A method for screening large quantities of 7 and 9 pin miniature tubes

[wa2ise]

If you have a few boxes full of a hundred or more miniature tubes, here's a quick method I used years ago to quickly weed out the bad ones:

Using an emissions tube tester like this one below

Note that it has twist knobs, not rows of slide switches for the tube pins. Start by sorting the tubes by heater voltage. You'll probably have a big pile of 6,3V tubes, so set the heater voltage to 6.3V on the tester. I can't recall if I needed to set a heater selector to pin 4, this covers both most all 7 pin and 9 pin tubes, but anyway... Set the "sensitivity to around "40" as some tubes call for "30" and some "50" so I split the difference. Insert a tube, wait for it to warm up, and rotate the meter selector knob to do leakage. You'll find a short indication on the heater on pin 4, that's normal, ignore it. Assuming no other shorts then set the slide switch to emissions and rotate that knob to find the cathodes (or grid 1's). The meter may go backwards on cathodes, but give a normal reading when you find the grid 1's. You can see thru the glass of the tube how many cathodes there are. Occasionally you'll have a tube that has multiple grid 1's sharing the same cathode, but those aren't real common. Anyway, bigger cathodes would usually make the meter move further. Strong tubes will all get past the meter mid-point. Set aside the tubes that don't seem to cut it aside for further review later.

Sure it's crude, but you can spot obviously bad tubes quickly. and screen tubes without looking thru the booklet.