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Re: [Amps] 3CX3000A7 on 144 MHz

Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: Steve Thompson <>
Date: Thu, 6 Feb 2020 22:52:59 +0000
List-post: <>
DC switch mode PSU is not good with directly heated valves such as 3CX3000, 3-500Z etc. One end of the filament ends up with different bias to the other so all the emission happens at one end. It might be ok if you reverse the polarity regularly.

With a transformer with centre tap, the average bias voltage is the same everywhere on the filament, so the emission happens evenly over the whole length.

Steve G8GSQ

I have just acquired 2 x  3CX3000A7 and I would like to put one on 2m.

I have a few designs for but before I start I have 2 questions.

The Heater current is 7.5V @ 51A - does anyone use a switch mode PSU? Not sure if Meanwell have anything suitable but that is where I would probably start. Otherwise I have no idea where to get a custom transformer wound in the Netherlands, which is where I live these days.

I do not want to run in AB2 all the time am I correct to assume that I can just switch in a Zener with a pass transistor to bias to class C? I have never used a zero bias tube before.

I could use a GM3SEK triode board of course!

I already have a 4.5kV supply that will drop down to 4.2kV on load, if anyone is using a 3CX3000A7 on VHF I would also like to know how much gain to expect. I do not expect the 144MHz performance to differ from that of 110 MHz based on observations with a YC-156.


Conrad PA5Y

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