On Tue, 16 Dec 2008, Paul Keinanen wrote:

Date: Tue, 16 Dec 2008 09:47:22 +0200
From: Paul Keinanen
Newsgroups: rec.radio.amateur.homebrew
Subject: Doubling

On Mon, 15 Dec 2008 23:29:35 -0500, "NoSPAM"
wrote:

"Telstar Electronics" wrote in message
...

You mean to tell me that you take a clean sine wave... pass it
through... say a single-ended class A amp... and you can put a tank on
the output of that amplifier... and tune for a harmonic? You will get
nothing.

Of course you will. No active device is perfect.

I decided to illustrate the fact that a single ended triode operated in
Class A can produce harmonics. For a tube, I used a 6C4 (1/2 of a 12AU7)
operated with 300 volts on the plate, a grid bias voltage of -7 volts,
driven with a pure sine wave of 14 volts peak-to-peak. The high driving
voltage was chosen to illustrate my earlier points, but the stage _IS_
operated Class A with the plate current between cutoff and saturation.

Did you bypass the cathode resistor or not ?

I did the same experiment that he did (6C4) only ran the cathod at
chassis, and grid through an RF choke, and 100 vDC on plate, and drove at
about 1/2-1 volt and that is zero bias, no need for cathode cap bypass, and
I got gain and second harmonic.

All active elements are more or less nonlinear, so if you need more or
less linear amplification, you need to use feedback/feedforward.

A non-bypassed cathode/emitter resistor will greatly improve the
linearity of a single stage.

I'm still waiting for any "expert" comments from anyone who would care to
speculate on the contributions, from oscillator harmonic content vs
contribution from harmonic distortion in the amplifier.

Paul OH3LWR