[Boatanchors] Dropping Resistor Question

[Jacques Fortin]

Hi Mark,

Why a zener with each diode: to limit the power dissipation of each zener.
175mA (mean) current at 21V equals 3.675W if only one zener is used, but using two reduce each zener mean current by two, so 1.835W per Zener.
Reliability wise, it's better.
But the real advantage is supply impedance: regulation will be better than using a 5V4 tube.
The zener orientation you describe is correct.

73, Jacques, VE2JFE