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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