To: <amps@contesting.com>
> Date: Sun, 24 May 1998 14:30:25 -0500
> From: Jon Ogden <jono@webspun.com>
> Subject: Re: [AMPS] where to find info
> To: w8ji.tom@mcione.com, amps@contesting.com
> Yes, true. However, all the "specs" I've seen both from Eimac and
> several editions of W6SAI's book say the IM at 4000 V is -34 dB and at
> 5000 V it doesn't even list IM specs. I suppose that's cause it's
> getting worse? Hmm...interesting.
What Eimac data sheet lists IMD for grounded grid 4-1000A's?
> Although, how can one measure intermod without a spectrum analyzer? I'd
> like to check mine, but have no idea how to do it and I no longer have
> access to an SA.
The tough part is driving the PA with a clean signal. That takes two
exciters with very linear output stages driving a combiner through
attenuators. Unless the signals source is a whole lot cleaner than
the PA, you can unintentionally cook the data. Remember, it is
possible to add distortion that cancels distortion.
If you get a clean test source, you can even use a conventional
receiver with a narrow CW filter for level measurements. All you need
is a calibrated attenuator pad to confirm the levels you are
measuring on the meter. As a matter of fact, most receivers are far
better than inexpensive or older surplus analyzers like the HP-141's
and such.
A typical receiver has a dynamic range of 100 dB, and many spectrum
analyzers (even the very expensive ones) only have about 60-80 dB
range.
A two tone test is also NOT a good indicator of poor dynamic
regulation added distortion. I can make a class C PA look pretty
good if I tune the thing carefully and keep a constant two-tone
level, and virtually NONE of the power supply dynamic problems
will show in a conventional two tone or broadband noise test. For
dynamic regulation tests, the two tone drive has to be varied in
level at a very low rate (a few Hz or less).
Two tone tests aren't useful for determining voice performance,
unless they are made correctly with sylbalic level variations.
> You're correct. Yes, it's probably not a good beginner's project.
> You're right.
The problem is suggesting a good tube. Ceramic tetrodes are too
complex in circuitry if they are operated *properly*, and other
tubes are less stable (572B's, 3-1000Z, 3CX1200A and D7's) or too
hard to find (3CX1200Z7). Although W6FR did have a very good 4CX1600
article.
I think MRI pullouts offered by several people selling pulls are a
good option, another option might be 3-500Z's (Amperex
only) . I'd stay away from high voltage or less stable tubes for a
first "learning" project.
73, Tom W8JI
w8ji.tom@MCIONE.com
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