> The tube is being tested in AB1, ZERO grid current,
> therefore Eimac is not
> overdriving it.
Wrong.
It is certainly possible to overdrive a tube without grid
current or excessive grid current. Odd-order products are
caused by flaws or imperfections in the transfer
characteristics, and having zero grid current does not
prevent that.
>They are simply showing the degradation when the tube is
>run
> at its TYPICAL rating of 2KV. I have no idea how they
> computed the PEP; the
> spec sheet is dated 1961 and the procedure may have been
> different. If you
> read back to my prior post I stated 2KV @ 245ma which is
> only 490W INPUT.
> Let us both agree that the PEP is a typo.
Not true.
Having two tones raises the PEP by a factor of four over a
single tone. The way it REALLY works is:
One tone by itself:
average, peak envelope, and per tone is the SAME.
Two equal level tones:
1.) Each tone is 1/2 the average and 1/4 the peak envelope
2.) Average is the sum of power of the two tones. This is
the heating power and the power that would show on an
average reading meter.
3.) Peak envelope power is four times the power of each
tone, or twice the average power. This is the power that
shows on a Peak Envelope Power meter.
Now that we see how it works, if we look at the 490 watts
and assume the meters were average or true RMS reading
meters we would have:
490 watts AVERAGE input power
980 watts PEAK Envelope Power input.
The single tone contribution to input power would be 245
watts
> When run at only 1500V the IMD is acceptable. Im again
> making the assumption
> that the screen is VR tube regulated in Eimacs1961 test.
Eimac only gives one particular set of test conditions for
one test fixture and in general one particular setting that
someone selected. There are an infinite number of variables
involved, and we can't conclude based on the data published
that the "IM gets worse as HV is raised".
A statement like that is taking the data sheets far beyond
what they really say. Even Eimac, if we take the time to
read, warns users to test in the actual application and the
data sheets only represent a few randomly selected operating
points in one particular hardware configuration.
What we find in an actual circuit can be much worse than
Eimac shows, or it can be significatly better. It doesn't
even have to follow the trends. As a matter of fact if the
screen is regulated and adjusted to optimum values and if
output power remains the same, and increase in HV almost
always improves IM performance.
> OTOH, I know of no commercial ham amp that runs any 4CX250
> at 1500V and many
> hams push them above 2KV which just makes matters worse.
Not true. That statement is most likely based on either
misunderstanding data sheets or not reading Eimac's cautions
that the actual circuit needs tested.
73 Tom
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