Adding Bluetooth to Antique Tube Radio

[PaulyWally]

I've got this little project I'm working on. I am trying to add a Bluetooth module to an old Philco AM (tube) radio.

I didn't want any batteries to replace or recharge. The Bluetooth module uses 5VDC. So I decided to try using a USB charger. I removed the case of the USB charger so I could solder it direct and hide the PS circuit inside the Philco radio along with the Bluetooth module. The Bluetooth audio output signal goes straight into the volume pot. This is just prior to the 1st audio tube (which then goes to the power tube). USB and Bluetooth grounds are going straight to the radio ground.

The problem is... noise. Lots of it. Some hissing, "clicking", low-frequency "farting", and mid-frequency whistling/whining (like interference from back in the day). When I physically touch ground, 60-70% of it goes away. Same when I physically touch the audio signal wire going to the volume pot. And when I power the Bluetooth module with a battery, the noise is just about completely eliminated.

I've tried coupling caps and resistors. Coupling caps are no help (although I'm lazy and only tried a 47uF). A 47K resistor in the Bluetooth/USB ground path helps some of the low frequency "farting" noise. But doesn't totally eliminate it.

I have a strong feeling that this is due to ground reference/offset between the USB PS and the radio PS... which I'm guessing I won't be able to alleviate. But I thought I'd ask here anyway.

Closing thoughts:

1. I'd like to keep the radio in original working order. Plans are to add a switch to select AM or Bluetooth. In general, I'm not a fan of gutting an antique item that works perfectly fine. I have it planned out so the Bluetooth can be removed with a few solder joints, and does not impede the original circuitry.
2. I'm not dead set on a 5VDC Bluetooth module. There are others I can get that will take anywhere from 3.6-30VDC. I was just hoping the USB power supply would work well, and they are cheap/easy to come by.
3. As with many of these old tube radios, the tube heaters are wired in series direct to 117VAC. Otherwise I might have attempted to piggyback a DC supply off the heater voltage.

Thoughts? TIA!

 

[Pano]

I've had the same problem with BT. However mine came when I shared a ground with the Class-D amps I used. When using a wall-wart for the BT there was no noise. If I put a signal isolation transformer in the audio line then sharing a ground didn't matter.

The only thing I could get to work was a little 24V to 5V DC-DC convertor. Although the trouble is sort of the opposite, maybe that will give you a little help. Definitely a grounding issue, a tough one.