[ARC5] [Milsurplus] Fw: Cool New DC-DC Converter

[Kenneth G. Gordon]

On 15 Mar 2018 at 17:25, Jim Whartenby wrote:

> It seems that there is no simple answer to this question.  Depending on the 
> impedance of the power transformer, that is the equivalent series resistance seen 
> by the tube filament, up to ten times the rated filament current can be drawn on a 
> cold startup.  In other words, the higher the V/A rating of the power transformer, 
> the higher the cold start current.

What this statement implies is that the voltage out of the transformer remains at its rated 
value from the get-go, and thus the excessive current problem is mostly due to the reduced 
resistance of the filament. A lower VA-capable transformer would, due to the 
lower-resistance and thus higher-current draw of the cold filment result in lower voltage due 
to internal resistance of the transformer.

>  See: 
> https://www.pupman.com/listarchives/2018/Feb/msg00091.php

I find this statement of particular interest:

"A 1994 study of transmitter tube longevity found that each off/on cycle reduced filament life 
by 0.2% from its maximum life of 30,000 hours. This doesn't sound too bad, but it implies 
that 500 off/on cycles will destroy the filament, so if you switched the tube off and on 
everyday, you could expect it to expire in less than 17 months."

...since this says that in any amplifier using, say, a pair of 3-500Zs, turning the amp off when 
not needed would cause the tubes to fail sooner than if it was left on permanently....yet the 
reduction in filament emissivity is also due to age: i.e., the longer the tube is left on, the 
lower emission becomes, sooner. I.e., the single-molecule-thin layer of Thorium on the 
surface of those filaments "goes away" with use, yet turning the tube on and off reduces its 
life.

So, what is the trade-off? Do we leave our amps on constantly, ready at an instant's notice, 
or do we turn them off in order to "save" emission, while at the same time, reducing their 
life-expectancy?

At this point, I think some form of filament in-rush limiting would solve the above problem.

NTCs maybe?

Ken W7EKB

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