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[Amps] filament with AC, no CT TSPA

Subject: [Amps] filament with AC, no CT TSPA
From: "John T. M. Lyles" <>
Date: Thu, 2 Jun 2005 10:31:00 -0600
List-post: <>
I believe that your analysis is probably on target, Bill. Would there 
be any cancellation of 60 Hz component of modulation *hum* in the 
output if the tubes were wired so that the AC feeds the opposite side 
of the filement in each? I wonder if you can cause such a 
cancellation in a push pull amplifier if you try. Similar to 
improvement in 2nd harmonic that you would get.

When i built a 100 kW push pull 2800 KHz amplifier here in 1998, at 
first I ordered both filament transformers with no CT. Arrgggh!  I 
considered using a tapped resistor across them, but having over 40 
Amps of cathode current made this difficult. Before we finished 
construction I got new transformers made, from a less expensive 
source, American Magnetics out in Illinois. They had the CT on the 
secondary so I didn't have to worry about hum. These were custom 
460VAc input/15 VAC output transformers, rated about 4.5 kVA. 
However, as you suggest, it wouldn't make it through the RF netorks 
at 60 Hz, but could certainly show up as low level noise sidebands on 
the carrier, depending on the tube design. Basically consider it as a 
small ripple voltage on the Grid bias, where one end of the filament 
is being modulated and the other held fixed. With 120 volts of 
negative bias to get 40 amps of plate current, hum modulation would 
be approx 10%.

Present project is a TH781 tetrode amplifier, 500 kW peak at 200 MHz. 
I tested it last summer with AC filament and one side grounded, no 
CT. The cavity has one conductor of the input circuit grounded, and 
it is a cathode driven linear, grounded grid and screen (for RF 
only). 60 Hz ripple can be seen when sweeping the amplifier with a 
network analyzer (S21) under power. We are going to move to DC 
filament now, as recommended by Thales for the tube. Using a AC/DC 
switcher I think.

The biggest DC filament we use is 5 VDC at 6.8 kiloamps. Yes, thats 
over 30 kW of filament heat. It is in the Burle/RCA 7835 triode, 
which has 96 individual unit electron beam structures radially 
arranged, each running about 70 Amps. We do not reverse the direction 
of current for the life of the tubes, typically 25,000 hours.


>Message: 1
>Date: Wed, 01 Jun 2005 11:52:30 -0400
>From: Bill Fuqua <>
>Subject: Re: [Amps] Heathkit HA-14 filaments
>Message-ID: <>
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>I am a bit surprised that I did not get any response to my email about the
>non-center-tapped filament in the HA14. The two 572B tubes have their
>filaments wired in series one end of the string grounded and the other with
>12.6 VAC applied.  After thinking about it  I realized that since the
>amplifier is operating in a linear mode or class the  60 Hz signal and an
>RF signal would simply be like an extreme two-tone test. The 60Hz would be
>amplified but not pass thru to the antenna and the RF is amplified as
>normal. Except for the intermodulation products due to the odd order terms
>in the characteristic curves. And even that would be insignificant due to
>the fact that the 60 Hz is so much smaller than the RF.  So instead of
>being 30db or so down as in a two tone test with equal level signals it
>would be very much lower.
>    This would  be like Grid Modulation in aClass C amplifier because  the
>second order terms would dominate producing 60 Hz upper and lower sidebands.
>     So, as long as you don't over drive the amplifier into non-linearity
>there should be no 60 Hz sidebands (no 60 Hz modulation of carrier).
>      It would be interesting to put it to the test in some homebrew
>amplifiers. Sometimes our long held assumptions get turned around and we
>have to stop and think a bit.
>Bill wa4lav
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