there will be a significant amount of DC there, indicating blown output devices. If the outputs are within 100mV of ground, it's likely the front ends that are dead.

Can't tell more than that without more info.

Quote from: bvh123 on October 19, 2009, 10:06:20 PM
I can't seem to find what I need. is there any places you  would suggest looking for one.I tried searching on google and found a few things but nothing with the 15+15. thanks.

I didn't think you would find a replacement with the 15V windings. That's why I posted:

QuoteYou may be able to replace the transformer with *two* generic transformers; one a 68Vct 2-3A device for the power amp and one a 28? 30Vct? small toroid for the preamp, which is almost certainly a set of opamps. There was plenty of room in the chassis of the Rogue for trickery like this. I'm guessing there may be in that one.

You could put in this one for instance:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=237-1326-ND
and bolt it right to the bottom of the chassis. You parallel the primaries with the existing transformer primary, and connect the secondaries where the old transformer's 15v/15v secondaries went. It's another $20, but it's better than junking the amp, right?

Notice that you're contemplating doing things that require messing with rewiring AC power line circuitry. This is major dangerous to yourself and others if you don't already know what you're doing and how to do it safely. Do not try this if you don't already know how to do it right. It's not worth dying for - or having someone else die because you didn't understand something.

Looking at the pictures and description of that amp on line, I think it's the successor model to the Rogue combo I scored for $40. The pictures sure imply what I found, and the power transformer nominal voltages look right.

You may be able to replace the transformer with *two* generic transformers; one a 68Vct 2-3A device for the power amp and one a 28? 30Vct? small toroid for the preamp, which is almost certainly a set of opamps. There was plenty of room in the chassis of the Rogue for trickery like this. I'm guessing there may be in that one.

A Really Good trick is to put in a 68Vct toroidal transformer for the main. Much lower hum emission.

Or, see my post on a "Rogue" 2x12 100W SS combo for $40 for a "soul transplant".

To the original poster: Please don't take this the wrong way, but insisting on building your own SS amp with no more electronics background than you show may be dangerous. Spend some time learning, build some smaller devices where the costs of mistakes are smaller before you jump into building something from the ground up.

It took me a few years, but I do understand how those amps work now.

Compare the power amp schematic to that of the Thomas Vox Super Beatle. Notice any similarities? They could almost be the same amp, with only minor differences. The Acoustic is a cleaned-up version, but is very much like the Beatle.

The red-hot emitter resistor indicates that the thing is trying to drop the entire power supply voltage across one emitter resistor, with poor results.

The secret to fixing it is probably divide and conquer. First off, remove any loading significant loading from the amp. This amp will not go crazy if unloaded, like many tube amps will. I would remove the output capacitor and also open up the driver transformer primary. It will help a lot if you have a light-bulb limiter like I'm always preaching about. With the output connection open and no signal to the output stage, the output stage is going to try to simply sit at a DC level. The voltage dividers set the bias for the output devices: 400 ohms and 6.8 ohms just barely trickle the bases on. If the 6.8 ohm resistor goes open, the output devices are turned full on and death ensues as all of the base current from the divider goes into the bases. If the 500uF cap shorts, that side is permanently underbiased and bad crossover distortion results. Check the values of each part - there really aren't that many - and also the continuity of the wiring/pcb. Bring the power up slowly with the primary of the driver transistor open and watch the junction that would feed C311. This point should rise with the power supply, following approximately 1/2 of the total power supply voltage. If it doesn't, something is wrong with the 400/6.8 dividers, the driver secondaries, the output transistors or the wiring.

Once you get the output stage producing about 37Vdc where C111 would connect, you can start debugging the input/driver side. Reconnect the primary and disconnect both secondaries, being sure to note how they're wired so you can get them back in the right places. The driver can put out a few watts from one of the secondaries, so you can put 8 ohms across one of the disconnected secondaries and get the input side working.

If it were mine, I would arbitrarily replace every electrolytic cap in the amp. I have sometimes been called a miracle worker for getting Thomas Vox amps running, when all I did was to replace them all. They're old enough that if a cap doesn't fail today, it might tomorrow.

There is one thing you always need to keep in sight when you're thinking about how to get tube sounds out of solid state gear, or nicely compressed and/or distorted and/or whatever that you think a good tube amp does.

What we mostly listen to guitarists on is a solid state amp. It's the one in our stereos, or the one in the group's PA system.

Whatever mystical system of tubes, power supplies, transformers, exotic wiring, turret boards, point to point wiring, or voodoo-based deals with the devil that has gone into making some magic guitar sound, it is by far more common to listen to that through a solid state PA system at a concert or through a stereo amp from the recorded CD or other medium.

So it is clearly **possible** that a solid state amp can sound like a tube amp if it's fed the right signal and drives that signal through nice, linear speakers. The whole trick is what to feed that SS power amp at its input so that its response times the response of the speakers has the right tonal quality and you get to hold and play the guitar at the front end that ends up in the signal to the power amp. The entire tube vs solid state argument can be reduced to this.