Re: Filament help!

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: David Speck <dave-at-davidspeckmd-dot-org> 

Jim,

5 volt, 6.5 amp filament transformers are not very hard to find at hamfests.

You definitely would not want to use a PC PSU -- too many sensitive 
components in the switching power supply section to use in the vicinity of 
HV RF.  You don't need DC for a filament supply, anyway.

You can make a custom filament transformer for next to nothing by removing 
the high voltage secondary from a small scrapped microwave oven 
power  transformer, and winding about 5 turns of 12 gauge wire in its 
place.  There is already a filament winding on most MOTs that may be close 
to the voltage that you need.  You might improve its efficiency if you 
knock out the magnetic shunts between the primary and secondary windings.

Can't comment from experience on the relative suitability of the tubes, but 
I suspect that the 4-125 is the more robust tube.

HTH,
Dave

>I'm using a 4-125A in my VTTC, and I cant find a filament supply!  I was
>thinking of using a PC PSU but they have too many wires and I don't know
>which ones to cross for power on, and which to load to keep regulation
>proper.  The 4-125A needs 5v at 6.5A for the filament.  Also, any idea if
>thise tube is better than the 813? I have both and am wondering which I
>should use.