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Re: [Amps] Heathkit HA-14 filaments

Subject: Re: [Amps] Heathkit HA-14 filaments
From: Bill Fuqua <>
Date: Wed, 01 Jun 2005 11:52:30 -0400
List-post: <>
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

Bill wa4lav

At 12:35 PM 5/31/2005 -0400, Joe Giacobello wrote:
>Bill, I once used one for about a year.  I never got any reports of a
>bad CW note nor did I detect any problem when I personally monitored the
>73, Joe
>Bill Fuqua wrote:
> >     To my surprise I found that the filaments of the HA-14 were not driven
> >by a center-tapped AC filament transformer when using the AC power supply.
> >I can see that this can be a real source of 60 Hz modulation on a
> >signal.  Has anyone used these for CW using the AC power supply?
> >
> >73
> >Bill wa4lav
> >
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