[AMPS] Plate load charts

[Ian White, G3SEK]

Jim wrote:
>"Have you ever noticed"  (Now I sound like Andy Rooney)  how different the plate 
>load impedance/component value charts (C1, C2 L1 L2) for a given plate load  
>change markedly between different publications ?
>
>I have noticed in the 23 edition of the "Radio Handbook" the chart for the Pi-L 
>network has values for the Q of 10. In the 1998 "ARRL Handbook" the chart for 
>the same plate load impedance has about the same values (within 5 % ) and that 
>chart is for a Q of 12.  I have also noticed that different publications give 
>the formula for calculating the palate load impedance different as well. Some 
>books give a "K" value of 1.8 (23rd radio handbook), and the some give the "K" 
>value of 1.57 ???
>
There are various methods for calculating the Pi and Pi-L network
values, some more approximate than others. 

As has been pointed out already, approximate methods are actually OK for
this application. There are always at least two adjustable variables,
and the real-life load impedance is never exactly the design value of 
(50 +j0) ohms. It doesn't matter if the loaded Q is a bit different from
the design values.

The current ARRL design method looks pretty accurate when analysed
backwards, calculating how the 50 ohm load impedance is transformed
through the network and presented to the tube.

BTW, I have an Excel 5 spreadsheet that implements the ARRL design
equations and also includes the effects of stray inductance, parasitic
suppressors etc. It's about ready for "beater testing", so I will load
it on my web site tonight (may be Wednesday before it becomes available
world-wide).


>In the information sheet that I have from Eimac on the 3CX800A7,  this sheet 
>says that the plate load impedance value for the tube operating at 2200 VDC and 
>a plate current of .5 amp is 2700 Ohms. This makes the "K" value 1.56. 
>
>All of the values for "K" are for the tube operating in class AB2......
>
When we talk about "plate load impedance", we mean the load impedance
that is being PRESENTED TO THE TUBE. That is the antenna impedance,
transformed backwards through the output network.

It's important to understand that RL is not a property of the tube
itself - it's about the way you are USING the tube.

The value of RL specified by the tube manufacturer is a compromise value
taking into account several different factors:

* anode voltage - must stay within flashover limits

* cathode current - must stay within emission limit of cathode

* power dissipation - must stay within limits

* efficiency - a function of other operating condx, and also frequency

* power gain and/or drive level

* linearity - probably the most complex area of compromise, because each
different order of IMD reacts spearately to the other operating
conditions

* tube life

* power output - get what you can, but recognise that you may pay for it
in other areas of performance, especially linearity and tube life.


There isn't any "One True Optimum" value for RL. It depends on how
highly you rate each of the above factors, which all interact in a very
complex way. The tube manufacturer's recommendation is usually good -
after all, they should know more about the tube than anybody else - but
it is only a recommendation.

The K values are a highly simplified way of guessing at the optimum RL,
so it probably isn't worthwhile to get too concerned about the exact
values implied. 

The next best way is using the characteristic curves - but even here you
need to judge where to position the ends of the load line.

The best way of all is to experiment with the tube, using a variable
output network, measure all the various parameters, and find the
settings that give a good compromise between all of them. Then
disconnect the tube and measure what value of RL it was looking into.
That is precisely what the tube manufacturers did.



>Just curious and its time for a "new"  discussion here on the amps 
>reflector........

Good idea - it's certainly time for something a bit less sterile than
"amps versus antennas".

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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