The Vox Supreme
(Have a look at the Vox Conqueror page on this site
to compare the two)
1) Another well used Vox, this time a 1971 Supreme by Vox Sound. All solid state, it has two channels with various combinations of reverb, tremelo, top boost, mid range boost and distortion. Output power is 100watts rms into 15ohms. It was designed to run into 15/16ohms and, although the output stage will actually work into a lower impedance, there's no guarantee how reliable it would be long term so speaker options are a bit limited.
The basic preamp is shared by the lower powered Defiant and Conqueror and can be found in at least two different versions, the main change being to the MRB circuit. Power comes from four 2N3055 transistors with virtually no protection. Short the output cable for a while or use the wrong speakers and it can all go very quiet.
For schematics and layouts of the individual preamp pcbs CLICK HERE
This spares-or-repair head brought back a few memories. I owned one before with it's proper 4x12 Beatle style cabinet and it was a nice sounding, versatile rig. Unfortunately you needed a big vehicle and arms like Mr T to move it around.
2) The handle is unusual but original. There are four punch marks in the vinyl next to it which don't go right through. They line up exactly with the standard plastic Vox handle brackets so the factory seem to have been in the middle of a type change. This handle is much stronger than the plastic ones and apart from being very rusty it's still serviceable. It looks horrible though so it's got to go.
The top control panel is suffering from terminal corrosion and ideally needs to be replaced. They have to be specially made so that'll have to wait.
3) In later versions, like this one, you can see Vox trying to keep down the cost. Early versions had a very attractive back panel covered in diamond cloth to match the front, but then you can't usually see the back on stage so why bother. This thing was expensive enough already.
The standby circuitry has disappeared by now. Notice that there's only one indicator lamp on the front panel.
The earlier double row of metal ventilation grills has been replaced by a single row of plastic mouldings and the knobs, previously solid metal, are now composites with much less metal content.
However, cost cutting aside, the circuit is virtually identical to the earlier version and the sound is the same. From the audience you wouldn't know the difference and this is still a rare, desirable amplifier.
4) For repairs the head can be split into three parts, the case, the power amp and the preamp.
Use a nut spinner or small socket set to remove the four nuts holding the preamp to the front panel. There should also be two wood screws, but they'll probably be missing.
The case was completely stripped and scrubbed with warm water and washing up liquid. Before that the front trim panel was carefully pushed out. It's held in with glue and staples but it's not too difficult to get off.
With the old cloth and piping removed you can see why the logo was crooked. Someone has drilled the holes for the locating pins at an odd angle (must have been a Friday afternoon). It's difficult to see in the photograph but I've plugged and redrilled the right hand hole and the logo is now straight (nearly).
The new grill cloth is stuck on with 3M Scotch-Weld Craft Mount adhesive. This is great stuff for the job. Spray it onto the wood initially and centre up the diamonds top to bottom and side to side. The glue stays wet long enough to get it right and you don't need to bother with rolling the cloth round sticks and so forth. At the edges, spray glue onto the back of the cloth before folding it over and cutting away the excess material at the corners. The white piping then goes back on with a label tacker (stapler). The odd see-through effect in the photo is caused by the camera flash.
5) The power amp is a stand-alone chassis that contains the power supply. It connects to the preamp via a detachable multicore cable and can be powered up and tested on it's own.
This one has suffered from such severe corrosion that it's had to be completely stripped back and replated. Rubbing down very old plating is a bit dodgy because it may contain cadmium, but this one was zinc and yellow so it just needed gloves and a dust mask.
Again, there is no assembly drawing available so it was a case of documenting everything before stripping it.
The printed metal back panel appears to be self adhesive and peeled off after a struggle.
6) These paxolin boards carry the bias chain for the output transistors. The burning looks dramatic but it may have been long term rather than a sudden failure. The tag boards are the larger size used in the power section of AC50s. Very difficult to get hold of so I've made two new boards from glass fibre. (Edit: Found some proper boards - see section 21 below)
New electrolytics in this position will be Multicomps rated at 105degC.
7) Pre-amp
8) Progress so far.
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The case is finished.
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Preamp components are all documented and stripped out.
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The power section metalwork is finished, but the preamp chassis is still being cleaned up before re-plating.
9) Control Knobs
The grub screws securing the knobs were welded solid by corrosion (aluminium bodies and steel screws). After bending or breaking half a dozon new allen keys I had to drill out the middles of the screws and wind the knobs off the pot shafts with brute force (don't try it if your nervous or haven't got some spare pots). Don't drill out the whole width of the screws or the hole will be too big to rework later.
A pillar drill chuck is too big to get close to the panel so I used a small hand-held battery drill with a 2.5mm bit. Even that was a bit too big and took over an hour and a lot of swearing. I've since successfully re-tapped a couple of the knobs to M3 as an experiment and fitted new grub screws, so we're back in business when the rest get done. A taper tap actually cut out the remains of the old screw without any other work. The thread was just right without running a plug tap through it.
The photograph on the left shows the three stages of repair.
Left. Original (this one was OK).
Centre. Corroded grub screw drilled out.
Right. Hole retapped and fitted with new grub screw.
These later knobs have a very flimsy alumunium skirt. To hold them in a vice, use soft jaws and a spacer. This one's an M8 nut.
10) Power amp reassembly.
The other consideration is the cost. The plating bill should be around £50. Add about £10 for materials to prep the metalwork and it starts to add up.
In this case I think it was worth it because there's no point in rewiring a heap of rust.
The rest of the metalwork has now been re-plated in zinc and yellow and I've made a start on re-assembling the power stage components. This is a dry run to find replacement screws and so on since most of the old ones are too badly corroded to use again.
Re-plating a Supreme chassis isn't for the faint hearted. You need to be quite aggressive removing every trace of rust or it'll be back in a few weeks. There are hours of work rubbing down and re-finishing some very awkward shapes with welded in board supports and you should be prepared to lose some skin on the sharp bits.
11) Thermal protection
The only protection device in the system is this mechanical cut-out mounted on one of the heatsinks.
The details on the case are:
Otter
G Ministat
Buxton England
Patented
5/250
05096U
It's a G Series Thermal Cutout made by Otter Controls Ltd. Otter are still very much in business and I'll try and find out the temperature spec and a modern replacement. Meanwhile I've checked that it works with a hot air gun. It needs to be done gently or the case will be damaged before the heat gets through to the trip.
This is it mounted on the left hand heatsink. The sockets are being rewired before the mounting plate is fitted.
12) Power chassis assembled.
New boards assembled and new wiring loomed in. Those RS 10R wire-wound pots aren't original, they're not sensitive enough, they're in the way and they'll have to go.
13) Mains Circuit.
I did some resistance measurements around the chassis while I was buzzing out the new wiring and got some interesting results. A meter across the live and neutral pins of the mains input read over 100ohms. After cleaning the voltage selector and the mains switch this came down to about 4ohms. The switch on this particular amp is screwed together and very easy to take apart. The voltage selector is rivetted.
Mains voltage would probably punch through the corrosion but it could still be noisy. Best to clean it up.
The power input is via a Belling Lee L1722A/S receptacle and a mating, cable mounted L1722A/P plug. Both are available from CPC/Farnell in the UK. Earlier versions may be fitted with the more familiar Bulgin SA1861 chassis inlet used on most vintage British amps.
14) Vox Supreme Power Amplifier Schematic MkIII.
This is the later version without a standby switch.
Notice that you can't just connect a grounded scope across the speaker output or you'll short out the feedback resistor R122. Options are to lift the ground connection from the scope (not recommended) or to monitor the output between the jack socket tip and the chassis. The actual rms value can be measured straight across the load with a battery powered meter.
The most obvious difference between this and the lower powered Conqueror and Defiant is the number of output devices. With rail voltages of +/- 70V, single 2N3055s would be outside their maximum VCE rating and are therefore doubled up in series.
After pestering several people cleverer than myself (thanks Wally), this is how I believe the circuit operates. If you know better please feel free to argue.
Consider the top (positive) half of the chain, TR16, 18 and 19 and Ignore the low value R109 and VR13 for the moment.
With no signal there will be 70v across TR18 and TR19 ( the speaker outlet will be at 0v ). DC current through the chain of bias resistors will set the junction of R106 and R107 (and therefore the base of TR16) to half of this value. Allowing for two base - emitter voltage drops (TR16 and TR18) the junction of TR18 emitter and TR19 collector will also be at this halfway point, neatly sharing the supply voltage exactly between the two 2N3055s.
The speaker output point is tied to the centre of the bias resistor chain and so the base of TR16 will track the output voltage. At DC and low frequencies it will always be at half the voltage across R105 to R109. As the signal frequency rises, however, R105 and R108 are bypassed by C64 and C67. The ac voltage seen by TR16 is now proportional to the difference between R106 and R107.
TR16 forms a Darlington Pair with TR18. R115 reduces the Darlington gain by bypassing the second base-emitter junction while R119 provides a path for TR18 emitter current as TR19 is driven towards cutoff on a negative going cycle. C66 is included to roll off the high frequency response and maintain stability.
R118 provides stabilisation by reducing the base-emitter voltage of TR19 as the current increases with temperature.
VR13 sets the quiescent current by adjusting the base current supplied to TR19 via the secondary winding of the driver transformer.
The bottom (negative) section works in the same way but with an anti-phase drive signal.
Although the quiescent current IQ is stated as 20mA per side, this may need to be unbalanced slightly to set the DC offset at the speaker terminal to zero. The other adjustment available is VR12. Although the factory schematics refer to it as 'gain' it actually sets the operating point of TR15. It should be adjusted to give symmetrical clipping at the speaker output at full power.
15) Vox Supreme Power Amplifier Schematic MkI.
The drawing below shows the extra wiring for the standby lamp fitted to the earlier versions.
16) Vox Supreme Preamplifier Schematic MkIII. (left click to enlarge)
This is a cleaned up version of the MkIII preamp schematic. Note the difference in the MRB circuit between this and the MkI.
Below: The wiring layout for the MkIII preamp.
17) Vox Supreme Preamplifier Schematic MkI. (left click to enlarge)
This is a first shot at a MkI. (Update when I get a minute). Notice the different MRB and two resistors missing from the distortion circuit.
June 2020: Issue 4 has a correction to the MRB section.
Below: The MkI wiring layout. The component numbers are from the MkIII drawing and just for convenience.
18) Just made a start on re-assembling the preamp.
Left is the Supreme MkIII circuit board arrangement.
Meanwhile, thanks to Paul Johnson for sending in a photograph of the missing Vox logo. (Sorry about the delay Paul).
Metalwork drawings and screen print artwork for both front panels are done but replacements will have to wait until the lottery comes up.
19) Pots
Later Vox amps use two stamped spacers to stop the pots rotating in the panel (1st picture). They need to be assembled so that the raised pips of the rear spacer fit into the holes in the front one. The pot lug then fits into one of four slots, chosen to get the terminals in the right place. The front spacer locates in a pre-drilled hole in the chassis (fourth picture).
It seems a bit complicated, but makes sense in a production environment. All the chassis locating holes are identical and the pots can be positively located in four different positions with no chance of rotating while the nuts are fitted. It also guarantees that the lugs wont dig into the back of the overlay panel.
20) Reverb
The preamp boards now have a new set of electrolytics and I've done a quick re-assembly job to check it out. Apart from some of the pots feeling a bit rough (don't we all), the only major problem is the reverb unit. It doesn't work and the problem is down to the transducer at the receiver end (the photograph is the drive end just to be awkward, but they're both the same).
Although they look specially made, the driver and receiver in the spring unit are standard Sonotone 2T ceramic phono cartridges.
A great web site owned by Roger Russell (ex Sonotone engineer among other things) covers the history of the Sonotone Corporation and it's products.
(Edit Oct 2023: Roger passed away in 2021 but the site is still available at the moment. It doesn't have an SSL certificate so copy & paste these links if you want to)
http://www.roger-russell.com/sonopg/sonopg.htm
The rest of Roger's site is well worth a look.
The 2T comes with all the mounting metalwork rivetted on and finding a replacement isn't easy. Output is about 1volt with a load of 2-3 megohms. Drop-in replacements include the Astatic 751 and Pfanstiehl P-136. All three are available from specialist dealers on the internet if you can arrange a second mortgage.
Incidentally, one lead of the driver cartridge is connected to the metalwork of the spring tray and needs to be wired to the ground side of the drive transformer.
Update:
Just found an unused 2T for a reasonable price.
The first picture above shows the cartridge as it would be used normally with it's flip-over stylus fitted. The metal part on the left is the clip-on needle protector.
In the second picture the stylus has been removed, showing the cantilever which it rests on.
If your absolutely certain that the old cartridge is dead, you can just unsolder the end of the spring and pull the cantilever out still glued to the wire. The glue can be pared off with a scalpel if the assembly is held over a straight edge (turn the blade the other way round if you want to keep your fingers).
With the spring hooked onto the new cartridge, use a very small amount of Araldite Rapid to secure it to the cantilever.
The hook was originally soldered on, but it's difficult to tin this old metal without using acid flux or a lot of heat and so I've used Araldite here as well.
With the tank out of the chassis, the cartridge can be tested by plucking the springs (gently) and looking at the output on a scope. It doesn't matter if it's the send or return end, they're identical. This ones good so we're back in business.
21) Bias Chain Overheating.
I haven't got time to write this part up at the moment but the power section is up and running and making a good 100watts rms into 15ohms at less than 1% distortion. The electrolytics that weren't replaced were reformed before switch on and then the whole chassis was powered up slowly through a variac over about an hour.
Now that the power section has been on for a while, the old problem of circuit board burning has reappeared. The output transistor bias chain runs hot and, while the components are within their rating, they're too close to the board to dissipate the heat. The photographs below show my attempt to cure this. On the left are the marks starting to appear next to R111 and R112 ( R106 and R107 are the same on the other board). The other photograph shows the slots cut into the circuit board to increase the air flow. These were done with the boards stripped out of the chassis (again) but with the components in place, so the caps look a bit the worse for wear. They'll be replaced at the end of the project. It's not an ideal fix because the caps are subject to radiated heat.
Decided that this didn't look right so tried a different approach. The existing power resistors are equivalent to the Welwyn W22 series. They run at extremely high temperatures, so it's not surprising that things start to burn. Replacing them with W23 series brings the temperature down by almost 50degC at 5watts. (That's assuming there's somewhere for the heat to go - the big problem with Supremes).
It seems to work. Ditched the fibreglass and found some NOS paxolin tag boards from RS - not made any more but available on ebay now and again. Look for 18way boards measuring about 172 x 54mm with open sided tags. The wirewound pots are mounted through the boards to stop them waving around.
18 way tag board. Note the open sided, wrap around connections. Not to be confused with smaller, 24way, 155 x 38mm boards with closed, through the hole tags.
Those are a short version of the ones used in an AC30.
Welwyn W Series temperature characteristics
Incidentally, the 18 way boards are suitable for AC50s.
If I was lucky enough to have the genuine foot switch this is what it would look like.
The colours probably varied over time, but the circuit is always the same.
23) Back together.
This is the amp back in one piece. It's still waiting for the Supreme logo to be glued on and the control panels are still well worn.
