[AMPS] Filament Inrush Current Limiting

[measures]

>
>Specifically, is it necessary to limit inrush current at turn-on in a 8877 
>tube?  
>
  In one word, no.  When cold, Oxide-cathode heaters have much higher 
resistance than do thoriated tungsten filaments. Eimac says to limit 
3-500Z/8008 inrush current to no more than double the normal current of 
14.7A.  No such limit is specified for the 8877.  

>If some filaments do not require current limiting, what is the 
>characteristic 
>of a filament which requires limiting the current (I understand that a cold 
>filament has less resistance than a hot one) ?
>
Thoriated tungsten has about 8.3 times as much conductance when cold 
compared to hot.   Thus, if driven from a 0-ohm source, the 14.7A 
filament of a 3-500Z/8008 initially consumes  c. 122a peak.  However, 
many thoriated tungsten fil. tubes do not specify inrush current 
limiting.  The difference seems to be in the way the filament is 
supported.  Tubes with little filament support - such as the 3-500Z/8008 
- seem to be the most vulnerable to filament warping.  I have never heard 
of a 3CX1200A7 with a grid/fil. short.  

>I have not seen a definitive answer although I have not been subscribed here 
>for all that long.
>
This subject was discussed here in 1999. 

cheers, Dave

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


--
FAQ on WWW:               http://www.contesting.com/ampsfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm