[DigitalNoMore]

I have a masters degree in electronic engineering and before I retired worked as a principal design engineer and design team leader for a large telecommunications company so can't possibly know what I'm talking about of course.

All I'm interested in is passing on advice to people if I can. Not trying to play the smart a**e like some people on here it seems.

Regarding guitar pickups, the output voltage can vary dramatically.
Single coil pickups such as on my Stratocaster typically output well under 100mV and so will be unable to directly drive a valve radio PU input sufficiently to get much usable volume. I know, I've tried it.
Humbucker pickups such as on my Les Paul typically output at least twice as much as single coil and are more successful but still less than ideal.

Just for completeness, early crystal phono cartridges, which a valve radio PU input would be designed to accept, would typically output as much as 1-2V.