[Hallicrafters] Testing tubes

[John Brooks]

Barrie,

The filament flash at turn-on is common for Mullard
and Amperex tubes in the 12A__ series (12AX7, 12AU7,
12AT7) in particular.  It does not hurt the longevity.
 Mullard and Amperex are also arguably some of the
finest-sounding tubes for audio.  I have heard reports
that is not unusual for these to test higher than 100,
maybe even as high as 120 for transconductance.  Not
sure how to expand the range of your tester to
accomodate this, though!

Your problems may not be in the tubes.  I have some
experience as a tube-audio hobbyist.  This is probably
a little OT for a radio group, but I will try to help
you if you want to write to me directly.

Regards,
John
KG4JZT

--- 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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> assistance**
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