Crystal tester using a vacuum tube

**wa2ise** · Sun 20th Dec 2009, 03:21

My father WB2JIA built this way back when, in the vacuum tube era. It works with about any crystal you toss at it. Has a meter to display "crystal activity" and an output jack to feed a frequency counter or other device that can accept a low level RF signal. This may have been published in QST or such magazine, but I don't really know where my father got it, or maybe he designed it himself. He's a silent key, so I can't ask. Uses a 6AH6 vacuum tube, and you'll need about 100V supply. The power supply consisted of a "plate transformer" that produces 120VAC, and in series a 1K resistor and rectifier, then a filter cap to ground. When no crystal is inserted, the supply sags down to 90V. Don't know if that's by design, if it's necessary for it to work. I don't think it's necessary, but maybe the original design specified a selenium rectifier, which tend to be lossy, and my father substituted a modern rectifier diode and used the 1K resistor to lower the B+ voltage to spec. oops, spotted an omission, the trimmer from the tube's control grid to ground. Not sure what it does, though; maybe it helps the osc circuit oscillate? Updated the diagram.

**KM3F** · Sun 1st May 2011, 05:06

You have a common form of a vacuum tube oscillator ckt. with some metering and controls to see relative circuit action and a jack to look at the frequency.
No doubt the controls are to see how far circuit parameters can be taken before the crystal stops vibrating internally.
There is more to this than meets the eye once you get into how the crystal is cut, what type of ocillator circuit is used and the application etc.
If you check a good handbook you will see at least several types of ocillator circuits that can be used with a crystal both in vaccum tube and transistor versons.
Another parameter is the cut of the crystal for use in an 'overtone' mode.
That is, a 20 mhz crystal of the right cut will oscillate and be "harmonic rich" at 2 and 3 or more times it's fundamental. Here is one instance activity may want to be known on a older crystal.
For example, 50 MHZ crystal ocillator who's output circuit is tuned to a mutiple of the crystal frequency will output that higher frequency but of course with less power.
Other parameters are how far can it's frquency be moved for equipment calibration and how temperature stable it is.
This is a whole specific subject once you get interested.
Good luck.