On Jul 19, 2004, at 8:45 PM, Will Matney wrote:
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.
This is guaranteed to spoil dynamic regulation, both with zeners and
with gas regulators. The caveat with zeners is, except in Antartica,
don't run them at more than 1/3 of their dissipation rating. // The
easiest way to make a zener string adjustable is to make the tap
movable.
>
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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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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