[ARC5] [Milsurplus] Cold Filament Inrush Current

[Tom Lee]

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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