> la7sl@online.no writes:
>
> No, I do not think the results would be the same at 3.5 as
> on 50 Mhz.
> Those curves are traced below 30Mhz and if you try to make
> a set of curves
> for 50Mhz they would be very different !
Lou Writes:
> What am I missing here? What does frequency have to do
> with operating a
> tube along a curve based on grid current/plate current?
> Lou
Lou,
IMD is much more complicated than the characteristic curves.
As a matter of fact thinking the only thing that matters is
a flat operating line is where a lot of well-intentioned
people go wrong.
Also important is the shape of the line, and that is heavily
influenced by feedback. The correct slope, even if the slope
changes, will not produce harmful odd-order IM products. The
smallest "bad" shape can produce significant IM, at least in
terms of what we look for.
This is a major problem with uncontrolled unplanned
feedback, especially feedback that changes phase and/or
level with tuning.
Remember all those articles about how clean tetrodes were?
When you look at them in real life they never meet those
claims. The reason is they lack the heavy negative feedback
of a grounded grid stage.
Now when you get at a frequency where the grid isn't well
grounded, you introduce all sorts of variables.
This isn't saying an amp will have excessive IM when
operated above a certain frequency, but it is a fact that
the IM is much less controlled.
73 Tom
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