I once used another but much smaller center-tapped filament transformer
with its secondary in parallel with the much larger one.
Primary was not used. This provided a low resistive path for the cathode
current and the inductance of the small transformer's secondary canceled out
for common mode currents. I no longer have that homebrew amplifier but it
worked very well for me.
Very little heat dissipated as well compared to using a pair of low value
resistors.
73
Bill wa4lav
________________________________
From: Amps <amps-bounces@contesting.com> on behalf of Jim Thomson
<jim.thom@telus.net>
Sent: Friday, August 11, 2017 1:01 AM
To: amps@contesting.com
Subject: [Amps] Fil transformer
Date: Thu, 10 Aug 2017 07:08:54 -1000
From: Merv Schweigert <k9fd@flex.com>
To: amps@contesting.com
Subject: Re: [Amps] Fil transformer
<How about the old time trick of putting two low value resistors to
<ground, one from each side of the
<filament, used to be used for a center tap many times in days of old.
## This isnt 1956 anymore. The B- has to float above ground a bit on a gg
triode amp, in
order to measure both dc plate + dc grid current.
The two resistors would have to be wired in series....across the sec of the oem
fil xfmr.
The junction point of the resistors becomes your new CT.
## two resistors in series will work, provided they are each real low value.
If say
a pair of 10 ohm resistors used in series...and perhaps 600ma of plate current
flowing,you will
end up with 300 ma flowing through each of the 10 ohm resistors. 10 ohms X
300 ma = 3 volts.
Thats another 3 vdc of unwanted bias on each side of the fil, yo-yoing up and
down as you talk on ssb.
Probably no big deal, moot point. You could also put a small electrolytic
across each resistor. That
would provide for a near constant 3 vdc, as both caps try and charge up when
the plate current peaks.
## Its also .9 watt dissipation in each resistor, so if u go that route,
use a 3 watt mof resistor.
## I cant fathom why dentron could miss such a fundamental concept. No
surprise really. I had one of
their dentron 160-10L 4 x 811 amps back in 1970. Its was a dogs breakfast
right out of the box, new.
Jim VE7RF
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