An inside look at a tube tester

This is just a very quick look inside a tube tester, a TV-7D/U to be more specific.  I recently listed this one on eBay and naturally got some inquiries that resulted in my doing some further checks to verify its operation and answer the bidders questions.

Most tube testers work in pretty much the same fashion, and you may remember using one at the drug store to check radio and TV tubes.  Some models had different features and degrees of automation…but they all pretty much do the same thing.

They basically consist of a power supply, sockets to accommodate an assortment of types, a bunch of switches to set the various test parameters for each type, and a meter to show the transconductance–sometimes simplified into a “Good-Bad” range rather than actual numerical values.  Not much to it, really.

The user would look up his or her tube type on a chart (in the book under the lid in the case of this one) and set the filament, grid, screen, plate, bias voltages, and metering ranges as shown in the chart for that type…then insert the tube to be tested, and press the buttons to check for shorts between elements, followed by the actual function test.  If you had a good one, you stuck it back in the set….if bad, you bought another to put in.  This was standard practice in drug stores across the nation even into the late 1970’s.  I remember checking many radio and tv tubes this way.

Eventually I ended up having need of my own tester….the ones in the stores were long gone…and I still run enough tube gear to justify being able to check a tube here and there.  It is also handy for when I end up with a box of tubes and want to do a quick sort of which to keep and which to discard.  (yes, in my world, boxes full of vacuum tubes still come my way)

A tube tester will tell you if a tube passes a given set of test parameters, but it may not tell you if that tube will actually perform correctly in your application.  So they are not always fool proof.  Nonetheless, a good tube tester can save some time in weeding out obvious duds from the pile.

This particular tester, the TV-7D/U is a surplus military model.  They are regarded as being among the best ever made.  Period.  The current sale prices they command reflect this too.  I put this one on eBay….reluctantly…but I am sure it will get to a good loving home.  And I am not without a good tester, as I have a WWII vintage I-177 model that I use as well.  It was a tough call to decide which to keep.  Painful, really….as I alluded to in my last post (TMS…) I made the decision based on the year of vintage.  The I-177 is pre-1950 and the TV-7 is a late 50’s/early 60’s model.  So I am letting the TV-7 go.  But since I had taken these photos this morning I figured I’d share them here….just in case you ever wondered what was inside that tester down at the drug store when you were a kid.