[ARC5] [Milsurplus] Cold Filament Inrush Current

[Tom Lee]

not only craxy, but crazy, too! (where's autocorrect when you need it?)

-- 
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 12:33 PM, Tom Lee 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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