[ARC5] Filament Question

[Sheldon Daitch]

Dennis, 

yep, much to be said about controlling current inrush on filaments, especially tubes were the inrush current can be measured in hundred of amperes.

In some cases, the current inrush is handled by the impedance of the filament transformer(s) involved.  Other systems may use variable transformers or other current reducing devices, resistance switching - all depends on the transmitter manufacturer.  Granted, this is geared more to transmitters in 50kW ranger and higher.

VOA's Walter Johnson, "Mr. Triode." did a lot of work on the issue of tubes and proper care, especially in regards to filament current control.

An interesting read is something he wrote some years ago:

http://www.photonis.com/uploads/literature/pt/Voice-of-America-Paper.pdf

Walter has long retired from VOA and frankly, I don't even know if he is still alive.

73

Sheldon



________________________________________
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Dennis Monticelli <dennis.monticelli at gmail.com>
Sent: Saturday, October 3, 2015 9:07 AM
To: Leslie Smith
Cc: ARC-5 List
Subject: Re: [ARC5] Filament Question

Ordinary incandescent light bulbs fail with repeated on/off due to the high
current surge at turn-on.  The positive tempco of the cold filament causes
the surge and the rapid change in dimensions that results shocks the
filament into failure, usually at a support post.

I would think that vacuum tube filament failure would be similar.  If so, a
controlled inrush is the remedy.

Dennis AE6C

On Fri, Oct 2, 2015 at 9:53 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

>   Hello Wayne,
>   Lacking any specific knowledge about your question, my guess would be
>   a definite "yes".
>   I guess that thermal cycling would reduce the life of the filaments
>   and therefore the tubes.
>   I suspect (from your question) you think the same as I do.
>
>   In an editorial in "Ham Radio", the editor Jim Fisk, wrote about a
>   surplus receiver he had operated from the mid-40s.
>   I may be wrong in detail here, but as I recall his editorial, his set
>   was wired so the filaments were always on.
>   He switched the B+ line when the set wasn't in use.  He believed (as I
>   recall) that the thermal cycling of metal to glass (in the tube base)
>   was the main source of failure of tubes.  He pointed to the 30 year
>   life of his set, with the filaments constantly on, as evidence of his
>   hypothesis.
>
>   I don't know if he's right, or not, but his theory seems to have
>   reason on-side.
>
>
>   73 de Les Smith
>   vk2bcu at operamail.com
>
>
> On Sat, Oct 3, 2015, at 12:27, Robert  Eleazer wrote:
> > I have recently gotten interested in firing up my RT-70/GRC, mainly
> > because I started testing the tubes in it with my tube tester in attempt
> > to identify the cause of missing transmit audio.  By the way, the first
> > clue I found when I took the set out of its case was a broken 3Q4.....
> >
> > The RT-70 was designed to be powered by batteries in the PRC-16 version
> > and so I guess that is why the tubes are DC powered instant heating
> > filaments that cycle on and off depending on transmit and receive.
> >
> > Would it be better for the longevity of those tubes to rewire the
> > filaments so that they are ON at all times when the set is powered up
> > from the AC power supply I built for it?
> >
> > Thanks,
> >
> > Wayne
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