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Old 15th Oct 2018, 12:27 pm   #1
greenstar
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Default 1930 HB portable power requirements?

I have a home made 1930 ish screen grid portable, won at the March Harpenden this year.
Probably because it was fairly rough I felt sorry for it - and I like home made sets, and also am not damaging anything rare in my inexperience. It is nearing the point where I can apply some power and I would appreciate any comments and advice. I have roughed out a circuit. My main question is what battery/power needs it has. There are four HT plus leads, which seems a bit excessive. Looking up each of the three valves I see they all have 150v max anode volts, with V1 screen at 80v. But I see most such sets have something like 90/60/40 on anodes. Also there is a 'C' battery, so am wondering about the procedure to find the best GB voltage.
So far I have repaired, partially rewinding, the frame aerial, fitted a 40's speaker and transformer in the absence of the original reed speaker, repaired and tidied the case, restuffed the capacitors bar the grid leak mica which measured almost exactly the marked value, replaced the big Ferranti intervalve transformer which was open on both windings (fitted a 5:1, original 7:1), replaced the rubber covered wire.
One bright idea I have is to run the HT from a 12v lead acid, via a DC-DC nixie supply ebay converter and string of zeners, screening it and poss with extra smoothing.
Comments much appreciated.
Tony
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Old 15th Oct 2018, 7:53 pm   #2
Brian4radio
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Default Re: 1930 HB portable power requirements?

Hi Tony, I have played with these early sets and they can be quite tricky to get working. What I do is to break the sets cct down to basics. It depends on the type of valve provided in the first stage. Did you mention it was a screen grid type. I would take the wiring off the anode of this valve and insert a pair of "old type " headphones (4000 ohms) between the anode and a ht battery positive of say 60 to 90 volts .Add 60 volts to the screen grid and 2 volts to the heater of the valve. Connect the aerial and earth to the set and start adjusting the two variable caps I see on the front of your radio. You may need a high frequency choke between the HT battery positive and the head phones. If you manage to get the first stage to work you can then progress to the next stage . The adjustment of the tuning cap and the reactance cap can be time consuming with a little movement on either can be critical in getting a station. Good luck Brian.
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Old 15th Oct 2018, 8:51 pm   #3
greenstar
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Default Re: 1930 HB portable power requirements?

Brian, thanks. I've been looking at some of the commercial suitcase sets, and they seem to use 60 and 90 HT. Mike at the Kolster Brandes museum restored such a set, and makes some interesting comments. http://kbmuseum.org.uk/kb_images/kb1...6_restore.html
Thanks for your idea - hope to get on to this soon.
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Old 17th Oct 2018, 5:41 pm   #4
Mike. Watterson
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Default Re: 1930 HB portable power requirements?

See batteries on this one
https://www.radiomuseum.org/forum/mc...tteries.html#1
I do have grid packs with 6 x AA alkaline.
Start at most negative voltage.
Basic principles:
The output valve will have full HT (maybe 120V)
The detector will have lowest. You can move lead for best detection.
RF and preamp that's not a detector might be 80V to 108V
You can just set the GB (usually for last two valves on a 3 or 4 valve set as RF (& sometimes Detector) use "grid leak" principle, the space charge causes a negative voltage) according to datasheet of actual valve fitted. Typically -3V or -4.5V on the detector or audio preamp according to datasheet. Often the output triode or pentode was coded with letter or coloured dot for grid bias setting, you can start -9V and change tap, WHEN LT OFF, to set Anode current to ideal for the HT and load resistance.
Make sure grid pack 0V is good connection and the -ve taps can't come loose or transformer / headphones burn out.
I open base of caps and cut one wire and fit ceramic, or cut one external wire and tuck ceramic or plastic out of sight. Often there is no on/off on HT and GB. Disconnect LT first and connect last. Connect GB first and disconnect last.
See also https://www.radiomuseum.org/r/unknown_lwmw_sg4_kit.html
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