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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 22nd Sep 2018, 4:54 pm   #1
martin.m
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Default Unidentified single ended valve amplifier

This amp chassis was inside a console type 78 rpm record player that was very much home made. However the amp chassis seems too good to have been home constructed from scratch and I wondered if it had been made from a kit of parts. It has a nice set of Marconi valves, U50 rectifier, KT63 output stage and DH63 triode. Both transformers are "Elstone" types and the smoothing can is dated 9/47. The amp was working when I acquired it around 5 years ago. Now on the bench, the valves lit up but there was no sound. Zero voltage on the anode of the DH63 triode led me to a short circuit capacitor (0.0002uF / 500v) which was wired from the anode pin to chassis. It was a paper covered tubular capacitor sealed with bitumen made by "Franklin" and looked like it should have lasted 100 years. I replaced most of the other resistors and capacitors with modern components as well as fitting input and speaker sockets. No attempt was made to preserve the "vintage" look underneath. However the top side of the amp was left untouched. Antique dealers call it Patina. I may build it into a cabinet with a speaker and use it to improve the awful sound on our flat screen TV. There's progress for you.

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Martin
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Old 22nd Sep 2018, 5:24 pm   #2
GrimJosef
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Default Re: Unidentified single ended valve amplifier

I could well believe that this chassis was constructed from scratch by a competent amateur. The sort of metal bending that home constructors were expected to be able to manage can be seen throughout, for example, Mullard's Circuits for Audio Amplifiers which contains detailed chassis drawings for every amp in the book e.g. here http://www.r-type.org/articles/art-003d.htm. Perhaps access to small bending machines, say at school or college, or to the sorts of small engineering companies who would do this kind of thing quickly and cheaply was also wider back then.

I haven't followed the wiring through but I would recommend making sure that the chassis is securely connected direct to mains earth. Once upon a time it was considered acceptable not to do this as long as the chassis was in a sealed wooden box and precautions were taken e.g. with the control knobs and with any wires that came out of said box. But if you plan to re-purpose it today then you wouldn't want other users to get a nasty bite in the event of a fault. We've got out of the habit of treating electonic equipment with a healthy respect.

Cheers,

GJ
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Old 22nd Sep 2018, 5:50 pm   #3
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Default Re: Unidentified single ended valve amplifier

Thank you for the advice GJ. I will connect the mains earth directly to a sturdy solder tag bolted to chassis. The circuit is a conventional 2 stage amp as used in most post war domestic radios. I agree that the amp was probably home constructed. It looks a lot better made than the ones I used to put together as a teenager in the 60s.
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Old 22nd Sep 2018, 5:52 pm   #4
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Default Re: Unidentified single ended valve amplifier

There does appear to be a wire link between the tag that green/yellow is connected to (HT CT?) and chassis, via another tag. That would have been a fairly common and acceptable recourse "back in the day" but I prefer to make sure that mains earth goes directly to its own chassis tag- there's more expectation of formal practice around ensuring integrity of earthing now.

That's a nice find there- the classic bulbous valves look the part, a "proper" amp, and it sounds (!) as if it will have worthy continued employment.

Crossed with martin.m!
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Old 22nd Sep 2018, 6:59 pm   #5
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Default Re: Unidentified single ended valve amplifier

Hi!

Almost certainly home-built from a Practical Wireless or Radio Constructor design during the 1950s era with the sloping diagram lettering!

If you search "DH63 and KT63" in the Practical Wireless & Radio Constructor sections of American Radio History and tick the "1950s" box you stand a good chance of finding the design - many, many small amplifiers of this type were published during this period in P.W. and R.C!

Chris Williams
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Old 22nd Sep 2018, 8:43 pm   #6
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Default Re: Unidentified single ended valve amplifier

Turretslug and Chris, thank you for your replies. I love equipment that uses Octal valves. It's not been possible to assess sound quality yet as my test speaker is quite small. I took the liberty of adding a little negative feedback from the speaker output back to the cathode circuit of the DH63 triode. Combined anode / screen current for the KT63 is around 36mA and even after several hours use, the mains transformer is barely warm. I could always do with a little more gain from these two stage amps as I listen to Internet radio using the headphone output of my mobile phone. Plenty of current there to drive headphones but valves are voltage operated devices and there is a definite mismatch which means the phone has to be turned to full volume.

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Old 22nd Sep 2018, 8:56 pm   #7
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Default Re: Unidentified single ended valve amplifier

Quote:
Originally Posted by martin.m View Post
Zero voltage on the anode of the DH63 triode led me to a short circuit capacitor (0.0002uF / 500v) which was wired from the anode pin to chassis.
You were lucky the output transformer was OK.

Mike
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Old 23rd Sep 2018, 1:28 am   #8
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Default Re: Unidentified single ended valve amplifier

I have also found that head phone outputs are very low.
My main HiFi is based on a pair of cinema mono block amps and they have an extra pentode gain stage and bring tablet outputs up to full power easily thru the optical sound track input. They also have a triode before the phase splitter.
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Old 23rd Sep 2018, 12:22 pm   #9
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Default Re: Unidentified single ended valve amplifier

If you need more gain from this amp, why not rebuild the front end using an EF37A pentode?

You may even be able to up the NFB a bit with this, though unless you can measure the open loop gain/phase response, adding NFB may be a bit fraught.

If you happen to have a decent old microphone transformer kicking about, you could try that between the phone and the amp input- it should provide a useful voltage step up without sounding bad.
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Old 23rd Sep 2018, 1:49 pm   #10
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Default Re: Unidentified single ended valve amplifier

The EF37A is what my old amps have in the front end.
They will almost work with a magnetic gram cartridge with a passive RIAA circuit.
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Old 23rd Sep 2018, 5:51 pm   #11
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Default Re: Unidentified single ended valve amplifier

Thank you both for the advice. The problem seems to be lack of voltage from headphone sockets when using these basic two stage amps, though if the phone's volume control is turned up full (ignoring the warning about hearing damage) results are acceptable. The impedance matching transformer sounds a good idea though and it could be incorporated in the lead from phone to amp.

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Martin
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