[Amps] 3CX800A7 Grid Current

[n4is at comcast.net]

I switched to a pair of 3CPX800 20 years ago and they are full power today. The 3CX800 is sensitive to plate load.  The way to tune is to  increase the load ( open the capacitor) and drop the grid current, peak again the plate capacitor to max output and repeat the process, increase load to reduce grid current and peal plate capacitor, you  should find a load good for max power with lower grid current, max 50 mA per tube, my tubes run at 20 mA grid current each for full legal power.
My two cents,

73'
JC
N4iS

________________________________
From: Amps <amps-bounces at contesting.com> on behalf of Dennis W0JX via Amps <amps at contesting.com>
Sent: Saturday, August 17, 2024 12:02 PM
To: amps at contesting.com <amps at contesting.com>
Subject: Re: [Amps] 3CX800A7 Grid Current

I have an Ameritron AL-800H here that uses 2 of these tubes. It is a great idea to measure the grid current vs output of each tube if you can. I don't know enough about the control circuitry of the Alpha 86 and if it will allow you to do this before kicks off line.
In the AL-800H, a grid current of 150ma (approx 100 watts drive) produces the maximum rated grid dissipation of 4 watts per tube and will disable the amp via a safety circuit within miiliseconds. The operating manual says "never exceed 60 ma of grid current on CW carrier." I think these parameters would apply to the Alpha 86 also.
When I received this amplifier from an SK estate, the tubes were already quite old. After a few years, the main line fuses began to blow. The tubes began arcing over. Another friend gave me a set of soft 3CX800's from his Alpha 87. They lasted about 6 months before they too began flashing over. So you can expect that if your tubes are soft, and need a lot of drive, sooner or later, as they approach end of life, they too will begin arcing over and either blow your fuses or the glitch resistor or both.
73 Dennis W0JX
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