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[Amps] 3CX800A7 cathode short

Subject: [Amps] 3CX800A7 cathode short
From: Steve London <>
Date: Fri, 15 Mar 2019 13:50:20 -0600
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I converted an Alpha 76PA to a pair of 3CX800A7's a few years ago. It's been a great amp since then.

Last night something strange happened - It started drawing plate and grid current even before the 3 minute warmup period had completed. I'll spare you the troubleshooting details. When the event occurs, the cathode voltage drops to 1 volt, rather than the usual cutoff of 28 volts. Removed the tubes, and all was well. Put the tubes back in, one at a time. Sure enough, it's one of the tubes. That would make it a cathode-to-filament or cathode-to-grid short, right ? Anything possible to clear the short ?

BTW, it was a pair of Chinese 3CX800A7's, sold by Alpha under the previous management.

Steve, N2IC
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