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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