[Amps] Fil transformer

Fuqua, Bill L wlfuqu00 at uky.edu
Fri Aug 11 02:14:59 EDT 2017

    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.

Bill wa4lav
From: Amps <amps-bounces at contesting.com> on behalf of Jim Thomson <jim.thom at telus.net>
Sent: Friday, August 11, 2017 1:01 AM
To: amps at contesting.com
Subject: [Amps] Fil transformer

Date: Thu, 10 Aug 2017 07:08:54 -1000
From: Merv Schweigert <k9fd at flex.com>
To: amps at 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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