Hi all,

I've got a lovely 1930’s radiogram console (brand unknown). Fortunately, the wooden case will polish up beautifully with only minor repair. Unfortunately, the radio chassis is completely beyond repair, most of the wiring and components have been chewed by mice, most of the metal has rusted and it’s missing it’s IF transformers. It would be almost impossible to rebuild. What is amazing is that the tuning capacitor still looks OK and appears to be in really good condition and meters out to be ok.

I have an old Mullard 5 Valve tube amp in the garage that I've restored which I plan to install in the radio but I’m wondering if it is it possible to use the radio’s original tuning capacitor and connect it somehow to a transistorised AM radio which I would mount on top of the original radio chassis and then run its audio output into the Mullard 5 amp. That way I can preserve the look and functionality of the original tuning gang.

Has anyone ever been successful in wiring a large old tuning capacitor into a transistorised AM radio for this purpose? How would you go about doing it so that it would work?

Your only limitation that I can see is the capacitance value of the tuning gang. If it is within a suitable range then it should work. Otherwise the tuned circuit in the radio as a whole would need some modification to accept it.

Another idea would be to build a crystal set and connect it to the amp.

Another suggestion I might add is that you can buy valve radio kits brand new off ebay ( from China ) and I'm sure you could make these fit without too much trouble. These are Four or Five valve radios.When I get the chance I will share the link with you if interested.

There are quite a few chasses lying about minus cabinet. One could acquire one that will fit & clean it up. I have a few cluttering the place up.

The tuning gang & coils were usually a matched set, so unless you get it right, it may not track well.

Marc

I've done this many years ago. Usually transistor radios used smaller (in terms of max capacitance) vs those used in tube radios. One way to "convert" the tube radio cap to substitute for the transistor radio's tuning cap is to use a series capacitor. One such cap for the antenna LC circuit, and another for the local oscillator LC circuit.

The equation for two caps in series


The tube antenna tuning cap section was usually 365pF, and transistor radio tuning caps were about 2/3 that. If I did the math right, a series cap of 700pF would make the 365pF cap look like 240pF. You should use a trimmer (variable) cap for the 700pF cap. Similar thing for the local oscillator cap.

To align it: we want to get the local oscillator set right. Select the lowest frequency strong local station, say 570kHz. Add 455 to that, and I'd get 1025kHz. Tune in a second radio (which is sitting close by) to hear the first radio's local oscillator. Set the first radio's tuning cap to where the station at 570 should be located at. Adjust the series trimmer cap to make the local oscillator show up on the 2nd radio at 1025kHz. Then adjust the antenna cap series trimmer to make this station come in, and adjust for a max signal. After this is done, then tune in a station around 1400kHz (hopefully you will hear one at a reasonable level) and use the trimmer cap on the tube tuning cap, make that station land at or near the correct spot on the radio's dial, and then using the antenna trimmer cap mounted on the original tuning cap, peak it for max signal. It may take a few iterations to get decent reception across the AM MW band, but you may never get perfection, but it should be "good enough".

Many thanks everyone for your input. Great to read Wa2ise that you have been successful in doing this before. I'm looking forward to giving it a go! I'll let you know how it goes. Many thanks.

It definitely works. In the particular pic I did want the osc outside of range for a weather beacon, but was able to trim it back.

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