In response to various comments made on my original posting.
I think that some hams misunderstand the basic principles of of rf
amplifiers!
The load impedance of a tube is defined by the anode voltage and the output
power and is calculated using the formula: (assuming that 80% of the dc
supply is available to develop the rf voltage swing, which is pretty close
to what happens in practice)
RL = (0.57 x Va2)
-------------------
Po
Taking an amplifier with, say, 2kV anode supply and an ouput power of 100W
gives RL as 22800 ohms.
Using the same anode voltage but now running 1kW output the tube load line
(RL) is 2758 ohms.
Obviously the Pi tank network will not be able to provide matching at both
of these load impedances without some drastic changes to the L & C's.
This is an over simplified example as in a practical amplifier the anode
supply will not remain constant - but allowing say a 10% variation in the
anode supply between the no load and 1kW case does not change the result
that dramatically.
If the anode supply is 2000V for the 100W case and 1800V for the 1kW case
(which is a pretty normal case) the resulting RL values are now 22800 &
1846 ohms. Which is even more severe but serves to illustrate my point.
As you can plainly see RL does change significantly between a low output
power and maximum. This is all simple stuff which the handbooks contain in
plenty of detail!
It seems to me that some people choose to modify the laws of physics as it
suits them !!!
Maybe that is why some hams have more than their fair share of "blow-ups"?
John ZS5JF
----------
> From: Rich Measures <measures@vc.net>
> To: John Fielding <johnf@futurenet.co.za>; Amps Contesting
<amps@contesting.com>
> Subject: Re: Fw: [AMPS] tuning capacitor & insulator
To: <amps@contesting.com>
> Date: 19 December 1998 03:37
>
>
> >snipped...............................
>
> >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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