[Amps] Regulated filament current

[Saandy]

...definitely!
12.6Vdc is just an indication that the correct amunt of current is flowing there. Don't forget : what woks is P=I^2 and this  defines the temperature. Even the Volt is in essence derived from a current function.
While on the subject: modern switchers don't.give a damn whether they start in current mode or in voltage mode so that is a moot point.
What is more important,is that you can use DC heating only on indirectly heated tubed, which are intrinsically more tolerant about heating voltages than directly heated  types
At Tadiran , many.years 
ago, we modified kW amplifiers for the GRC -106 to regulated heating voltage foe a pair of 4CX350, reasoning that in vehicular use, it would improve something, lifetime or performance... It didn't do anything when the operating voltages ran from 22 to 32 volts. The only bright point was that it saved filaments if the vehicle's generator's voltage regulator  gave out. The voltage could the rise to 40, 45 volts.
Worth it on a mains based amplifier with a ±10% variation?.....



-------- Original message --------
From: Steve Wright <stevewrightnz at gmail.com> 
Date:  
To: amps at contesting.com 
Subject: Re: [Amps] Regulated filament current 
 


On 05/09/15 04:00,

nemo zilch <nemozilch at gmail.com>

  wrote:
>  My inclination is to adjust the current so that the
> voltage across the tube is 12.6 volts.  Does this seem like the correct
> thing to do?
>

I was looking at similar using a programmable switching supply.  Once
the tube warms up fully, the current will normally drop off - except in
your case it won't be doing that.  Also, if "other" undefined RF
conditions cause the heat in the tube to drop then I think you might
discover some uncharted territory the tube designers never imagined.

Constant current heat-up time will be longer than constant voltage as well.

Any it has to be asked, why?  Just simply cold-start the filament in the
traditional manner.  It's robust and inexpensive, and it doesn't provoke
the designers into adding another chapter to their do's and don'ts for
this device.


Steve

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