Just got a Vox Supreme from the original owner

[Mick Bailey]

I had a call a week ago from a long-standing customer who was moving house. He said "You know that Vox Supreme - do you want it?". I immediately said yes and it's now mine; 1970, one owner from new (he picked it up directly from the factory) and with all the original tags with serial numbers matched to the amp. There can't be many surviving with that provenance. I had it in for a very minor issue about 15 years ago and was struck by how good an amp it was. It now switches on but hums, so some work to do as it wasn't used at all in those intervening 15 years.

It came with the knurled knobs and fittings to attach to the tilt-back cabinet, but sadly the original handle and fittings have been lost (the handle broke decades ago). Oddly, the cabinet is drilled for both the traditional strap handle as well as a later strap handle. I understand that some transition amps had both sets of drillings/markings, but need to establish if this is one.

Hopefully the reverb still works, as this is one area that's tricky to restore, being based on gramophone cartridges as transducers. I'm hoping that there isn't too much wrong with it as I recall it being quite difficult to work on with the fragile wiring and PCB.

[R.G.]

Congratulations, Mick! Great amp there. I've worked on a Defiant and a Conqueror, as well as doing a preamp-replacement PCB for the series, so I may have answers to a few questions if you get stuck.

Best advice if you want to actually use the amp to make sound - replace every electrolytic cap in it, then see if it works. 15 years without bias voltage is pretty damaging to electros. The reverb uses a step-UP transformer to drive the piezoelectric phono cartridge with a high voltage, which I found odd. Last time I looked, replacement phono cartridges are still available, but don't go there unless you're forced to.

These things are not nearly as hard to work on as the Thomas Organ Vox amps from the USA. Oddly, though, certain parts of the circuitry are near-relicas. This had to have been an echo of the time that UK Vox was cooperating with Thomas Organ in the US.

[Mick Bailey]

The last time I heard it the reverb worked but was weak and I expect by now it may not work at all. Its had a 3 pin IEC socket fitted but the original Bulgin plug and socket are with the amp. Also it has the foot switch. I'll take some pictures.

I'm hoping there's nothing too badly wrong with it - I haven't yet tried it myself and was going off what the original owner told me. I want to give it a good look over and assess the electronics before powering it again.

[R.G.]

Good luck with it!!!

[J M Fahey]

Sadly those DIY reverb tanks were weak even when new.so don´t expect much even if properly restored.

Being out of production for decades guess NOS cartridge prices will be all over the place.

[R.G.]

They run about US$40 to $50 fromaudio replacement places not trying to make a killing. The sky is the limit if they think you're working on a guitar amp.

I once converted a Defiant (Supreme's little brother) to a regular spring style tank insert. Worked.

[J M Fahey]

They run about US$40 to $50 fromaudio replacement places not trying to make a killing. The sky is the limit if they think you're working on a guitar amp.

I once converted a Defiant (Supreme's little brother) to a regular spring style tank insert. Worked.

That´s the way to go.
You need to buffer drive circuit because any coil driven tank will be MUCH lower than any crystal/Piezo one; recovery side will work reasonably well.

Or plain ditch the Reverb section and use signal out/in to drive and recover from any modern board, even a digital Belton "tank"

[R.G.]

I left the transformer in for the built in driver's load, and shunt-fed that to the reverb tank coil through a capacitor. Worked.

I agree with the Beltone tank. I did a PCB aimed at holding a BTDR module and having the opamps and power supply conditioning to bolt inside a gutted reverb shell, so you could replace any shell/tank guts with the module. Worked, and is adaptable for the common input and output impedances for tanks. Several people have used these in reverb replacements for home-grown Supreme/Conqueror/Defiant or UL700-series clones, and think well of it.

[Mick Bailey]

I had a Bird Golden Eagle combo in for restoration a few years ago and it used a similar arrangement to the Vox. I gave up trying to get hold of transducers and converted it to a Fender-type arrangement. Although this worked much better than the original setup, it still bothered me that I never got the original tray working, though it would have been more than the owner wanted to pay to find those 1958 cartridges.

I'd be happy to fit a Belton unit and leave the amp in a state where some future owner could refit the original tray if they really wanted to. I've only ever heard this type of tray at the end of its life and don't know what it would have sounded like when the amp was brand-new. Without a compelling reason to chase down the original cartridges I'd opt for something more effective and reliable.

[R.G.]

PM me Mick

[Mick Bailey]

I've had a good run with the amp today. Opening it up refreshed my memory on how well laid out the amp is. I think I'd confused it with some of the other SS Vox models I've worked on and this one is fairly straightforward by comparison.
The main problem was the preamp coupling cap C60 being open. After replacement and checking ESR for the rest of the electrolytics the amp came to life, but weak reverb and no distortion. The reverb came back strongly after rotating the blend control through its full rotation a few times and the absent distortion was down to the footswitch cable being wired so that only the trem switch worked and the reverb was on all the time, plus no distortion. Just three wire swaps in the pedal and all was good .

Overall I'm happy with the sound. The reverb is fine, and at full depth is quite splashy but not like a Fender or Ampeg reverb. I think in part this is down to the single spring and how it's driven. The amp is in remarkable condition inside and the additional holes where a Marshall-type strap was fitted can be dealt with and sometimes I've achieved an invisible repair.

There are a few things to attend to:

1. The distortion reduces the volume quite a bit and needs investigation.
2. Both volume pots are crackly and the reverb blend pot could do with cleaning. I'm not sure whether the plastic covers are removable without damage, as they may have gone brittle by now.
3. There's a little more background hum than I'd like.
4. Viewed from the front, the RH power transistors run warm, whilst the LH side are stone cold when idle - could need rebiasing.
5. The ceramic resistors that sit behind the power amp tag boards get quite hot on both sides. As yet I haven't taken any measurements to determine if this is normal.

[Mick Bailey]

Well, the bias was quite out of spec. It should be set to 20mA each side, but the side that was running hottest was 125mA and the other 14mA. This was causing a DC offset of just over 5v on the speaker output. Setting to 20mA now gives just 2mV DC on the output. The idle hum has dropped considerably, though the hiss has increased, maybe due to the original offset damping HF response at low levels.

Two remaining problems; crackling controls - more information or experiment needed to get the shells off. The other is the considerably lower volume with the distortion engaged, though I don't know if this is a characteristic or a fault.

Overall though it's got an excellent sound. The three position mid control is less useful than I remember, giving a resonant and nasal fixed-wah type of effect.

[Mick Bailey]

I'm looking at why the gain of this circuit is lowered considerably when SW3 is closed (distortion switched in). After listening to examples of this amp, I'm certain that there's a fault and it's not just a characteristic.

Looking at TR4, TR5 and TR7 they're strapped as diodes, though I'm not clear why they would not have used Ge diodes rather than transistors. TR4 and TR7 just look to behave as back-to-back clippers. I'm pondering though, what's the purpose of C10/TR5?

The variable gain increase needed for the distortion is provided by switching in VR1 (5K Ohm). I'm thinking this is possibly where the fault lies, as the volume further decreases as the distortion control is increased, but I'd like to understand the overall circuit a bit more.

I've measured the gain and leakage of TR4 TR5 and TR7. Leakage is 93uA, 310uA and 114uA, though I'm not sure whether leakage and hFE are too important as they're not used for amplification.

[Helmholtz]

Looking at TR4, TR5 and TR7 they're strapped as diodes, though I'm not clear why they would not have used Ge diodes rather than transistors. TR4 and TR7 just look to behave as back-to-back clippers. I'm pondering though, what's the purpose of C10/TR5?

A first attempt:

I don't know what SW3 does, but assuming point 6 is grounded, then TR4//TR7 are the upper part and TR5//TR6 the lower part of an almost symmetrical clipper, essentially one anti-parallel diode pair stacked on another one.
C10 shunts the upper pair for HF and will make clipping of upper half a bit softer.
It is interesting that (only) the lower pair is DC biased which changes threshold(s).
Also the transistors TR4/5/7 are reverse-conducting by the base-collector diode, because when base and emitter are shorted there shouldn't be forward conduction.

[Mick Bailey]

SW3 open = clean, closed = distortion.

SW3 Point 6 isn't grounded and is connected to +ve via R14 with SW3 closed. I checked back to the original Vox drawings and the redrawn circuit is accurate. TR6 shunts TR5 when SW3 is closed and connects TR4/TR7 (and C10) across the signal path as a clipping pair, but why would clipping diodes be present when the distortion is off? As I see it, TR5 is being switched in or out of circuit but there's always clipping whether SW3 is on or off.