Jack said:
"Much of the literature contends that a tuned input is better for IMD,
however, that leads to possible requirements for neutralization and other
complicating factors."
Either you are reading the literature wrong or it is wrong.
There is nothing to be gained by using a tuned input when the amplifier is
in a grounded cathode configuration. It is only when it is cathode driven.
With a grounded cathode and a resistor load on the grid, the only concern is
adequate grid voltage.
Sometimes hams take facts out of context. When driving a cathode, usually
the full 100 watts (more or less) is required to drive the amplifier.
Without a tuned input, the amplifier does not present a proper load over 1/2
of the cycle - which reflects back into the driver and increases driver
distortion - leading to increased distortion out of the amplifier. The
pi-network or variation acts as a flywheel to reduce distortion by 3 db more
or less. But, the same thing could be done a number of other ways. A diode
and load resistors on the alternate half cycle would do the same thing, or a
specific coax length (which makes it a single band amplifier).
Using a grid driven amplifier with a passive input presents a proper load to
the driver at all times. Changing from a swamping resistor to a tuned input
in that case will not improve distortion at all. The passive grid is
perfect because of the excess of drive when driving the grid. You could
actually use a passive cathode input to eliminate any tuned circuit in the
cathode - but the drive requirements would at least double.
The question is one of choosing between grounded grid (with a tuned input)
or a passive grid. The grounded grid circuit has one advantage in that it
has inherent feedback - which may reduce distortion products by up to 10 db.
Knowing that, you would then look at the tube characteristics. The 4CX-250
series would be a poor choice for grounded cathode operation because it is
rated for approximately -22 db distortion products. Assuming -30 db is a
minimum desired goal, you would have to use the 4CX-250 in grounded grid or
provide negative feedback or degeneration of some sort if used in a grounded
cathode configuration.
The 4CX-800 tube is cleaner than the 250 series, so it is a good candidate
for grounded cathode with a resistive grid input. George Daughters has
designed a few amps using the Svetlana tubes and inserted degeneration in
the cathode by using an unbypassed resistor. They perform nicely. The
degeneration consumes some of the excess drive and decreases distortion
products a bit. The articles are available on the ARRL website.
One of the advantages of passive input is that the amplifier can be used on
a different frequency simply by tuning the output. When we consider there
are 8 hf ham bands where you would use the amp, the passive grid circuit has
a lot to recommend it.
73, Colin K7FM
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