[ARC5] [Milsurplus] Cold Filament Inrush Current

[Michael Clarson]

Wasn't the original post about command set rcvrs? These were used in
planes running on batteries. The filaments were hit with a solid 24
volts when turned on, and, they would last quite a while. Inrush
current did not seem to be a concern. --73, Mike, WV2ZOW

On Thu, Mar 15, 2018 at 3:33 PM, Tom Lee <tomlee at ee.stanford.edu> wrote:
> Yes, blooming filaments = bad!
>
> If you are using the gear pretty heavily and are concerned about the on-off
> stress on filaments, there are several solutions. NTCs have already been
> mentioned, and are probably the most expedient solution in most cases.
>
> Broadcast transmitters generally keep the filaments warm when not in use.
> Since the lifetime of a filament is proportional to something craxy, like
> the 12th (!) power of voltage, it doesn't take much voltage reduction to
> prolong lifetime to whatever value you want. Then the shock of going full
> out is moderated, taking care of that concern.
>
> "Instant-on" TVs simply inserted a single rectifier in series with the
> filament string so that the tubes were kept warm even when the set was off.
> The reduced filament temperature had negligible impact on tube lifetime.
> Bridging across the rectifier turned the set on "right away" without much
> thermal stress. This trick might be a viable option for some usage
> scenarios.
>
> --
> Prof. Thomas H. Lee
> Allen Bldg., CIS-205
> 420 Via Palou Mall
> Stanford University
> Stanford, CA 94305-4070
> http://www-smirc.stanford.edu
> 650-725-3383 (public fax; no confidential information, please)
>
> On 3/15/2018 12:22 PM, DSP3 wrote:
>
> One of the factors mentioned in the Eimac power tube book is to address
> in-rush by ensuring that the filament transformer (especially in thoriated
> tungsten types) is rated for the actual current of the tubes, and no more.
> That way the lower, in-rush resistance it initially mitigated by sag in the
> transformer.  The opposite scenario is that an overrated transformer will
> sink whatever current necessary during the cold, heat-up period.  I have
> seen tubes "bloom" when power is applied where the extra current is
> available.  Not a good situation...
>
> Jeep - K3HVG
>
>
> On 3/15/2018 2:37 PM, Tom Lee wrote:
>
> Exactly. Resistance is a function of temperature, so tubes that operate at
> white-hot incandescence (pure tungsten) will have a larger hot-to-cold
> resistance ratio than oxide-cathode tubes. For the former, a 5:1 ratio is
> not uncommon, where for the latter, something around 2:1 is more typical.
>
> --Tom
>
>
>
>
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