[Amps] filament voltage question

[Radio WC6W]

On Wed, 27 Oct 2004 12:34:22 EDT TexasRF at aol.com writes:

Hi Gerald,

> I am looking for some advice on adjusting the filament voltage on a
TH347  
> tetrode used in my 1296mhz cavity amplifier. The TH347 is a directly
heated  
> cathode tube.

<snip>

> Does anyone out there have input on the effect of elevated filament
voltage  
> during periods that the cathode is not emmiting? In other words, is
tube life  
> compromised during standby periods or is it only for operating periods
that  
> matter?   

   Yes, standby time counts!
  
> Adding a filament voltage regulator is complicated by the 34 amp
current  
> requirement for the tube.

   Not necessarily.  

   One could build a simple primary side regulator using a suitably
heatsinked power FET, as the control element, connected across a diode
bridge placed in series with the filament transformer primary.   The gate
of the FET may be driven via an optoisolator from a TL431 connected in a
commonly found secondary sense circuit.  

> It would be fairly easy to switch in a small value  
> resistor at the filament transformer primary to reduce the
receive/standby  
> filament voltage but I wonder if this is a viable solution?

  That feature would be an easy addition to an electronic regulator, as
would a soft-start.

  This almost sounds like something that Ian should add to his product
line.  :-)
  
> Any comments/suggestions are appreciated.
>  
> Thanks/73,
> Gerald/K5GW

You're welcome & 73,
   Marv  WC6W 









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