[Amps] 3CX3000A7 on 144 MHz

[Jim Thomson]

##  That  sorta  works. 2 x 5  ohm  resistors  results in  .5  amp  draw 
from  the  fil  xfr.
2.5  watts  total,  1.25  watts per  resistor.   800  ma  plate  current   + 
300ma  of  grid  current divides..with  550  ma
flowing  into  each  half. That’s  another   1.51 watts  per  resistor. 
It's  also  2.75  volts  of  additional
bias...  per  resistor,  5.5 volts  in  total  of  yo-yoing  bias.....  in 
addition  to  normal  5-7  volts of  regulated  bias.

##  he  shoulda just  removed  equal  turns  from each  end.  New  hammond 
5v  @ 30A ..... or  7.5 V  @  21A  fil  xfmrs  were  dirt  cheap  back then.

##  Plan  D  .......esp  if  the  fil  xfr  does  not have a CT, is  to 
wire a 5V- 12Volt  @ 3-5A  CT  xfmr  across  the  oem  xfr....  then  use 
the CT  of  small
xfmr as  the  artificial CT.    And  also  tape  up  the  Un-used 120 v 
leads  on  the  small  xfr.

##  2 x 2 ohm  resistors  would  work  real good.   (5-10  watt  rated)

##  I  used  to know  why  the 60hz  hum  appeared  with  an  un-balanced 
CT,  but  have  forgotten  the tech  explanation.   Does  anybody  remember 
???

Jim,   VE7RF



-----Original Message----- 
From: dj7ww at t-online.de
Sent: Friday, February 7, 2020 9:43 AM
To: 'Jim Thomson'
Cc: amps at contesting.com
Subject: RE: [Amps] 3CX3000A7 on 144 MHz

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.

73
Peter


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

Date: Fri, 7 Feb 2020 11:35:34 -0500
From: MU 4CX250B <4cx250b at miamioh.edu>
To: Steve Thompson <g8gsq72 at gmail.com>
Cc: amps at contesting.com
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.
<73,
<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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