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Re: [Amps] SB-220

To: "R.Measures" <r@somis.org>
Subject: Re: [Amps] SB-220
From: Vic Rosenthal <vic@rakefet.com>
Date: Mon, 01 Nov 2004 18:27:13 -0800
List-post: <mailto:amps@contesting.com>
R.Measures wrote:

The high grid-I during a vhf parasite creates a lateral EMF on the hot filament helix. This occasionally bends the filament far enough sideways to short against the grid -- which is grounded through the choke. Since the filament xfmr is connected to the +110vdc power supply on Rx, it shorts the supply to ground, causing a large current flow-- which smokes the choke.

3-500Zs (perhaps certain vintages and manufactures -- I'm not sure) sometimes fail with grid-to-cathode shorts. The short often only appears when the tube is hot, so removing the tube and checking with an ohmmeter does not show it. I have seen this myself a couple of times, and have heard of several other cases.


Without speculating as to the cause of the short, I will say that such a tube is likely to repeat the performance the next time you turn it on. So if the circuitry is such that a g-c short will damage components, it's a very good idea to replace the tube after you replace the components.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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