Paul said:
"Except perhaps on the air? Rated third-order IMD of the ACOM 1000 is
only -35 dB below rated output. I suspect real-world performance from this
tetrode-based amp without a tuned input may be worse than stated.
I owned the ACOM 2000A and it was a great amp until I began receiving
splattering reports. I confirmed the performance of the exciter(s), and it
was definitely the amp. Going back to my old triode-based amps fixed the
problem."
I want to question some of the comments made by Paul. I have never used an
ACOM 1000 so I am shooting from the hip. -35 db 3rd order distortion is
not bad. Better than most exciters on the air these days. That sounds
about right for a 4CX800 tube. However, a tuned input will not help a grid
driven amplifier at all and it makes no sense to install one. Cathode
driven amplifiers can benefit from a tuned input because it will provide a
proper load for the driver. In other words, the tuned input cathode driven
amplifier will be better than an untuned input cathode driven amplifier
only because the linearity of the driver output stage is improved. The
amplifier does not care. Grid driven amplifiers, however, use a resistor
that provides a consistent load for the driver. A tuned input cannot do
any better than that.
The 3rd order distortion of the grid driven amplifier would be set by the
tube and the regulated supplies. I question whether the splatter Paul had
was related to the amplifier at all. I suspect that the amplifier was
overdriven by the driver and flat-topped. If ALC was used, this would be a
almost certainty. It is possible that by going back to the cathode driven
triode, drive was increased and the triode was not overdriven.
I tend to find complaints of splatter when the 3rd order distortion gets
down to about -22 db or so. Most amplifiers at -30 db are ok (not great
though). A few amateurs run at over -40 db and it is easy to tell, but I
can live with -30 db. I wish my neighbor would get to -30 db. He
overdrives an SB-200 to overdrive a 4-1000. He may be getting less than
-20 db.
Did not mean to stir things up - but what the heck.
Colin K7FM
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