[AMRadio] Testing tubes

[GGLL]

Hello Barrie:

Barrie Smith escribió:

>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.
>
Which scale is this?, plate mA, relative performance?

>
>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.
>
I have a few of them with the same behaviour, I figure it's because the 
heater is a very low resistance at startup (cold), and so there's a high 
inrush current when heater voltage is applied, bringing them nearly 
white for a short period. These work fine anyway.

Good luck
Guillermo - LU8EYW.

>
>The new tubes that I tested did not exhibit that characteristic.
>
>Any thoughts on this subject?
>
>73, Barrie, W7ALW
>
>
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