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
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