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Re: [Amps] 3CX3000A7 on 144 MHz

To: "'Jim Thomson'" <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz
From: <>
Date: Fri, 7 Feb 2020 18:43:14 +0100
List-post: <>
He should have used an artificial center tap.
Two low ohm resistors in series across the heater winding, between them
connect the ground, works very well.


-----Original Message-----
From: Amps [] On Behalf Of Jim Thomson
Sent: Freitag, 7. Februar 2020 18:37
Subject: [Amps] 3CX3000A7 on 144 MHz

Date: Fri, 7 Feb 2020 11:35:34 -0500
From: MU 4CX250B <>
To: Steve Thompson <>
Subject: Re: [Amps] 3CX3000A7 on 144 MHz

<     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

##  electrician buddy  across  town  re-wound a 24 volt  xfmr....and  used
new 10  ga  wire.
5.6  volts....  loaded  with  30A.....for a hb  2 x 3-500Z  amp.....complete
with  CT.

##  worked  good,  but loaded  V  at  the  cathodes  was  way  too  high.

##  he  removed  turns  from  ONE  end  only,  instead of  equal  amount  of
turns  from  both  ends ! 
That  effectively put  the  CT  off  to  one  side a bit.   We  could all
hear  the  60hz  hum  across  town  on  75m  ssb.
Go  figure. 

Jim  VE7RF 
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