[AMPS] Adjusting fil voltage

[Rich Measures]

>Simple yet so valuable, this information. This is the opposite of the guys
>who want to squeak just 50 watts more by using a variac on both HV and
>filament to RAISE the voltage!
>Controlling filament voltage (via the primary) is a worthwhile endeavor. It
>could make your tubes into "lifetime" tubes.
>
>I can't imagine why anyone would do this in the secondary?
>
€  When the amplifier has a combination hv / fil. transformer, like the 
SB-200 and 2KD-5, such resistors must go in series with the fil. sec. 
winding.  

>I used this method (primary) with a 25 ohm 50 watt wirewound reostat in the
>primary of my old Henry 4K to lower the filament voltage on an expensive
>5CX1500A tube.

€  The reported filament inrush  current in the Henry 4K was 400 peak 
amperes.  Did you add a step-start? 

>Actual numerical voltage value is not that important, I just turned it down
>to where power output dropped off and went back up about .1 or .2 volts
>(reading secondard voltage).

€  This is the right way to adj. the fil. potential for a thoriated 
tungsten cathode.  One really doesn't need an accurate fil. voltmeter.  
Howeven, this method can destroy heater/oxide-cathode type tubes.  

>This also acts as an inrush current limiter as a bonus.

€  ... only when the rheostat happened to be set at max. R at startup.  . 
 A step-start  always limits fil. inrush current.  
-  later


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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