Precision 10-12 "Tube Master" Tube Tester (1947)

This handsome Precision Model 10-12 "Tube Master" tube tester looks like new and works like it, too.

I bought it for $10 from a guy whose basement was crammed with tubes—thousands and thousands of tubes. Somewhere in his travels he had come across this tester, which he didn't want at the moment. He obviously didn't need it, either. Also stored in the basement were a couple of dozen other testers.

Housed in a blonde wooden cabinet, this tester has enough knobs, switches, buttons, levers and sockets to put Dr. Frankenstein on Cloud Nine. Visible through windows on the bottom are the paper roll charts that tell you how to set up the tester for hundreds of different tube types.

Stowed in a little compartment in the top you can see a Precision G-140 adapter, which allowed you to test tube types that were novel at the time: nuvistors, novars, and compactrons.

Here is the operating manual for the Precision 10-12 and 10-15 testers:

I use this tester once in a while because it can handle older tubes as well as more recent ones. The next photos show the tester in action, checking an 807 tube, a five-pin type used in my 1947 DuMont RA-102 television/radio.

If a given tube type doesn't appear in the built-in roll charts, you can look it up in a separate chart of obsolete and oddball tubes. The following links let you download PDF versions of both charts, which were compiled by Wayne Hertel:

Precision 10-12 roll chart tube data

Precision 10-12 obsolete tube data

Calibration is the Achilles heel of vintage test equipment, but this one compares reasonably well against my newer Sencore tester.

All tester results must be taken with a grain of salt, of course. It's common for two testers of the same model to give slightly different results, and certain tube functions, such as oscillation, can't be accurately evaluated by any tester. I mainly use tube testers to get a basic go/no-go reading. The best tester of all is a working circuit for which the tube was designed.

If you don't own a tester, or you have a tube type not covered by your tester, you can make a basic dud/not-dud test by checking the tube's filament with an ohmmeter. See First Steps in Restoration for details.

This schematic is provided for any 10-12 owner who needs to service the tester:

To download it to your computer, right-click on the icon and choose Save Target As.