Making an Audio Input Adapter Cable for a 1950s Vacuum Tube Radio

I recently fixed up my first vacuum tube radio, a 1956 Philips 353A, which now works well and sounds fantastic and is set up in our front room.

The AM/FM signal reception in our house is relatively poor, however, and I noticed that this radio has a “Gramophone Pick-Up” input, which is basically just an audio input directly into the amplifier stage of the radio, bypassing the RF demodulation stage – this is selectable using the band selection switch on the front of the radio.

The only problem is that this is a non-standard connection, so I had to be a bit “clever” and make up an audio input adapter cable to be able to use the radio with a modern smartphone or any other device with a 3.5mm stereo audio output.

Radio Safety

This is only possible on the Philips 353A radio because it has a mains input transformer, and the chassis ground (which is also the negative for the audio input) is not connected to live or neutral, which it commonly is on transformerless radio sets.

AS SUCH, THIS WILL NOT WORK SAFELY ON JUST ANY RADIO! You need to make sure that you clearly understand the design and operating principles of your radio before trying this, and you could put yourself or others in danger of a serious (potentially deadly) electric shockas such, you do so at your own risk.

This is not a guide, and is for reference purposes only.

Adapter Theory of Operation

The main problem with connecting a modern device to this radio is that most modern devices output audio in stereo (two channels, left and right), whereas the Philips radio expects a mono (single channel) input. We therefore have two choices: only connect a single channel, and lose a significant amount of audio quality, or we attempt to merge the two stereo channels into a single mono channel.

The latter is certainly the most attractive choice, so that’s exactly what I decided to do.

I followed this excellent video guide from M Caldeira – this advises using a 1kOhm resistor in series with each input channel to prevent damage to the driving device (most likely an operational amplifier with a very low output impedance).

Audio adapter schematic.

Adapter Assembly

I ordered all of the necessary parts to make up the adapter, in this case:

Building the adapter was a relatively simple process: first, I cut the 3.5mm audio cable in half, and stripped some of the external and internal insulation; then, I cut down a length of screened audio cable, and stripped some of the external and internal insulation at one end.

Then, I prepped and fitted a resistor in series with each audio channel as per the 3.5mm audio pinout standard, and covered these in some heatshrink.

3.5mm jack pinout (image credit: etechnog).

Then, I joined the output of the resistors together and attached this to the positive side of the screened audio cable, and covered the joints with heatshrink.

Then, I joined the ground from the 3.5mm cable together with the negative side and screens of the screened audio cable, and covered the joints with heatshrink.

I also de-cored a ferrite bead and used the case to clip around the soldering, to neaten the adapter cable up a little bit.

Then, I stripped the other end of the screened audio cable ready for fitting the banana plugs – I connected the screen and core on the negative side, but cut off the screen on the positive side and added some heatshrink to prevent any shorts.

Then, I used some cable lubricant to fit the banana plug sleeves, soldered the plugs themselves into place, then fitted the sleeves.

With that, the adapter cable was complete!

I tested it out with the radio (taking care to get the correct polarity), and it worked perfectly. Happy days!

Published by themightymadman

My name is Adam Wilson - I'm an electronics engineer based in the North East of England, UK, and I like tinkering with old junk. In my spare time, I collect, repair, refurbish, and (sometimes) sell vintage computer systems and peripherals, typically from the 1980s (the likes of Commodore, Sinclair, Acorn, Apple, Amstrad, and Atari).

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