[Arran]

If running tubes with the heaters on, but with no B+ is harmless, then why is it that in any AM/FM set I have encountered, where they use an FM tuner, and the tubes in that tuner have the B+ switched off, are the tubes almost stone cold dead emissions wise? These were in sets where the tubes were all seemingly original, assuming that the tubes were not all replaced at some point, and the tubes used for the AM broadcast, and shortwave bands were all functional. In fact it was suggested in at least one magazine article I read that it would be best to operate such tubes with a reduced B+ voltage, via a resistor, then to switch it off altogether.
I'm not sure where that theory came from that having plate voltage on a tube with the tube heater off would damage the tube, the flow of electrons from the cathode ceases, but there is really nothing stopping one from switching both the B+ and heaters off. Again the front end tubes are not the main current hogs in the radio, the main current draw would be from the tube heaters, approximately 300 ma each, the B+ is in tens of milliamps.
Getting back to the Bluetooth modules, the fellow behind the Electronics Old and New channel often adds these to his German sets, but he pairs them up with a power supply model, tapping voltage off the heater circuits. Basically he uses the tape or phono switching circuits to connect the Blue Tooth, something most German radios have. I'm linking to his odysee channel here, I think Y.T is getting enough traffic they don't deserve: https://odysee.com/@ElectronicsOldandNew:c?view=content
Regards
Arran