[Amps] filament supply options

[jeff millar]

Things to think about
- Cold resistance much lower than hot, PS may fold back on overcurrent 
and never light up the tube
- Add some series resistance to limit surge current when cold...For 
example, a 6V supply with 1V of drop across the RF choke
- Alternatively could arrange a soft start circuit to ramp the voltage 
and minimize cold current.
- With DC on the filament, the Grid to cathode bias varies along the 
filament the same way, which causes more emission from one end of the 
cathode than the other.  Some people talk about reversing the filament 
connections periodically to even out the wear.
- Very easy to adjust filament voltage lower for better life at your 
emission needs.

jeff, wa1hco

Steve Jackson wrote:

>Let's say I have a 3-500z which wants 5 volts at 15
>amps for the heater.  
>
>The spec doesn't say AC or DC -- and according to Ohm,
>(given that one is 60 Hz and the other is zero Hz into
>what is essentially a resistive load on the other side
>of a BIG choke) it doesn't matter.  
>
>Watts is Watts.
>
>I have a 5 volt, DC, 20 amp, industrial-grade
>switching supply.  It cost $2 and weighs about 3 lbs. 
>Power-factor-corrected, auto-ranging 90-255VAC input.
>
>What do you say I use the switcher, instead of the
>$58, 22 lb. transformer that would provide the same V
>and I ... with poorer regulation, etc. ?  
>
>Before someone says something about switchers and RF
>noise, etc., I checked this one under load and it
>doesn't put out anything I could hear.  It also
>provides about 4500v isolation to the AC input, and it
>didn't flinch when I subjected it to a strong RF field
>at 14 and 28 MHz.
>
>What's wrong with this picture?  Why aren't more
>people going this route when building?
>
>
>Steve KZ1X/4 ...
>
>... who is also looking pretty seriously at that HV
>switcher from Watts Unlimited
>
>
>
>
>
>	
>		
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