Avery Fineman wrote:
. . .
Making practical, reproducible active multipliers in the home shop
is, practically, a trial-and-error process involving playing with cut-
off bias of the active device input, energy and harmonic content of
the source, and Q of the multiplier's output stage. In the past I've
made tripling-in-the-plate pentode crystal oscillators using
fundamental frequency quartz but those were highly dependent on
getting the highest impedance tuned plate circuit and needed
scope viewing to check output waveforms. Not very reproducible.
There's no "easy" way to do it that will "work every time" despite
the claims of many. :-)
. . .

While that's certainly true of multipliers in general, I've certainly
found it very easy to make repeatable doublers with a two transistor
push-push stage. Driving it with about zero bias and a large enough
signal to get it to conduct on at least a good fraction of each cycle
gives plenty of harmonic energy. A collector circuit with decent Q will
take care of most higher harmonics, although a simple filter following
the stage is usually adequate for more demanding applications. The
fundamental can be nulled out reasonably well with a pot between
emitters with a grounded center tap. I'd think a push-pull tripler would
be nearly as easy, but I haven't had occasion to make one.

Several simple diode and transistor multipliers are described in Chapter
5 of _Experimental Methods in RF Design_, which I heartily recommend for
the homebrewer and experimenter.

Roy Lewallen, W7EL