[ARC5] [Milsurplus] Cold Filament Inrush Current

[Bobby Drummond]

Here is my poor man's answer to inrush filament current potential problems
(especially with series string filaments)

I power the rig first through a 25 watt incandescent light bulb (bulb in
series with the 120 volt power to the rig) for 30 seconds (long enough for
the filaments to warm sufficiently at the lower voltage allowed through the
series incandescent light bulb), disconnect and then quickly power the rig
with full normal voltage.

It's the cheapest way I know of the reduce the effects of initial filament
inrush current on the rigs I use.


73 de AK4JA


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 94305-4070http://www-smirc.stanford.edu650-725-3383 <(650)%20725-3383> (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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-- 
73 de AK4JA
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