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[Amps] 3CX800 - a science experiment

Subject: [Amps] 3CX800 - a science experiment
From: Steve London <>
Date: Fri, 31 May 2019 08:37:56 -0600
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As I have previously mentioned, I have two Alpha 3CX800A7LG's that appear to have grid-cathode shorts that develop within a few minutes of applying filament voltage. After seeing VE5RA's posting about shorts clearing themselves after 6 minutes, I did some experiments with the two tubes. I hope someone can explain the physics/engineering for what I am seeing.

Apply 13.5 DC to the filaments. No other voltage applied to any pins. Initially the filament-grid and filament-cathode voltages are zero. Also, the grid-cathode voltage is zero.

As the filaments warm up, the filament-grid and filament-cathode voltage rises.

After a 20 minute warmup, I have these voltages:

+ filament to grid 9.5
- filament to grid 0.5

+ filament to cathode 8.9
- filament to cathode -2.9

The grid ring is extremely hot.

What's going on in the tube ?

Steve, N2IC
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