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Re: [Amps] 3CX3000A7 on 144 MHz

Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: Steve Thompson <>
Date: Sat, 8 Feb 2020 18:13:33 +0000
List-post: <>
Looking at the constant current curves, DC filament supply with a centre tap probably won't be a problem, maybe 20% variation in emission from end to end.

I still have some radios with 1.5V filament battery valves/tubes and more earlier ones with 2V DC filaments; they were specifically designed to run with dc powered filaments which might be a bit of difference from 3CX3000.

I'm curious, why does the low HT voltage make the emission variation worse? I'd always assumed it's the filament to grid potential which determined it.

Steve G8GSQ

Roger wrote:
Jim w8zr wrote:

"Seems to me this problem may  not be so troublesome......"

I have to agree with Jim on this, for various reasons:

There is no reason why a DC filament supply should not be centre tapped, most simply with a resistive bridge;

The bias on the tube is related to the average of the emission over the whole of the filament, and this is static with a DC filament supply;

Those of us who are old enough will remember battery tubes (such as the 1T4 and many others). These (obviously) had a DC filament, and significantly an HT voltage which was sometimes as low as 22.5V. Any problems due to a voltage gradient along the filament would likely be much worse than with a power tube with several kV on the plate. Yet they worked;

And most significantly. I cannot find any recommendation by Eimac or other tube manufacturers against using DC filament supplies. This is despite their specifications going into great detail about other matters such as permissible filament voltage variation.
73 Roger

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