> Re: Amplifier Linearity
>
> Does anyone besides me still use a 'scope in trapezoid
> pattern looking
> at input vs. output linearity?
Not really, because a scope tells us very little about the
quantity of splatter.
It might be a tuning aid for those who don't know how to
tune with meters, but it really isn't a good bandwidth
indicator.
> Seems like we're getting into "flat topping" tubes, etc
> just to get the max.
> output level etc.
It's actually pretty tough to flat top a tube when you tune
it correctly. Especially compared to a semiconductor like a
bipolar transistor.
Tubes generally have a very high current saturation point
and remain reasonably linear up to that point, especially in
grounded grid systems with the inherent high levels of
negative feedback. They don't happily live at 500 watts and
suddenly fall apart at 550 watts, the exception being grid
driven sub class 1 tetrodes that suddenly move into sub 2
(grid current) or fall out of screen limits. But a triode in
GG remains pretty clean long after rated power is passed if
the amplifier is tuned correctly and if there is no positive
feedback or regeneration.
That's in stark contrast to bipolar transistors or AB1
tetrodes, which can fall apart dramatically before the
meters tell you something is wrong. For example you see the
CB Hams shorting ALC lines on rigs or turning up the power
limit pots inside radios to make 100 watt radios run 120 or
150 watts. This makes transistors and some grid driven
tetrodes increase IM expotentially where the user might pay
a 15dB penalty in IM performance (splatter) for a 2 dB
increase in power.
> All the "it can't happen here" talk is really counter
> productive. If a guy
> says
> he can do it for heaven's sake let it go. Please don't try
> and prove how
> clever you
> are. It just doesn't impress me and wastes my time.
What doesn't impress me are grumpy people who take
everything personally, and who think just because something
works it is a good engineering practice. That's most common
with antennas, and largely why people learn so slowly when
dealing with antennas. Of course that is also why they are
so much fun, because when we are emotionally involved it is
like a romance.
Unfortunately what that does is stop people from thinking
and encourage living by pure emotion. It also makes me think
RF or too much fat meat and beer damages brains and makes
Hams grumpy old bitter farts with narrow minds and big
butts. :-)
73 Tom
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