CHAMP ELECTRONICS -" THE VALVE AMP HOSPITAL"

NOTTINGHAM, ENGLAND

 

OVERHAUL OF A STENTOR "SUPER 12" COMBO AMPLIFIER

 


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OK, CAN WE PLEASE CALL THIS ONE A "RAT'S NEST"!

It was brought to me by a guy named Steve from North Yorkshire, here in the UK. This Stentor “suit case” was one dreadful piece of junk!! It was apparently designed by a guy who worked at Automation Telephones, London, England and has a date stamped inside of 16th June 1960. I have very vague recollection of the brand name “Stentor” but that’s about as far as it goes!! This amplifier has to rank as one of the worst I have ever seen……especially as a “branded amp”! The designer has chosen simple tag (group) boards as his method of use, and then simply cut holes in these said tag boards for all the tubes’ bases. He basically continued on from there, utilising whatever wiring pattern emerges from thereon! Though this amp is far from “rocket science”, it nevertheless has been pretty thoughtlessly designed! Personally I wouldn't have given this amp house-room but…….Steve wanted to go ahead with getting it back to rights! OK, so now down to “brass tacks” and reality.

It is a push-pull output stage with a pair of EL84’s, about 12 watts. The phase splitter is pretty bog-standard too with an ECC83, but…….now here is where things take a dive! The tremolo section uses both sections of an ECC83..….Fair enough, but then half of an ECC81 (12AT7) is used for interconnection of the tremolo section to the main amplifier section! This now means one and a half tubes are being used for just the tremolo alone……very much an “overkill” for a tremolo circuit! As the designer seemed to have run out of room on the main panel and yet, still needed a twin input pre-amp section, it seems that he decided to add a separate piece of board and an extra ECC83 tube for this purpose; sticking out and soldered to the existing board at a 45 degree angle. What a heap of crap this thing was!

OK, so as in all my repairs etc I decided to start with the power supply, then the output and splitter stages, followed by the pre-amp, and finally the tremolo stages. That is the order in which I always do my repairs and investigations on amps. On power-up, the power supply started to smoke! In the end, this turned out to be a shorting output transformer on the primary winding to ground. This was corrected (rewound) first before any further investigations could be carried out. With the output transformer back in place, a glimmer of life started to appear at the output….though dreadful! Switching it back off with a view to replacing all the dead and leaky Hunts caps eventually saw a marked improvement. It was doing a good 10 watts now, but with a very uneven clip, so this necesitated a further replacement of virtually all the dead, duff or “way out of tolerance” 20% resistors. (As far as the amp's layout is concerned, it looked like someone had gotten a handful of resistors and capacitors, then thrown them at the amp and wherever they stuck was fine!) Anyway, changing all the caps and resistors was a long and tedious job. In carrying this out, I have made things a little neater, but other than a complete rebuild this is about as good as it gets.

Powering back up again now saw a perfect, just under 12 watts RMS…..good.

Next is the tremolo section. This was completely non-existent and it was blatantly obvious, because of the many changed components around the two tremolo tubes, that someone had been trying to get this going at some point in time. There were all sorts of components bodged in “here, there and everywhere”, and it was pretty much impossible to follow what they had done. This wasn't helped by the fact that there was no schematic available for this thing, though I hardly ever use a schematic anyway. I decided to rip-out everything from both the two tremolo tubes and then removed the stupid “add-on” pre-amp board and rebuilt this simple part of the circuit onto the main board, using the tube socket that was next to the splitter; previously being the ECC81 as part of the tremolo mixing circuit. Now we had both the EL84’s, the ECC83 splitter, and next to this now resides the ECC83 twin input pre-amp section. This was much more like it; all in a sensible line and working perfect. I now had just the one tube socket left on the board: previously the main tremolo ECC83 that used both half’s. I rebuilt this, still using an ECC83 but this time only one section and with my own design simple circuit. It all burst into life perfectly!

One of the best tremolo designs is that using a neon coupled to an LDR (light dependent resistor) like on most Fenders. This method is “noise free” as it simply takes the volume level up and down. Another commonly used method is to use the oscillator to modulate the grid two of a tube within the signal path (a lot of Selmer’s use this method). This is always very noisy and “thumpy”. My own design is not unique, as some amps also use the following method too. However it does only use one half of an ECC83, a minimum amount of components, and actually modulates the control grids of the output stage, being single ended or push-pull. This in effect is like turning the output tube(s) on and off at the given depth and speed settings. It isn’t completely noiseless but it is far superior to the previous example. It can also easily be added to any amplifier that doesn’t have the tremolo facility.

Everything working as it should now, so it was time to refit the whole thing back in the case. On doing so I found the speaker OC (open circuit)! I removed this and also what looked like an extra baffle. On doing so I could see that the amp originally had been fitted with a 10” x 6” elliptical speaker, most probably an Elac. These are very hard to find nowadays so I plummeted for a nice 10” round driver that I had laying around……not exactly a vintage model but nevertheless it sounded quite nice. This fitted perfectly OK on the original baffle without the need for the extra one that I had removed………Job done!!!

As much as I didn’t like this thing, I have to admit that as a 12 watt push-pull amp, and with the replacement speaker, it actually sounded really good and very quiet (noise wise). When Steve let’s me know his impressions of the little amp I will post any comments herewith.

Thanks for your interest, cheers, John.


UPDATE:

Steve (the amps owner) had written to the current Stentor company searching for some information on this unit? Below is their e-mail answer.

"We have received your pictures which confirm that this is a Stentor Super 12 Amplifier. The designer was Mr G.Pal. They were built for us in Battersea, South London in a factory owned by the Robophone Company whose main product was automatic tape answering machines. The measured output was 12 watts RMS."

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The “Rats Nest” as received.



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  The power supply & various fly-leads.



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What a dreadful “codge-modge”! Note the 100k resistor in the top left hand corner, paralleled to a 560k resistor. This is on the depth control. I have seen this done many-a-time by tech-guys who just don’t have the knowledge of tube amps. When the tremolo is dying they always attack this section of the circuit, trying to bring some depth back into life. However this is completely the wrong place to be sourcing the fault!

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The dreadful speaker wiring. In fact both the fly-leads were ripped off the cone!

  

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The…… well……sort of……tremolo components!

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Out & ready to start the painstaking task of rebuilding.

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The faulty output transformer off for the rewind.

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Close up of the tremolo components. I lifted that “big, black wax thing” (yes it was a cap!) to get the better photo. Note the Mullard/Philips mustard caps and the circuit board mount type too. None of these would have been original in this amp. How on earth is one expected to follow this!

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Virtually everything “ripped-out” & a tentative start made on the de-coupling caps.

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All finished.

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As above, all ready to go back in the case.

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Back in-situ. Note the original baffle for a 10” x 6” speaker, most probably an Elac.

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I’m glad I don’t have the previous 4,032 units to get back to life!!

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The date, 16th June 1960.

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Completely finished.............Phew!

  


 

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