[Amps] 4-250A question

[Richard]

>In <200207041336.g64Da0KF001984@contesting.com>, on 07/04/2002 
>   at 06:38 AM, Richard <2@mail.vcnet.com> said:
>
>
>
>>>the filiament voltage ........ for the 4-250A  tubes can you go over a 
>>>bit, say to 6 volts?
>
>>The emissive life of a thoriated tungsten cathode/filament varies as 
>>[e1/e2]^23.4.  Or, in other words, each 3% increase in fil. V over what 
>>is needed to produce normal emission, reduces emissive life by half.  A 
>>healthy 4-250A produces normal emission at c. 4.8V.  If the filament is 
>>operated at 6.0V, life will be reduced to [4.8/6.0]=0.8^23.4=0.0054, or 
>>0.54% of the normal emissive life.  (''Care and Feeding of Power Grid 
>>Tubes'', Eimac Co.)
>
>>>will this adversly affect the tube  what i really need to know will it 
>>>kill it right away.
>
>>No.  The tube should last c. 80-hours instead of the normal 15k-hours.
>
>Not good at all!
>
>How hard is it to buck the input voltage to your filament transformer?

Piece o' cake.  One can buck it in the primary or add R is series with 
the bifilar fil choke to bring it down to c. 4.8v.  I use small gauge TFE 
wire for the latter purpose.  At 15Arms #22 TFE runs warm to the touch.  
#18 TFE will carry 30Arms.  However, some breathing room around such 
wires is obviously needed -- i.e., such wires should not be neatly 
bundled.  

>Drop it down by  12vac and you should be in the 5.0v range. 

yes.

>You do this by
>placing a 12v filament winding  to act against the mains supply voltage.
>Add a small, heavy-duty resistor in series with the  primary to make
>smaller adjustments if necessary. Also, you can get relatively small,
>air-cooled  buck/boost transformers to correct your mains supply to the
>hamshack. These generally offer a  6v or 10v correction; you could put one
>on each leg. 

However, this will also drop the HV, which is not wonderful.
>
>>>...

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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