Re: MOT mod.

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

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

Chris,
Most MOT's put out a volt per turn, so you would need 5 turns through the 
core for 5 volts.

However, the ratio is not cast in stone.  Try a temporary winding with 4 
turns of #12 or #10 wire, and see what voltage you get across your tube 
filament.  (Do not apply high voltage to the plate while you are measuring 
the filament voltage).  Measure the voltages directly at the tube pins.

Add any additional turns you need to get the working voltage you need.

For two tubes, you can pull two parallel windings through the core at the 
same time, and use one winding for each tube.

I've heard conflicting opinions on whether the shunts are beneficial when 
using a MOT core for a filament transformer.  My inclination would be to 
remove them to save wasted power, but it has been proposed that they would 
limit the inrush current when the tube is first turned on, and the 
filaments are cold, with lower than nominal operating resistance.  Standard 
filament transformers do not usually have shunts.

HTH,
Dave

>The tubes are a pair of 4-125A (Sylvania) radial beam
>power tetrodes, wired in triode mode.  The specs say the
>filaments require 5 volts -at- 6.5 amps.