## 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@t-online.de
Sent: Friday, February 7, 2020 9:43 AM
To: 'Jim Thomson'
Cc: amps@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@contesting.com] On Behalf Of Jim Thomson
Sent: Freitag, 7. Februar 2020 18:37
To: amps@contesting.com
Subject: [Amps] 3CX3000A7 on 144 MHz
Date: Fri, 7 Feb 2020 11:35:34 -0500
From: MU 4CX250B <4cx250b@miamioh.edu>
To: Steve Thompson <g8gsq72@gmail.com>
Cc: amps@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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