[R-390] Tube Failures

JM/CO [email protected]
Thu, 24 Apr 2003 17:03:42 -0400


Heater to cathode leakage is an insidious problem which the vast majority of
tubetesters out there are not capable of detecting. Even the venerable TV-7
and TV-10 series cannot detect this problem. This is one very good reason
why serious tube people will own ( and be prepared to pay for ) the Hickok
539 series.
Chuck N1LNH
----- Original Message -----
From: "Todd Bigelow - PS" <[email protected]>
To: "Gord Hayward" <[email protected]>
Cc: <[email protected]>
Sent: Thursday, April 24, 2003 11:18 AM
Subject: Re: [R-390] Tube Failures


> Gord Hayward wrote:
>
> > The heater to cathode short is a good one to know about.  I had a
> > humongous amount of hum when I
> > turned the limiter on, but not when it was off.  I eventually found
> > the problem to be the tube.  It tested
> > OK (previous posting about testers with forked toungues is true!) but
> > when I replaced it the problem
> > vanished.  With the high impedance circuit, even miniscule AC leakage
> > into the cathodes was enough.
> >
> > 73 de Gord, VE3EOS.
>
> Surprised it didn't show a short on your tester, Gord. But this is an
> excellent example of why a tube tester is a great tool for getting you
> into the ballpark, but also how/why swapping tubes is still the only way
> to be certain. Even a good tube tester can't give you the critical
> results that the hands-on approach gives.
>
> Someone on the Collins list was just mentioning how a complete re-tubing
> of their KWM-2 was not the great idea they thought it was. Apparently
> two new Phillips 6AZ8s were too hot for the rig and caused oscillations.
> His revised view is to replace one tube at a time, remember where each
> tube came from, and have some known-good replacements to use as
> substitutes. Replacing the Phillips with RCAs cured the problem.
>
> Good stuff....
>
> de Todd/'Boomer'  KA1KAQ
>
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