RSC Guitar Amplifier.

[Neil Purling]

I looked in on Mr Chambers Web site while looking for information on Linear amps.
They were made down to a price. Mr Chambers does not like those metal capped resistors, by Erie I believe. Those blue capacitors look like Dubilier. I won't make comment on them, no personal experience.
Was the amplifier bought ready-made or as a kit? Group board really makes for a neater under-chassis layout, if you have the room. If it was a kit, I assume you just followed the instructions they gave you.
Do you intend a full re-build after Mr Chambers efforts?

[maninashed]

Hi Mike, I'm not familiar with that particular amp, but from what I can see, if the output pair were EL84, then the ECC83 would need to be the phase splitter. So I think that theory can be discarded. A guitar amp needs several stages of pre amplification so it is feasible that the output pair may be ECL type valves, 82s or 86s. These were very common in the 1960s/70s. If the valves in the amp are the originals, post a close up of one of them, the lads on here will soon identify it. Also some close up photos of the wiring of the output valve bases would be helpful.
I used the RSC shop on Dale St, Liverpool. They were great in their day and a mecca for me as a schoolboy. They also did a range of Baker loudspeakers which were rather inefficient. I bought several of their kit amps, but never the ready made guitar amps. All the kits were made by Linear, of Armley, Leeds. The Linear concord with 807s on John Chambers excellent website is actually mine before it was restored by me and now back to full working order.
Bill

[sparkymike]

I am finding that the HT voltage is up near 400. This seems a bit high to me. The a/c volts before the rectifier is around 350 . Could the mains transformer be at fault here?
Mike.

[maninashed]

Sounds ok to me, especially if there is no load.
Bill

[sparkymike]

There is no output at the moment. I had thought that the output transformer was faulty ,but just tried a known good one in its place and same result. Nothing at speaker.
There is no click when touching either output valve anode (with the meter to chassis) through the speaker, but with set unpowered, the speaker will click given same test.
Perhaps I should have left the amp in the loft. !!
Mike.

[maninashed]

Hi Mike, don't despair we ll get it going. I'm satisfied the ecc83 is correct, with heater on pins 4,5 and 9. I'm not convinced we have correctly identified the o/p pair. That's if it is a pair. Do the primary connections of the op transformer go to two separate valveholders? If not we may be barking up the wrong tree! Another possible line up would be 2 ecc83 and EL84 we need to establish for sure what the amp is.
Bill

[sparkymike]

Yes There are seperate wires to each output valve ,which as far as I know are ECL 82's. I have another record player push pull amplifier and this has same valves and very similar circuit. (Mullard 7 watt design).
Anode to output transformer are pins 6. Don't think that would work with EL84's
Mike.

[Neil Purling]

What pins on the two bases are the two outer wires of the o/p transformer Primary wired to? Then look and see if the cathode is pin 3 or 2. I don't know if the cathodes are bypassed or not. You should then be able to tell what sort of output valves should be there: ECL82 or EL84. You have the heaters on the same pins (4&5) with both types.
Not a lot of gain if the correct o/p valves are EL84.
With ECL82's you'd have one of the triodes for extra gain in the pre amp & the other as a phase-splitter.

[sparkymike]

Output primary outer wires are not used and are just made safe on the ends of the tag-board. (See thumbnail in earlier post) Output primary goes to pin 6 on each output valve. This tallies with the Mullard circuit. (see thumbnail)
I have just found that the HT feed to the fuse was a dry joint, in fact it looks like they missed soldering it at all !! (Looks to be original workmanship, or lack of it.)
Mike.

[Neil Purling]

That right? The o/p transformer has a Primary with 3 wires, so the ECL82's could/should be in Push/Pull. They are both in parallel?
You have ECL82's? There are some unbelievably cheap ECL82's on-line, if you don't have a prejudice against Svetlana that is. Bag yourself a spare pair as well at those prices for less than forty beer tokens inc p&p.

Are you a guitarist? I wondered what you were going to do with it when it is all fixed.

[sparkymike]

I have now found a faulty electrolytic. One of the tappings is down to around 400 ohms to chassis so needs to be changed. A bit of a strange one, to me, it has two wires one end and one the other, all insulated. On the can it says 50mfd red and 50mfd yellow and can not insulated, but no mention of the other end. The is clipped to the chassis.
Is the single wire a common connection ,or is that third cap ? It looks like this might be part of the feedback circuit.
Mike.

[maninashed]

Mike, it sounds like a double electrolytic, but if its got 3 inside it will say on the case. Generally there is a common ground which will be connected to chassis. You may not be able to get exact device these days, but you could just use 2 modern electrolytics under the chassis and disconnect the original leaving it in place so it 'looks right' . You must use same voltage components or higher but never lower. The modern day equivalent will be 47uf. Make sure you connect the right polarity + to ht.
Bill

[Neil Purling]

About that electrolytic: You can get a very good 47/47uf double-section by F&T of Germany. The negative is to a tag, in which case you will have to connect the black tag to chassis.
They are excellent electrolytics.
If you have the means to 're-form' electrolytics it may 'come back'. I wouldn't count on it though.
Buy a pair of ECL82's & the electrolytic if you want. I'd change the grid-couplers and protect your fifteen quid investment in a pair of ECL82's.
I have not come across those blue Dubilier capacitors. I would be wary about leaving the grid-couplers in situ, even if they weren't liable to leakage.

[maninashed]

Those blue Dubilier capacitors were very often used in RSC kits of the time 60s/70s and in my experience experience do go leaky, but they are not as bad as Hunts! .
Bill

[sparkymike]

Hi Bill, can you check them with a megger?
Mike.

[maninashed]

Mike, yes of course, that's the best way there is to check them. Make sure you use a range under the rating of the capacitors and you will need to disconnect one leg. Keep your hands clear and make sure you DISCHARGE the cap after the test. I personally would discard any with a reading of less than 50meg ohms in that application.
The ones connected to the control grid pins of the valves I would replace anyway.
Bill

[sparkymike]

My meg is cranked with a handle(no electronic megs here !! Its a nice old Vignales and Evershed in Bakelite case. !!
I have started to re-draw the circuit using the Mullard circuit as a template.
So far I have only got up the the pentode section of the output valves.
The cathodes are just strapped and link back to the triode section of the ECL 82's. See thumbnail. You will probably have to save the photo and then enlarge it to see it clearly.
Mike.

[Valvepower]

Hi,

When I rebuilt the A11 featured on the John Chambers site I discarded all the blue Dubilier coupling capacitors as they were suspect - I think I discarded nearly all the original capacitors! Personally I’d replace all the coupling and any other suspect capacitors if you want a reliable amplifier. I did go to the extreme and completely re-build the A11 as is was a bit of dogs breakfast, but this was a kit amplifier, whereas yours looks to be a factory built unit and is subsequently better built.

Terry.

[sparkymike]

Unfortunately, these amps were not very neat in the wiring department !!
Just take a look at a Quad valve amp wiring and you will see what I mean.
I spent several years wiring outside broadcast trucks for companies like the BBC and we had neatness drummed into us. We also built OB's for the German market and they were even more precise.
Mike.

[ColinTheAmpMan1]

Hi All,
I have one of these RSC A11 amps which I constructed about a million years ago. I have changed it about since to make it more suited to guitars.

The original valve complement was 2xEL84, 2xECC83/12AX7 and 1x5Y3/5U4. The circuit comprised two triode pre-amp stages with a Baxandall-type tone control network between them. The second triode stage had negative feedback applied to its cathode from the secondary of the output transformer. Next came a standard differential pair phase-splitter and then the two EL84s in a push-pull, ultra-linear configuration. The high- and low-gain inputs simply had different input resistors going to the grid of the first triode stage.

To give an idea of the age, the entire valve complement was priced at 46/6!

This set-up is not ideal for a guitar amplifier and I have modified mine since building it originally. Baxandall tone controls are not exactly voiced well for guitar amplification; the tone-stacks favoured by Fender, Marshall and co. are better suited. Negative feedback is not always a good idea, either, if you want a bit of "grit" in your sound. A "presence" control can be included to reduce negative feedback.

Shortly after building the amp originally, I bought a Linear TP1 Tremolo Pre-amp to add to it.

I find it a bit odd that the amp being referred to has an EZ81 rectifier, as the high-tension fuse in mine is rated at 250mA and the EZ81 will only cope with 150mA current. Perhaps this beastie was tamed a bit by using triode-pentodes which wouldn't give the output of EL84s? Another benefit might be the lack of a need for a 5 volt heater winding on the mains transformer, meaning a cheaper transformer. Both of my transformers are half-shrouded, unlike the one being discussed here.

Maybe the circuit is rather similar, with one ECC83 employed as in my amp, followed by the triode sections of the triode-pentodes configured as the phase-splitter and the pentodes as an ultra-linear output stage? One worry that I might have over the output transformer's extra, unconnected leads, is that the output stage is not ultra-linear, but the screen-grids are simply returned to the HT via resistors.

The comments about wiring tidiness deserves comment, I think. These amplifiers were wired "point-to-point," that is, without too many tag-strips. They also had a bus-bar earthed only at the input sockets. Positioning of some components on this bus-bar are important, especially the power-supply reservoir and smoothing capacitors. There are a couple of points about this construction. While "point-to-point" wiring can look messy, it can also be the best way to avoid too much wire between components resulting in instability (both high-and low-frequency). Some of the best audio amplifiers ever built were constructed in this fashion. But, and it is a sizeable but, an amplifier that sits on a shelf working with a tuner and turntable is one thing, a guitar amplifier that gets carried from place to place is another. There is every possibility that the constant knocks and bangs that a gigging guitar amplifier gets will eventually strain some of those soldered joints. So, tag-board construction is likely to be a better option in the long run.

Incidentally, the manufacturer of mine is "Radio Supply Co. (Manchester) Ltd." although the address is 29/31 Moorfield Road. Leeds 12.

If you are interested, I have the original instructions and circuit-diagram which I could scan and post.
Regards, Colin.