Fw: [AMPS] tuning capacitor & insulator

[Rich Measures]

>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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