Using the PU jack
I am posting my experience connecting modern peripherals to 1930s radio sets via the PU jack, in case this helps anyone here. Most things were learnt through trial and error, asking questions here and googling. Below please find the things I wish I knew starting out.
1. On both my sets (GEC superhet 5 and Pilot u650) the PU jack requires an input via a 6.35mm mono plug. This automatically mutes the radio to allow sound via this 'line in' only. The problem is that not all 6.35mm plugs work. The overall length of the plug seems to vary by about 5mm. The shorter ones are a waste of money. I have found (after 3 attempts) that the kenable 6.35mm mono male to 3.5mm mono male audio cable (Amazon/eBay) works well.
2. There is a very well concealed mono mode on the later versions of android. This is accessible from
Settings->smart assistance->accessibility->mono
If you set this to mono and plug the 3.5mm mono plug in your phone's stereo headphone jack, you get great sound
3. If you can only get stereo sound from your device, you need a resistive circuit to convert stereo to mono. There is a thread here that explains how to make one. Alternatively, you can buy one from ebay, just search for
'smadt1 stereo to mono'. Its priced £7.99 but the seller accepted £6.50. Don't be tempted to use anything that shorts L+R - the sound will be terrible.
4. If you want to avoid cabling, there is a 3.6v 3 channel AM transmitter available on eBay or AliExpress (£10-£15) which i can verify works well provided it's very close to the radio.
Hope this helps someone
Gabriel
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