>But what about the transceiver that uses tubes for the output - like the
>many hf rigs which use a pair of 6146's or similar? Surely if a
>tube or, for that matter, a transistor/power module is tuned to match into
>the grid circuit for Class C operation the impedance it "sees" is very
>close to a VSWR of 1:1?
yes
>
>I think that what you are trying to suggest is that an amplifier which is
>normally biased for Class AB1 and is then over-driven into Class C would
>cause the impedance to change as the grid current starts to flow. How is
>this different to, say, a grounded grid Class B amplifier which runs a fair
>amount of grid current under normal operation.
If a Class AB2 amplifier is driven with less power, the amplifier is
still in Class AB2
>Surely in this case the input Z also changes from one state to the other?
> But doesn't the input
>impedance change gradually as the grid current increases from zero to its
>maximum?
>
Surely, things change as the grid is driven into the positive region.
>There is no doubt that some solid-state rigs do not produce acceptable IMD
>products when mis-matched into reactive loads. But from experiments that I
>and others have performed the degradation is not as bad as some would have
>you believe, provided the ALC circuitry in the transceiver is designed to
>gradually back-off the drive under poor VSWR conditions. It may be true
>that some solid state transceivers use the VSWR back-off as a "sudden
>death" type of protection, which is an exceedingly poor piece of rf
>engineering!
>.......
>My original comment related to an amplifier specifically set up for Class C
>operation for CW. This is how the "old fashioned" amplifiers worked and
>nothing was considered unusual about the tuning up or operation in any of
>the many ARRL Handbooks or text books on the subject that I have read so
>far. Drive was applied until the required amount of grid current flowed
>and the anode tank circuit was then tuned for a pronounced dip in the anode
>current before the loading was increased to bring the anode current and
>output power to the required. Surely nothing has changed as far as that is
>concerned? Today we have better tubes available, but the basic principles
>haven't changed - or have I missed something along the way?
Not that I can see. However, a Class AB1 grid circuit that is designed
to deliver a good match to the exciter is not going to do so in Class C.
In AB1 the grid is a high Z. In Class C the grid is a low Z.
>In the days
>before VSWR meters, hams where quite happy to have a high VSWR between the
>transceiver and the amplifier, one simply adjusted the matching networks in
>the transceiver and the amplifier for best power transfer.
semi-true
>It seems to me that too few hams understand the basics of rf engineering.
>They should take some time out and read Walter Maxwell's (W2DU) excellent
>book published by the ARRL entitled "Reflections". Once the fundamentals
>of impedance matching are fully understood it only requires a little
>application of common sense to resolve 90% of the problems that hams
>encounter.
If all of Maxwell's principles applied in linear RF power amps, the
efficiency would be 50% instead of 60%.
>
>As a final thought:
>
>Consider the anode impedance of an amplifier. One tunes up with maximum
>drive.
ok
>In this case for the anode voltage/curent in use the tube now
>"sees" its correct load impedance and transfers the maximum power to the
>load (antenna).
agreed
>Under low drive conditions the tube "sees" an impedance
>which is many times higher than what it was set up for.
I doubt that RL is that different at less drive. As delta-E decreases,
delta-I decreases, so RL remains in the ballpark
> Can some genious
>devise an automatic tuning mechanism that will provide the correct anode
>impedance for any condition of drive level, to ensure the best efficiency
>under all drive conditons?
>
Apparently, if the tank works satisfactorily at full throttle, it will
work ok at lower levels.
- later, John ZS5JF
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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