[Hallicrafters] Re: Testing tubes

[Neil Barnett]

Barrie

Most repondents seem to think the filament flash
is normal for a cold tube at switch-on. I agree,
but I don't think that's what you're reporting.

You state that it is the *cathode* lead which gets
hot. I suspect that the faulty tubes all have heater
to cathode shorts, and you are seeing heater
current flow into the cathode circuitry of the tester.

This would explain the anomalous meter readings.

You don't state whether the tubes survived their
time in the tester. Did they still work afterwards,
or did the tester actually burn them out?

73 de Neil, ZL1ANM
Auckland, New Zealand.


--- Barrie Smith <barrie at centric.net> wrote:
> Attempting to discover why an old Luxman
> tube-preamplifier didn't sound too
> good, I drug out the Hickok Cardmatic tube-tester
> this morning.
> 
> I noticed that 6 of the 7 12AX7s tested off-scale
> (to the right, above 100).
> New 12AX7s that I tested ran from 80 to 90 on the
> scale.
> 
> Is there a problem with an off-scale reading?
> 
> There was no indication of leakage or gas in any of
> the tubes.
> 
> Also, all of the tubes that tested off-scale had
> another characteristic:
> Upon powering them up in the tester, the wires in
> the base that evidently
> connect to the cathode lit up bright red to almost
> white for a short period
> of time, perhaps a second or two.
> 
> The new tubes that I tested did not exhibit that
> characteristic.
> 
> Any thoughts on this subject?
> 
> 73, Barrie, W7ALW
> 


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