Hi Jim,
What I said was, or tried to say, when a Class C rig is unloaded so
as to draw less current, that is, to tune the loading and plate circuit so
that the plate dip is lower current than it was when it is tune up for max.
The plate voltage will stay about the same but the plate current is less and
you have less RF output as well of course. In this scenario the ratio of Ep
/ Ip is greater. The Z that the modulator sees is greater.
Now if the plate voltage is lowered with out retuning anything, the
plate current will fall as the plate voltage falls and the ratio of the two
remains the same.
Basically when a class C rig is set and not retuned, the Ep:Ip ratio
is set and the plate current should follow the plate voltage up and down
linearly. The RF should follow the plate voltage up and down as well. Some
tubes and circuits need a little help with this. Such as using grid leak
resistance instead of a fixed supply if the stage is to be modulated. The
grid leak resistance will allow the grid voltage to fluctuate a little with
the audio as the plate current goes up and down. This actually helps to
keep the ration of IP to EP constant. The grid leak resistor is something
of a self regulator for the ratio. The screen grid tubes have a whole other
set of things that can be done to help the plate current to plate voltage
ratio remain constant.
There was a discussion awhile back about the plate voltage to plate
current ratios. Some one was saying that a circuit will lose efficiency if
the plate voltage is reduced. This is only true if the person changes the
loading or tuning. What they probably meant was that if you reduce the
voltage and try to retune to get the same power out that you would have less
efficiency. The only thing that should happen when plate voltage is reduced
is that the power input goes down and the RF power also goes down. If a rig
with a plate supply of 600V is putting out 100 watts into 50 ohms the RF
voltage would be 70.7 volts RMA. When the plate voltage is reduced to 300V
the rig should put out 25 watts with RF voltage of 35.35 RMS on the load.
Here is a chart.
600V EP
200ma IP
DC input power = 120 watts
RF output 100 watts
Plate dissipation = 20 watts
EFF = 83 percent
EP/IP = 3000 ohms modulator load Z
Now let's go down to 300 volts on the plate
300V EP
100ma IP
DC input power = 30 watts
RF output 25 watts
Plate dissipation = 5 watts
EFF = 83 percent
EP/IP = 3000 ohms modulator load Z
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Jim Wilhite
Sent: Tuesday, February 12, 2008 11:36 AM
To: amradio@mailman.qth.net
Subject: [AMRadio] RE: Using a Ranger as a driver
I forgot to mention John, you might explain how a Class C final sees
higher impedance when the plate voltage is lowered. As I recall Zp is =
Ep/(2X) Ip. So a Ranger with 600 V. on the plate with 120 ma. is seeing
~ 2500 ohms. With 300 volts at the same current, it should be seeing
~1250. Zp.
Maybe this will help some of these people and you are much more eloquent
than I.
Jim/W5JO
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