Crystal tester using a vacuum tube
 

This is more for the shortwave listeners or ham radio operators using vintage equipment, bit it's still radio related. This is a crystal tester built using a 6AH6 tube, and has a meter to display "crystal activity" and an output jack to feed a radio or counter. It pretty much will test any crystal you throw at it, usually it runs at the crystal's fundamental freq. My father built this back in the day, back in the 50's or 60's.
http://www.wa2ise.com/radios/crystaltester.gif

Thanks! I might try that. I have a bunch of used color crytals I should check out.

What's that symbol to the left of the 1K resistor that goes to 120 VAC? It looks like a transistor.

John

That's a diode, like a 1N4007

So the plate supply is only this half-wave circuit? I thought for a minute
that the purpose of the 120VAC was to add some kind of modulation.

Just a plain ordinary power supply. Use your favorite supply to supply 100 volts at something like 20 ma

Guess if I had paid attention to the "VAC" it would have been obvious to me. :o

John