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Re: [Amps] 3CX800A7 cathode short

Subject: Re: [Amps] 3CX800A7 cathode short
From: George Hall via Amps <>
Reply-to: George Hall <>
Date: Sat, 16 Mar 2019 20:27:55 +0000 (UTC)
List-post: <>
Hi Steve,

If you have access to a Hi-Pot tester it should be very easy to tell which tube 
or tubes are faulty by testing for filament to cathode; filament to grid and 
cathode to grid shorts.

73, George N2CG
On Saturday, March 16, 2019 Steve London <> wrote:
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 
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 

Steve, N2IC
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