[Amps] 3-500Z tube problems

[Vic Rosenthal]

I have seen a few of these in genuine Eimacs from a specific period. I don't recall the date codes, but I'm sure someone here knows. The problem was bad welds which allowed the grid assembly to move and touch the filament.

In the cases that I had, the short only appeared when the filament was hot. I doubt that this could be cleared by an electrical procedure, but maybe a slap on the tube in the right place would work, at least temporarily.
-- 
Vic 4X6GP

On June 11, 2016 7:55:55 PM GMT+03:00, "Jack C. Shutt" <w9gt at frontier.com> wrote:
>Hello all.  I don't post here very often, but always enjoy reading the
>posts and comments about the experiences of others.I'm looking for some
>info and advice on 3-500Z tubes.  I have used these tubes for many
>years in several home brew amps, as well as in a few commercially-built
>units.  I recently have had some bad experiences with bad tubes (fil to
>grid shorts?) resulting in the loss of some expensive components such
>as the fil/bias transformer, grid meter, and bias circuitry consisting
>of hard-to-find high power zener diodes, etc. I'm wondering if anyone
>has had any success in reviving bad tubes with shorts.  Is there a
>procedure that can be used for clearing these shorts?  Also wondering
>about design additions that you may have included in amps to protect
>the circuitry from catastrophic failures.  I have experienced failures
>of Eimac tubes and Chinese versions. Looking forward to your comments. 
>73,  Jack, W9GT
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