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Re: [Amps] 3CX3000A7 on 144 MHz

To: MU 4CX250B <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: Steve <>
Date: Fri, 7 Feb 2020 18:09:17 +0000
List-post: <>
What you say is true with respect to differences due to plate current but, from the grid's point of view, one end of the filament is at +7.5V with respect to the other end because of the filament PSU. I figure that's going to affect where the emission takes place.

Steve G8GSQ

On 07/02/2020 16:35, MU 4CX250B wrote:
      Seems to me this problem may  not be so troublesome. The dc
resistance of the hot filament is 7.5V/51A=0.15 ohms. If the B- return
is on one side of the filament, and if we assume the plate current is
1ADC, then the maximum DC voltage drop across the filament would be
only .15V
     Furthermore,  the filament emission is distributed across the
entire length of the filament and not just at one end, so the average
resistance is actually less than .15 ohm. What this means is that the
variation in operating bias from one end of the filament to the other
by using a switch mode power supply is at most a tiny fraction of a
volt. For practical purposes, I think it could be neglected, which
means a user would see no difference in performance, compared to
using a center-tapped filament transformer.
Jim w8zr

Sent from my iPhone

On Feb 6, 2020, at 3:52 PM, Steve Thompson <> wrote:

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 

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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