[Test-Equipment] tube question

[Bill Smith]

----- Original Message -----  <snips>
From: bonddaleena at aol.com
To: test-equipment at mailman.qth.net ; boatanchors at mailman.qth.net

Testing several of the tubes, the 'emissions' test was fine, but when
switching to 'leakage', some of the tubes showed very high leakage,
initially. However, the longer I left the tube in the tester, the leakage
came down to an acceptable level.
When testing a NOS spare of the same #, switching from 'emission' to
'leakage', immediately, shows ZERO leakage.

thank you in advance!

ron
N4UE

Hi Ron,

Generally tubes should not show leakage even when they are cold.  What pins
are showing leakage?  Most common is leakage between the filament and
cathode, which in some circuits can be ignored, in others will introduce
hum.  Plate or screen to grid leakage might be more serious.

The silver paint inside a tube is a "getter" material which is highly
reactive and is in place to adsorb gasses, some of which are emitted by the
metals in the evacuated chamber (the inside of the tube).  I have seen power
tubes "improve" with long periods of idle time, filaments on but no B+.
However, gassy octal and miniature tubes generally increase leakage as they
warm up and the instructions recommend leaving the tube in the tester for a
few minutes before making a gas test.

The Hickok 735 features a sensitive leakage test circuit.  I am not familiar
with the Sencor, but have not seen good tubes act the way you describe on
the Hickok.  Perhaps the Sencor is more sensitive.

You might try cleaning the tube bases with alcohol, perhaps the base is
contaminated.  I'd use the tubes for a while and retest them after they have
been in recent service.  You know where to look if the 602 acts strangely.
Or, if you have a good stock of spares, you might just replace them.

Bill