Identify this 3 valve amp. 1950's?

[vintage_8bit]

Can anyone identify this 3 valve amp please. I removed it from a home made record player today. Maybe it was a kit?. Colin

[qualityten]

If the clear glass valve is a rectifier (can you identify it for us?) then my guess is this is a single ended amp, akin to the Mullard 3-3. Definitely home brew, judging by the construction and the scratched input and output markings on the side of the chassis.

I look forward to the comments of those more knowledgeable than me.

[GrimJosef]

I think it's home-brew rather than commercial.

The input stage is a 6SG7 which is a vari-mu pentode designed for RF/IF work. It would be an odd choice for audio use but if you happened to have one in your bits box then it could be pressed into service at a pinch. It's got plenty of gain which would suit a high-output record cartridge.

The input is fed direct to the volume pot and thence to the 6SG7. The tone controls come between the input stage and the 6V6 output valve.

The chassis is used as a common ground with connections made to it all over the place (not really good practice if you want to keep hum and noise under control). The chassis even seems to be used to carry one half of the heater wiring, and if the two yellow wires from the mains transformer are the HT secondary then again one is connected to the chassis with the other going to the rectifier. I can't see the marking on this but it looks like a full-wave one, perhaps a 6X5GT or something similar. Pins 3 and 5, which would be the anodes in a 6X5, look like they're wired together and to the yellow secondary wire, so it's running in half-wave mode - crude but reasonably effective and all you can reasonably do if your HT winding isn't centre-tapped and your build pre-dates the availability of cheap silicon rectifiers for a bridge.

Cheers,

GJ

[Herald1360]

6SG7 is a vari-mu RF pentode- a slightly odd choice for an audio preamp valve. A 6SJ7 might be more suitable.


The tidy chassis and odd lineup suggest a kit using whatever surplus valves were conveniently to hand.

[Radio Wrangler]

screws and nuts securing valve holders and in/out sockets... a commercial unit would use rivets. Homebrew in a purchased blank chassis?

I wouldn't expect 6SG7 for low level audio in a magazine-published design or a kit. So likely a home-done variant of something.

David

[Boulevardier]

I can't make out the mains input wiring either - not how it gets to transformer nor how it's switched. It looks almost as though one side of mains lead is earthed through tagstrip mounting lug.

Mike

[Radio Wrangler]

The bottom tagstrip seems to be home to the mains primary tappings. Thin grey/greenish wires run from it, between the pot bodies abd the deck to the on/off switch, which is bridged to single pole.

I'm not sure it would be likely that the tagstrip mounting lug spacing would match the transformer bolt spacing, so maybe the bottom end of the mains tagstrip isn't secured. I hope it isn't because otherwise it does look like one side of the mains would go to chassis and that cable is unpolarised figure of eight as well. If you think back in the day, a lot of guitarists did seem to die by electrocution. Scary to see this nowadays.

Spot the Plessey red-and-black trying to hide behind a smaller (tubular ceramic?) capacitor at the top?

David

[vidjoman]

I think it might be from one of the radio shops like Stern, Clyne or G W Smith’s or even Laskeys and others as they all had kits available to build your own amplifier. The valves supplied varied from time to time, according to what was available. Most were war surplus. The chassis came punched out and I think they were mass produced by a sheet metal company as they were all the same no matter which shop sold you the kit of parts. I built several in the late 50’s and used them in record players. Obviously the build quality could be good or bad depending on the skill of the person making it.

[vintage_8bit]

Thanks for all the replies, much appreciated.

Looking at it again with your observations I can confirm the following.

The chassis does look like a purchased blank.

The rectifier a 6X5, is, as spotted, wired as a half wave.

The heater chain does indeed have one side to chassis.

The mains is wired to the 7 way tag strip. One end-terminal of the tag strip is used for the chassis connection of the H.T. secondary & heater chain. The other chassis connection on the tag strip is not used, so no live chassis. The other terminals are for the primary mains adjustment taps.

The use of an R.F. valve for the preamp is interesting.

The output transformer doesn't have much iron in it. I will probably look at the ratio here & see how matched it was to the now broken speaker.

I will report back after its 1st power up. I have already checked the output & mains transformers for continuity & all good. The usuall caps to check/change first. It will be interesting to hear how much hum this designs has.
See attached pic for the whole record player. Someone had spent a lot of hours making that case, unfortunatly it was beyond saving. The deck appears in good condition.

Colin

[rontech]

I once used a Sylvania Red EF50 for a tape head pre amp.

[Valvepower]

Hello,

After spending the last few months looking at Tripletone amplifiers and doing my next ‘Odd-Ball’ amplifier article about Tripletone amplifiers (its now with the Editor, Alex) and its follow up for the BVWS Bulletin I feel it maybe a home brew Tripletone amplifier?

But saying that, for a short while at the beginning of the life of the Tripletone valve amplifiers in the early 50s, the first Tripletone amplifier was available as kit – maybe it was a Tripletone kit?

I can 'just' make out the circuit of the triple tone controls - This is best I can make out with lowish resolution pictures on the forum.

The Mk1 uses Octal valves and from what I can ascertain from an amplifier I’ve seen online they were 6SJ7, 6V6 and 6X5.

I’ve attached the pictures I located on the net and a page from PW on the American Radio History site. I know fellow forum member has unearthed the PW page as well, but I can't recall his name...

If it is a Tripletone amplifier there isn't much Iron in the output transformer!

Hope this helps.

Regards
Terry.

[Valvepower]

Hello again.

This morphed into the Convertible amplifier which uses 6BR7, EL84 and EZ81. The circuit stayed more or less the same, all be it with a few tweaks for the different valves. If I remember rightly, Brimar have the 6BR7 down as a B9A replacement for the 6SJ7 - but don't hold me to it

Terry

[Marconi_MPT4]

Might be the RC 3/4 Watt amplifier published in Quality Amplifiers for AC mains dated 1958. It has bass, mid, and treble tone compensation. Attached first two pages.
Rich

[Edward Huggins]

This is actually the RSC Radio Supply Company (RSC) "3 valve, 4 watt" kit sold via advertisements in Practical Wireless up to the early 1960s. Yes it did use mainly surplus parts and the actual valve line up varied from batch to batch. And, yes, it was a crib of the Tripletone with a middle tone control. The early Tripletones with octal valves (but same 6V6 output) and full rectification, did not employ NFB, but I think these did.

[GrimJosef]

Staring at the under-chassis wiring I couldn't see any sign of global negative feedback, and given that the tone controls are between the two valves I wouldn't have expected any. But the 270R 6V6 cathode resistor is un-bypassed, so there's some local degeneration there of course.

Cheers,

GJ

[Valvepower]

Hello,

Marconi_MPT4 thanks for posting the RC 3/4 Watt amplifier circuit. Also thanks to Edward for giving the heads up regards the RSC Radio Supply Company (RSC) "3 valve, 4 watt" kit… as I wasn’t aware of these two amplifiers. This makes another two uses of the Triple Tone control. Looks like nearly everyone was on the triple tone control bandwagon back then.

Attached the rough scribbled circuit for the B9A Tripletone Convertible – this may help a tad.

Yes, no overall negative feedback, just the [current] feedback by omitting the output valve cathode bypass capacitor.

Terry

[vintage_8bit]

Thanks for all the replies & the pdf. I have been surprised by the amount of info it has generated. When I am next in the workshop (soon), I will do some comparisons & report back. Colin

[Valvepower]

Hello Colin,

There is interest in what I call 'odd-ball' amplifiers.

If it's possible, could you kindly attach a high resolution version of the picture of the underside of the amplifier, maybe zip-up the JPEG?

Do I spy Wish You were Here, Floyd on the screen in the far background in top left corner of the underside picture

Regards
Terry

[ms660]

Yep, "Welcome To The Machine"

Lawrence.

[vintage_8bit]

Terry, well spotted, the Floyd artwork!

The circuit in the above pdf for the (RC 3/4 Watt amplifier) does seem to be exactly the same. Even the knobs match up.

Ok. Despite having another job on bench, I have just changed the coupling cap to the output valve grid. Now seeing if the main smothing cap will reform, fingers crossed.

Later I will have a look back at posts & make sure Ive taken it all in & replied.

Colin