[Test-Equipment] tube question

[J. Forster]

> 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).

Not quite. Most getters react with ions, not neutral atoms. For a getter
to function well, you need both filament and plate and have the tube
biased to carry about 5 to 10% of it rated plate current. The flow of
electrons from cathode to plate ionizes the gas atoms and they then react
with the getter material.

If you go higher on the plater current, you will likely 'poison' the
cathode and the Gm will suffer.

Best,
-John


 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