[ARC5] Filament Question

[Dennis Monticelli]

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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