[Central-Electronics] 100V VFO questions?

[Jim candela]

-----Original Message-----
From: Eugene


3. STRICTLY ACADEMIC QUESTION: Has anyone ever tried to replace the 0A2 tube with a solid state Zener? One that comes to mind is NTE5226A which is a 10W 150V zener. Would there be any benefit to this? Stability? Lower heat generation? 

thanks!
Eugene



Reply by Jim, WD5JKO:

Eugene, Zener diodes in general have a positive temperature coefficient, so the voltage gets higher as the devices get hot. The most stable zener is around 6 volts where the rise in voltage is about 2.2 mv / deg C. This can be offset with a silicon diode in series which has a -2.2 mv / deg C decrease in voltage as the temperature rises. Those HV zener diodes drift upward like a SOB as they get hot, and are not useful when you want to stabilize a voltage. These HV zener's are better used as a HV clamp, or used in simple regulators where line and load regulation are not a big concern. 


I have built several solid state HV regulators. I integrated these into an old Sherwood S8000 tube type audio amplifier + FM multiplex tuner.

Here is one that drifts up as it gets hot, but offers excellent ripple reduction, and slow start-up:
http://pages.prodigy.net/jcandela/Sherwood_S8000/HV_REG/SS_HV_REGULATOR2.jpg



Here is another that has excellent regulation and ripple reduction using a compensated 6.2 volt zener:
http://pages.prodigy.net/jcandela/Sherwood_S8000/HV_REG/SS_HV_VOLTAGE_REGULATOR.jpg


   You could replace that OA2 with a 10 watt zener. Stability would be far worse (rise about 10-15 volts as it heats up), heat generation the same, and no benefit that I can see really. In fact you would have to heatsink this part (solder leads to copper heatsink) in order for it to dissipate more than a few watts. The OA2 will last for years and years, and a super hot zener will eventually fail. Old technology wins here for stability, reliability, and simplicity.

Regards,
Jim
WD5JKO

 

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