Rich comments;-
> The statement that I did
>not buy is that drawing grid current (Class AB2) decreases IMD. I use
a
>Class AB1 amplifier, and I know that such a statement is unsliced
>bologna. ..... AB2=doo-doo.
Then my spectrum analyser is a liar. I get better linearity in AB2 from
the 4-250s than in AB1 - probably because of the IMD cancellation with
grid current, as stated by Bruene, and Eimac. Of course, by doing a 2
tone test, it doesn't tell the full story on SSB, and you can get
cancellation. Looked at with the noise test, it's still more linear,
though. In both cases, I used series regulated screen and shunt
regulated bias supplies.
There is no inherent reason that AB2 should be more non-linear than AB1
- if everything else is equal. If the bias is stiff, if the regulation
of the RF grid drive is adequate, why should it be more non linear? But
where I do agree is that you have to make sure of these things, and that
can be where the problem arises in practical implementations.
Incidentally, I see Eimac state that generally, better linearity is
achieved with a low screen supply voltage, allowing a bigger maximum
plate swing i.e swing down to a lower value - this might be another
explanation of why my AB2 amplifier is more linear, since in AB2, the
screens are at 300 instead of 600, with a lower grid bias.
Tom comments about low mu triodes for efficiency. The biggest Tx I ever
worked on used 3 vapour phase cooled TETRODES in Class D, and achieved
just under 94% DC to RF efficiency - 800KW plate power in, 750KW RF
carrier output, plate modulated by two enormous vapour phase cooled
tetrodes. They paralleled two of these babies up for 1.5MW of carrier -
about 900KHz, somewhere in the middle East. This was over 30 years ago
now. At one stage, we had to work nights so that the rest of the lab
could have enough power during the day........they were running
(simultaneously) in that lab a 250KW output SW BC tx, a 50KW out VLF tx,
a 10KW out ground station satellite tx drawing about 100KW., and QRP
rigs - 30KW out HF, and 7.5KW out HF........glad I didn't have to pay
the electricity bill!
Rich, you want to see a TX with AB1 triodes. Try a Marconi SWB8 when
used as a linear - originally, many of them were used on straight CW or
FSK, but I have seen them running AB1, on MCVFT circuits. A pair of push
- pull water cooled triodes, running about 7.5KW or thereabouts - a
1930's design.
73,
Peter G3RZP
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