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@contesting.com> on behalf of Dennis W0JX via Amps
<amps@contesting.com>
Sent: Saturday, August 17, 2024 12:02 PM
To: amps@contesting.com <amps@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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