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

[D C _Mac_ Macdonald]

Perhaps leaving the filaments on at say half voltage when the amplifier is not actually in use (high voltage off) would maximize life span?  Turn them on a half voltage also?

I once had a National NCX-5 that ran continuously for years.  The only tube replacements were the 12BY7 driver and 6GJ5 final amplifier tubes.

73 de Mac, K2GKK/5
(Since 30 Nov '53)
Oklahoma City, OK
USAF Retired 61-81
FAA Retired 94-10


________________________________
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Kenneth G. Gordon <kgordon2006 at frontier.com>
Sent: Thursday, March 15, 2018 13:38
To: Milsurplus at mailman.qth.net
Cc: ARC-5
Subject: Re: [ARC5] [Milsurplus] Fw: 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://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.pupman.com%2Flistarchives%2F2018%2FFeb%2Fmsg00091.php&data=02%7C01%7C%7Ce7ba79e92f9c4438cda008d58aa4389f%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636567360278967511&sdata=98j6xhwQytjkkCH5h3hRe7CR4rewBjONaYnd%2BM2tcRw%3D&reserved=0

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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