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Chris, & all here,

Here is a fun exercise:

Assume that no commutating diode is present at L14 in QDX, and that,
for convenience, the PA is drawing 1A through L14. Then at the moment
the last BS170 transistors switch off, a large V=Ldi/dt spike will
take the transistors to avalanche at 60 Volts. At that moment, 60
watts of instantaneous power (60V x 1A) is being dumped into the four
of them. It will last about half a microsecond in duration.

If 47 volt Zeners are intended to absorb the spike, the calculation
results in half an amp flowing to each zener, and 47 watts being
delivered to the pair. The Zener resistance will allow for
considerable elevation of the drain voltage above 47 volts, and the 60
volt avalanche spec may still be violated in spite of the presence of
the Zeners, assuming they can even handle the instantaneous power
dump.

Hans has shown through bench testing that high voltages are extremely
unlikely as the result of high SWR. Simulations done by Evan and I
agree with his bench testing. It seems the Zener diodes offer no real
value here, and that the L14 commutating diode is a better overall
protective solution. It can handle the L14 current easily, which
becomes more important under low Z / high SWR conditions.

More musings from one of those pesky nerds...

JZ KJ4A