----- Original Message -----
From: "Mike(W5UC) & Kathy(K5MWH)" <w5uc@cox.net>
> There clearly are individuals on this reflector who are more knowledgeable
> than me, so I will ask this question again, but in a differently worded
> form. How does the impedance of the PA tank affect the over-all
> efficiency of the amplifier? W8JI said yesterday that a tank whose
> impedance is, say 2100 ohms, and connected to a Pa with a calculated load
> impedance of 1333 ohms would produce a very efficient PA but it would have
> poor linearity. BUT, what happens to efficiency if the tank impedance
> matches the load impedance? Again, how does the impedance of the PA tank
> affect the over-all efficiency of the amplifier? Inquiring minds want to
> know.
>
> Thanks & 73,
> Mike, W5UC
>
Hi Mike,
In principle for a given operating class (A, AB1, AB2, C),
matching the load impedance presented by the tank circuit
to the output impedance of the tube provides maximum
available output power and a good compromise between
efficiency and linearity. Working from there as a starting point,
you can improve linearity by increasing the loading (lowering
tank impedance), but this comes at the price of somewhat
lower efficiency and output power. Likewise if you go the other
way (lighter tank loading), efficiency will improve, but it will
come at the price of poorer linearity and lower output power.
I think Tom's point about loading slightly heavy is that the
linearity improvement you get is a good tradeoff against the
small hit you take in efficiency with the added bonus that the
heavier loading keeps the tank voltages a bit lower giving you
more margin on the tune C and the bandswitch. You can
only take this so far of course as if you get carried away
with bea coup heavy loading output power will suffer.
I suspect many people (myself included) make this tradeoff
unconsciously when we manually tune-up a PI network amplifier
(especially one using tubes with sensitive grids) since loading
towards the lighter side generally results in higher grid current
and greater sensitivity of grid current to small changes in drive
or VSWR whereas loading heavier keeps the grid current in
check and is a bit more forgiving to changes in VSWR and
drive level.
Hope this answer's your question.
73 Mike W4EF............................................
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