On Sun, 14 Dec 2008, Telstar Electronics wrote:

Date: Sun, 14 Dec 2008 08:20:56 -0800 (PST)
From: Telstar Electronics
Newsgroups: rec.radio.amateur.homebrew
Subject: Doubling

On Nov 22, 8:43*pm, Stray Dog wrote:
Despite what at least one other person responding to this said, I can rest
assure you that if you run a doubler/multiplier stage even in a linear
mode, AND if you tune the output of that stage to the multiple harmonic,
you will definitely get output at that harmonic frequency which is
stronger than the input drive voltage.

Huh? No way... you MUST have non-linearities to make a doubler.

All tubes (and transistors, etc) have non-linearities (if the transfer
characteristics are non-straight lines) if that is what you are talking
about.

However, I have observed output on a scope of second harmonics (and, yes,
the time base was set right and auto-self triggering) and the
amplifier was running no higher than Class B. You should actually try this
yourself and see for yourself. Tune the output to the second harmonic and
you will see grow out of the vally new "peaks" corresponding to that
second harmonic.

I don't know what the solid state gear is doing, but from many schematics
of the vintage tube gear I'm familiar with show, and measure, biasing for
linear operation, even in stages meant to multiply frequency.