[Amps] 4CX250B Screen Supplies

[Will Matney]

Since some have been discussing the 4CX250B supply voltages and the 
regulation thereof, I figured it would be worth mentioning older ways. 
Matter of fact, this older way is tougher than zener diode strings. 
There is still available in good quantity gaseous regulator tubes still 
being made. Typically used ones were the OA2 and OB2. The OA2 is a 150 
volt regulator, and the OB2 a 105 volt. Two OA2 tubes in series would 
get you right at +300 Vdc for the screen supply. I have also seen three 
OB2's in series for this getting 315 Vdc. They fit a standard 7 pin 
miniature socket which is still being made. These are tough as a Abrams 
tank and don't short easily like zeners could. Keep in mind that the 
voltage can be made adjustable by placing a potentiometer between the 
last tube and ground just like zeners. The formula for the dropping 
resistor is Es - Er / I . Es is the source voltage, Er is the regulated 
voltage, and I is the tube current which is usually 40 mA (0.04 amps). I 
used some 325 Vac transformers I have here (the ones for sell) to do 
this easily in the past. These can regulate a negative circuit by simply 
changing the cathode and anode around like a diode. Sometimes tubes just 
beat semiconductors when power handling is evolved. I think they are 
being made in China by 1-2 manufacturers along with the sockets. Use a 
varistor rated at 30-50% of the screen voltage with the tubes parallel 
to ground to clamp any over voltages-spikes.

Will Matney