[Hallicrafters] Re: Testing tubes

[Barrie Smith]

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.

Possibly, but the tester has a comprehensive group of indicators for shorts,
and no short was indicated on any of the tubes.

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?

Yes, all the tubes survived and have been tested several times, always with
the same results.  All but one will function  in three different pieces of
audio gear.

I replaced all the suspect tubes in the Luxman CL35 preamplifier with NOS GE
12AX7s and it now sounds much, much better.  These GE tubes are the ones
that DID NOT glow brightly upon startup, and tested about 85 to 95%.

I've tested certainly many hundreds oftubes, some old, some new, in this
Western Electric version of the Hickok 1234, and have never seen any tube
test higher than 90%, until this batch of 12AX7s that came with the preamp.

Granted, it's quite likely that I've never tested Mullards in the past.

An oddity, perhaps:  There was one Mullard 12AU7 in the preamp.  It does not
lightup brightly, and tested about 80%.  I've always believed that there was
very little mechanical or electrical difference between the AU7 and the AX7,
or, for that matter, the AT7.  There is a considerable difference in the
amplification factor.

The preamp sounds great with the NOS GEs in, so in they will stay!

73, Barrie, W7ALW

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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