[AMRadio] RE: Using a Ranger as a driver


Gary Schafer garyschafer at comcast.net
Tue Feb 12 22:57:06 EST 2008


Hi Jim,

I think what John is saying is that you do NOT want to retune when you lower
the plate voltage in order to maintain the same plate impedance.

If you were to half the plate voltage with a modification to the transmitter
then during tune up you would want to tune up to half the plate current that
you had with full voltage before the modification. That will give you the
same plate load impedance and the modulator tubes will see the same
impedance. You are now operating at 1/4 the power you were when you had
twice the plate voltage.

But maintaining the same load impedance on the modulator is not necessarily
important: See below.

When reducing power by reducing screen voltage the plate voltage is going to
remain the same and plate current is going to be less, so the plate
impedance will be higher and the modulator will see a higher impedance also.

Now the modulator will not be able to produce as much power as it did before
because the modulator plates will flat top (go into limiting) with the
lighter load on them. That is if you try to pull the same amount of audio
out of them as you did before when they were working into the lower load
impedance. 
But this is not a bad thing because with reduced power in the final you
don't need as much audio to modulate it with. 

The load impedance that the modulator tubes see is not important as long as
it is low enough to be able to produce enough power to fully modulate the
transmitter before the modulator tubes voltage swings to the saturation
point. If the load impedance is too low the tubes will work too hard trying
to produce enough power out and will operate in the non linear part of the
plate curve.

Some transmitters are deliberately designed to have the modulator tubes flat
top and act as limiters to keep the modulation percentage below 100%. The
venerable Johnson Viking 2 is such an example. The modulation transformer
that they chose provides a little too high a load impedance to the modulator
tubes so that they go into clipping before 100% modulation is achieved.
Their idea of a soft clipper.

73
Gary  K4FMX


> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Jim Wilhite
> Sent: Tuesday, February 12, 2008 10:00 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] RE: Using a Ranger as a driver
> 
> Ok John,
> 
> I have but a few observations and questions that might steer everyone
> through this.  I am having trouble understanding how the Z the modulator
> see is greater if the plate load impedance goes down.  If the Plate Z is
> = to the Ep divide by 2 times the Ip and you change one value how does
> the Z remain the same.  Even if you maintain the same ratio doesn't the
> plate Z change as a result?  In your example you change both at the same
> ration which maintains the same impedance.
> 
> Ep   600
> Ip    200 ma.
> 
> Zp= 600 divided by 2 times .2 = 600/.4 = 1500
> 
> Ep    300
> Ip    100 ma.
> 
> Zp = 300 divided by 2 times .1 = 300/.2 = 1500
> 
> The ratios stay the same, but what I question is if you reduce the plate
> voltage of a tube but load it to the same value of Ip or near it, the
> impedance changes on the secondary of the mod transformer, which I
> guess, is where the divide is.  I have seen many people make this
> mistake.  Because if you do, to maximize power or whatever reason, the
> impedance seen by the primary of the mod transformer is not optimum.
> 
> In the discussion of the way to drive the amplifier no one pointed out
> that you must maintain the ratio of Ep to Ip.  This is where I kept
> sticking.  Now the question becomes, why did Johnson say to load the
> Ranger to rated plate voltage and current but insert the pad between the
> transmitter and amplifier?  Just guessing here, but I would bet that
> amateurs of the 50s were much like those of today.  All knobs and stated
> reading must be to the right.  Or is there something that is seen on the
> primary of the mod transformer by the Class A or B modulator tubes we
> haven't discussed yet?
> 
> Jim/W5JO
> 
> 
> 
> 
> 
> > 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
> 
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