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Old 28th Aug 2017, 11:41 am   #1
greenstar
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Default 1930's baseboard set

I visited the MKRS rally yesterday and picked up this two valve set. It is beautifully made and I hope to get it running. When I have chance I will trace the circuit. It appears to have been made by somebody who had professional engineering skills and wired in a mid 20's style but with two B7 valves (no top caps). The panel and base are both bakelite or similar. The valveholders may be home made, as they are similar material and construction.
It has a Wearite branded tuner unit, again similar construction, looks like MW/LW switched.
I have checked nothing else. As it came without and there is no information with it, it would help to identify it and identify suitable valves.
It always seems a great shame to me that homebrew sets often have no trace of their constructors, particularly one as superbly made as this.
I realise it may be a magazine set, but if anyone can give me a lead it might cut down hours of trawling through.
Thanks,
Tony
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Old 28th Aug 2017, 1:36 pm   #2
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Default Re: 1930's baseboard set

Hello,
The two B7 valveholders are interesting as the only common battery valves I can think of with a B7 base and no top cap, of suitable age, are the class B double output triodes (or possibly double pentodes). I think these came out in aout 1933 which might give some idea of the set's date.
It is possible the first valve is a push-pull detector as there seems to be only one intervalve transformer.
If it is a magazine set and my theory is correct then I would look around 1933 and for something like "The Double Double Triode Two" or "The Push Me Pull You Two" etc.
Yours, Richard
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Old 28th Aug 2017, 5:38 pm   #3
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Default Re: 1930's baseboard set

I'm not sure Pushmepullyou's were as well known in 1933, although I see now the first story came out in 1922, so may be wrong! Hugh Lofting apparently began writing to his children - in the trenches. Thanks, Richard that's a start. I notice six pins only used in V1, and all seven in V2. I will have a search. Just to add, it does look like push pull for v2. I had wondered what that big transformer was doing in a two valve set!
Tony

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Old 28th Aug 2017, 6:13 pm   #4
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Default Re: 1930's baseboard set

Just to add, Practical Wireless March 30th '35 lists blueprints available for sets published in PW, Amateur Wireless and Wireless Magazine. Nothing definite, but there is a 'Class b detector 2v' in PW 24A (17.6.33) but unfortunately this issue is missing from the American Radio History website.
Tony
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Old 28th Aug 2017, 8:23 pm   #5
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Default Re: 1930's baseboard set

Hi Tony, see if you can locate any Telsen Radiomags. These often had slightly quirky circuits for home constructors.

The coil packs look the business as does the coupling transformer.

Ed
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Old 29th Aug 2017, 10:29 am   #6
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Default Re: 1930's baseboard set

Hi Ed, thanks there are three Telson mags on the ARH site - I'll check.
Both transformers seem fine, although the big one is an enigma. The intervalve is a Lissen Hypernisk (?) 'For quiescent push pull circuits' and is good. The output one is an 'Igranic CC Output Unit', and has odd resistance readings, seeming less between primary and secondary windings than within them, so I suspect it's not just a transformer.

Next step may be to trace the circuit around V1, which may take me a few days!

Tony
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Old 29th Aug 2017, 2:04 pm   #7
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Default Re: 1930's baseboard set

Hello,
Looking at the pictures the first valve has only six connections which is correct for a type B double triode (no connection to pin 6).
However the second valve has connections to all seven pins which would suggest a double pentode (the pin 6 screen grid connection could possibly be a later addition) and the earliest I can find (with a 7 pin rather than a 9 pin base) is the Marconi/Osram QP21 in 1934.
The Igranic CC Output Unit could be an output choke rather than a transformer and so would have a connection between "primary" and "secondary".
Yours, Richard
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Old 29th Aug 2017, 2:49 pm   #8
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Default Re: 1930's baseboard set

Richard, thanks, this is a help. I am not too familiar with the use of chokes in output circuits! Would you be able to suggest at this stage a couple of inexpensive valves?
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Old 29th Aug 2017, 5:01 pm   #9
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Default Re: 1930's baseboard set

After a look at the circuit I realise I won't be able to draw the first stage fully, as the maze of connections within and for the tuning unit are untraceable. But assuming the valve is a double triode, and similar to the 220B (couldn't find anything closer) it makes sense. The two plates go either end of what may be the reaction coil, which isn't connected to the other coils.
As the set is in such good condition (the two resistors are more or less correct value) I wonder if it may work straight off (possibly the big capacitor the most likely failure), so my next step, perhaps, should be to find a couple of valves?
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Old 30th Aug 2017, 10:08 am   #10
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Default Re: 1930's baseboard set

I've found a similar FJ Camm circuit, in 'Practical Wireless Circuits' 1941 reprint. It's helpful to me to see a push pull detector circuit! The coil is also similar, home made, very like the Wearite which has a LW coil and changeover switch added. It uses two single triodes though, so still need to identify a possible valve.
Tony
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Old 30th Aug 2017, 12:04 pm   #11
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Default Re: 1930's baseboard set

I seem to be answering my own questions - I looked a little further in Camm's book - a real gold mine - and find the previous circuit, no 16, was similar but has a double triode in place of the two 210 HL's. He uses a Cossor 240B, B7 base, so I guess that's the valve to seek.
The 240B is described as an AF output valve, so possibly two of these or two QP21's could be used. I could do with being a little more sure before I splash out. Apparently the reaction is likely to be affected.
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Last edited by greenstar; 30th Aug 2017 at 12:11 pm.
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