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

[Gene Smar]

FWIW:  The battery to the PRC-25 (BA-4386) includes a piece of thin nichrome wire (don’t ask me the gauge) maybe 2 inches long that serves to limit the filament inrush current of the only vacuum tube in the rig – the 2DF4 final amp.  The PTT switch applies filament voltage of 3.0 Volts to this tube in transmit.  My guess is that, in arctic climates, inrush current would snap the filament, so the nichrome wire limits the heavy inrush until the filament warms up sufficiently to self-limit the current.

 

Maybe one could devise a similar method to tame the inrush current until the filament naturally limits the operating current.  The IR drop across the nichrome in operation must be sufficiently low so as to allow proper voltage to be available to the warmed-up filament.  Selection of the nichrome gauge would then depend on the R needed at the filament operating current to keep the Voltage drop at some calculable level.

 

 

73 de

Gene Smar  AD3F

 

 

From: milsurplus-bounces at mailman.qth.net [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Jim Whartenby
Sent: Thursday, March 15, 2018 3:50 PM
To: Kenneth G. Gordon <kgordon2006 at frontier.com>; Milsurplus at mailman.qth.net
Cc: ARC-5 <arc5 at mailman.qth.net>
Subject: Re: [Milsurplus] Fw: [ARC5] Cool New DC-DC Converter

 

Yes, limiting the inrush current is the solution to the majority of the problems mentioned, as I understand them.

Jim

 

  _____  

From: Kenneth G. Gordon <kgordon2006 at frontier.com <mailto:kgordon2006 at frontier.com> >
To: Milsurplus at mailman.qth.net <mailto:Milsurplus at mailman.qth.net>  
Cc: ARC-5 <arc5 at mailman.qth.net <mailto:arc5 at mailman.qth.net> >
Sent: Thursday, March 15, 2018 1:40 PM
Subject: Re: [Milsurplus] Fw: [ARC5] Cool New DC-DC Converter

 

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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