[ARC5] [Milsurplus] Cold Filament Inrush Current

[Tom Lee]

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

-- 
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 11:31 AM, Bob kb8tq wrote:
> Hi
>
> You also get down to “what tube?” and “made by whom?”. If you dig around a bit, you can
> find rectifier tubes that have pretty major inrush currents. (like 8X). No, that’s not the type
> of tube you are going to find in an ARC-5. Some high power transmit tubes are pretty major
> inrush devices as well ….
>
> Bob
>
>> On Mar 15, 2018, at 2:06 PM, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
>>
>> I would think that it would be easy to figure cold-filament in-rush current for any particular
>> tube:
>>
>> 1) measure the filament resistance of the cold tube with a good ohm-meter,
>> 2) use Ohm's law to figure the current at that resistance and the applied voltage,
>> 3) calculate the hot resistance from the published figures for filament voltage and current.
>> 4) Extrapolate from those figures.
>>
>> For instance the 12SK7's published figures are 12.6 VAC at 0.15 amp, which figures out to
>> 84 ohms.
>>
>> I have not measured the cold resistance of a 12SK7's filament, but that should be easy to
>> do.
>>
>> So....are those two resistance values different?
>>
>> Other factors, of course, are involved: for instance, is the tube a "controlled warm-up" type?
>> Is it a "battery tube"? Etc.
>>
>> Ken W7EKB
>>
>> ---
>> This email has been checked for viruses by Avast antivirus software.
>> https://www.avast.com/antivirus
>>
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html