[ARC5] Filament Question

Sat Oct 3 11:09:46 EDT 2015

Thanks for the additional info on ultra high power tubes, Sheldon.

Early in my career I was involved in optoelectronics and one of my tasks
was to set up a light bench that was traceable to the National Bureau of
Standards.  I was handed a tungsten filament tubular lamp that had been
stabilized and then calibrated to deliver a certain output level and color
temperature.  It pretty much arrived on a velvet pillow and carried the NBS
sticker.  I was to run it at 8.349 amps DC and take 4 min to ramp it up and
4 min to ramp it down.  Talk about babying a filament!  I built a precision
current source with an automatic slow ramp capability to drive it.  I also
had to log the total number of hours on the lamp. After X hours it had to
go back to the NBS for a re-calibration while I used the backup NBS
traceable lamp in its place and run it at something like 8.317 amps.

Dennis AE6C

On Fri, Oct 2, 2015 at 11:49 PM, Sheldon Daitch <SDAITCH at bbg.gov> wrote:

> 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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