measures wrote:
>>First of all, I do agree with your statement. In reality your right, we
>>match the tube for our desired power output. But is "desired" power
>>output really the most efficient or is it the maximum available power
>>output?
>>
>? Who tunes for max efficiency?
You really can't tune for max efficiency. The efficiency part of the equation
is taken into account in the design of the amplifier, particularly the tank
circuit. You really can't "tune" for efficiency.
>>.......because the tube is not linearly biased. It acts more as a switch
>......
>
>? Undoubtedly unsliced bologna. You outta know better, Jon. The tube
>is typically biased linearly for over half of the 360-degree cycle.
>
OK, Rich. You're correct there. But so am I. Class AB is not purely linear,
you state it yourself - it's only linear for over half of the 360 degree cycle.
The tube does act as a switch as well. What I meant by linear bias was linear
biased over the entire 360 degree cycle (class A).
>
>>and therefore that impedance varies over the drive cycle.
>
>? The critical moment is the instant of max. peak emission when
>instantaneous anode volts reach a minimum. .
>
Agreed.
>>No, that's not correct. If the output of the amplifier tank circuit is
>>conjugately matched to the impedance of the feed line, you won't get any
>>power reflected back to the antenna. The fact that the tube might not be
>>matched completely to the tank doesn't affect that. The tube not being
>>completely matched to the tank means that you won't be able to deliver
>>all of the available power to the tank in the first place.
>
>? When the tank is tuned for max out into a less than perfect load, one
>is delivering all there is.
Not necessarily, Rich. You tune the tank for the best obtainable match you are
able to give. But it isn't necessairly optimal. Unless you are able to
conjugately match the output of the amplifier to the impedance seen looking
down the feedline, you won't get 100% power transfer. Some power will be
reflected back into the tank. The only way you'd really be able to do this is
likely with a roller inductor for your L.
> // Who tunes their amplifier for a
>conjugate match?
I would like to. Unfortunately, I don't have a roller inductor so I can only
get as close as possible. Plus, most folks design their tank components for an
output impedance near 50 Ohms. If you try to match a 750 Ohm antenna into that
kind of a design, you likely won't be able to get a conjugate match. So
instead you tune for maximum power output. But maximum power delivered to the
load is not the same as total available power. If your antenna is near 50
Ohms, your tank components can usually be matched to a conjugate match (or near
enough for practical purposes). In this case tuning for maximum power out will
likely give you a very close match to a conjugate match since maximum power
transfer occurs with a conjugate match.
If one were to use only a 750 Ohm antenna system with their amplifier, then
their tank circuit should be designed to match from the load impedance of the
tube to 750 Ohms, not 50 Ohms. This explains why Jim Reid needs more drive
power to get 1500 Watts out of his amp with his mismatched antenna then when he
matches the antenna with the tuner. The tank of the Henry (and all commercial
amps for that matter) are designed to match to 50 Ohms, not the 23 Ohms that
Jim is seeing.
Now, sure, you can vary the impedance of the output of the amp by adjusting you
tank components, but are their adjustment ranges enough so that you can tune to
an impedance vastly different than 50 Ohms? And even if you are able to match
to some impedance other than 50 Ohms, what is the Q then of the tank? Is the Q
too high? Too low? Or just right? Who's to say. You are trying to tune to
something for which the tank was not designed.
In the real world, you have to many times sacrifice optimal performance for
what you can really obtain practically.
This is why I do advocate antenna tuners. Your antenna tuner is designed to
match a wide variety of loads to 50 Ohms. And in the antenna tuner, Q is not a
critical component like in your tank. So your tank matches the tube's load
impedance to 50 Ohms, then your tuner matches the 50 Ohms to whatever it sees
at your antenna. This way you can have the most efficient power transfer.
>
>>In other words, a mismatch between tube and tank decreases gain and
>>efficiency.
>>
>? not if the tank is adjustable.
Well, correct. But how adjustable is the tank, really? Most guys don't use
roller inductors and their caps may not have the necessary range. Adjusting
the load cap for maximum power out into a load, does not necessarily mean that
the amp is matched to that load. Go through the mathematical equations and
solve for the best network. It likely isn't your tank.
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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