Karl-Arne Markström wrote:
>The principle used in the K4IPV power tube tester is very simple.
>
>Testing the tube involved providing it with proper DC potentials, and
>inserting a small and known AC voltage in series with the control grid bias.
>
>The corresponding AC current in the plate circuit was sensed with a
>series resistor or current transformer and the transconductance
>was calculated as gm = AC plate current/AC grid voltage.
>
>If the AC voltage was wisely chosen, the AC current indicator could
>be calibrated directly in gm.
>
>When a tube reaches the end of its lifespan, the gm decreases, which is
>a sensitive indicator
>for the tube condition.
>
>At SK6AB in the early 80's I was involved in constructing a makeshift
>tester for i..a. 4-1000A's
>along these principles. It required some careful attention to
>high-voltage precautions.
Along with these functional tests, has anyone looked at checking the
vacuum quality by configuring the tube as an "ionization gauge"? Someone
told me about this technique for checking small audio tubes, but it
ought to translate to larger tubes as well.
There are some hints about the technique on the web. The idea is to
operate the filament/cathode at ground potential and connect the grid to
about +250V so the cathode and grid act as a diode. In a real ion gauge,
the diode current is limited to a few milliamps by controlling the power
to the filament.
Meanwhile the anode is operated at about -20V (minus twenty, not a typo)
to collect the current from the positive gas ions, which would be
microamps at most. The more gassy the tube, the higher the ion current.
It doesn't seem too difficult to try, and gives a different viewpoint
from a hi-pot test.
--
73 from Ian GM3SEK
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