[Patrick Turner]

Two Zener diodes connected facing in opposite directions to each other
in parallel ? And a voltage dropping resistor in series . . .- Hide quoted text -

But not in parallel, because zeners act like ordinary diodes in one
direction of current, so the zeners must be in series, and then it
clamps. Using a series R might be OK to give a higher load once
clamping limiting begins to avoid the low Z when zeners turn on. It
should not matter in a tube amp though because the zener voltage
should just exceed the likely peak Vo max when normally loaded. The
zeners have some capacitance and are accused of being slow to act, but
they'll act quite fast enough, and the capacitance is orders of
magnitude below what might cause HF instability because of capacitive
loading of the amp.

I have used about ten 68V x 5W zener diodes in parallel as diode vari-
caps to generate +/- 40kHz of FM signal deviation of a 455kHz RF
signal. The modulation applied to the diodes can be a saw tooth, and
this gives a "wobbulated" IF signal which allows the selectivity curve
of a radio set to be seen on a CRO, so that when one aligns an AM
radio IFTs, or RF sections, or experiments with an IFT teriary
winding, the effects on pass bandwidth and skirt selectivity can be
seen easily. The amount of capacitance with each zener is only tens of
pF maximum. This form of FM modulation would be most difficult to
otherwise achieve unless one had a tuning gang which was driven at say
40Hz rotation by a motor. The electronic method of wobbulation has no
mechanical parts which tend to wear out.

Patrick Turner.