From my experience, having trodden this path, it's unlikely that the output level of an MP3 player such as an I-pod, i-pad, phone etc would be sufficient to give the required volume level when fed into the gram socket, but much will depend on the signal level required at the gram socket. Back in 2015, I wanted to connect an MP3 player to a Pye Fenman1 and Bush AC41. Even with the set volume and I-pod turned up high, all that could be hear was the faintest sound. I trawled internet looking for solutions, and found all sorts of simple circuits which it was claimed would do the trick, but they didn't. I've attached one such circuit at pic 1.
Rather than dwell on what won't work, I'll say what will. Namely, a small preamp. When I say 'small' I mean 1.5cm x 4cm. They're ludicrously cheap (three for £2.00 from China), a bit more expensive from the UK. EG:
https://www.banggood.com/3Pcs-LM386-...r_warehouse=CN
They have a pre-set volume control on board, so what I did was to adjust the set volume as I would if listening to a station, connected the MP3 player to the preamp and then adjusted the pre-set pot of the preamp to give the required output to the radio.
Velleman do a nice little cheap mono preamp kit No K1803, which I've also used for a set used by my (adult) son.
To quote Velleman:
"This kit was developed as module for a number of audio applications where the input signal is too weak.
Applications such as a microphone pre-amplifier or for signal level correction".
End quote.
£4.32 plus post plus VAT from ESR electronics. You'll find the construction manual at this link too:
https://www.esr.co.uk/velleman/k1803.htm
Pic 1: Useless suggested circuit.
Pic 2: How to connect your device to your radio whether directly or via a preamp.
The two resistors combine the stereo output from your device to mono.
The capacitor isolates your device from the radio, whilst allowing audio signals to pass.
Pic 3: A Velleman K1803 pramp I built into an ABS box.
Pic 4: Box with the lid on.
Pic 5: Showing 100mV 20 kHz p-p sine wave into the preamp, and undistorted 4V out.
It would of course be possible on an AC only set to put the preamp inside the cabinet, powered from the rectified 6.3V heater to provide approx. 9V DC with the preamp input to the gram socket, so that whenever the set is switched on, the preamp is powered up. I'm not suggesting anyone does that as it means modding the set a little, and a separate plug-in preamp can be used with different radios that have a gram input.
Having written this post, I've discovered that I covered this groud in a thread back in 2017! See post #9 here:
https://www.vintage-radio.net/forum/...+to+old+radios
Hope that helps.