[Amps] 3CX800A7 cathode short

[Nermin Bajramovic]

Hi Steve, back in 90s our club Alpha 77d have had 8877 cathode to grid short..we use an ‎transformer and "dewelded" short..rejunaveted tube hold for another year, then  we went to pair of 8877..though Steve  be carefull and check more sources.. kind regards, Nermin S58DX

Sent from my BlackBerry 10 smartphone.
  Original Message  
From: Steve London
Sent: Saturday, March 16, 2019 12:52
To: amps at contesting.com
Reply To: n2ic at arrl.net
Subject: [Amps] 3CX800A7 cathode short

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.

73,
Steve, N2IC
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