[AMRadio] Ferroresonant transformer revisited

Donald Chester k4kyv at hotmail.com
Wed May 10 22:22:00 EDT 2006

>oh, yeah, you betcha!  I've seen a few Sola filament transformers at 
>different hamfests.  "heavy duty" comes to mind, as the best way to 
>describe 'em.

I have a very "light duty" one to run my 75A-4.  Even a slight change in 
line voltage, < 1v, will cause the PTO to drift frequency, due to the change 
in filament voltage (B+ is regulated).  I found a Sola CVT rated at about 
100 v-a at a hamfest.  About the size of your fist and weighs maybe 3-4 lbs. 
  It was a modern unit, designed to supply power to a PC.  Unlike most of 
the older version, it is almost completely quiet; I have to really listen 
carefully to hear it.  It does run hot, but that's normal for those 
transformers.  My complaint has always been that they sound like a chainsaw 
running.  With that little xfmr, the A4 is stable as a rock.

I use a T-368 PTO as my main station vfo.  I stabilised it by  running the 
filaments off a regulated DC supply.  Also use it to run mic preamp.  Now 
have stable VFO and hum-free microphone preamp.  I'm not sure why a small 
fraction of a percentage variation in filament voltage will cause such 
noticeable drift.  Without the regulated filament voltage, neither the A4 
nor the T-368 pto are stable enough to work 40m cw using a narrow filter.

Don k4kyv


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