[Johnson] A question from A New BA operator.

Thu, 21 Mar 2002 12:24:36 -0500

The KWS-1 and Valiant are entirely different types of finals. The KWS-1 finals
run class AB1. That is how you control the output level on an AB1 amp. Reduce
the drive. When you run AM on it you need to reduce the drive until you have
1/4 the carrier output that you have at full power. You will notice that the
efficiency also drops as you reduce the drive. From full power you run about
58%. When you reduce the power to 1/4 for AM operation the efficiency drops to
around 30%. This is normal for AM operation. The amplifier still needs to be
tuned at the 550 watt level to handle the peak power of the AM signal.

If you wanted to reduce the power out for CW operation you could reduce the
drive and retune the final to get better efficiency at the lower power level.
However you could not operate AM like that as you would flat top the audio
peaks.

The class C amplifiers like those in the Johnson rigs work different. You must
always maintain the proper amount of grid drive. If you do not the efficiency
will drop and the tubes will dissipate too much power. They will not modulate
properly either. The power output will drop on them when you reduce the drive.
That happens mostly because they use a clamp tube on the screens of the finals
to protect the tubes if drive is lost. The clamp tube is a shunt tube with
it's grid driven by the grid current developed on the final grids. When the
clamp tube sees the grid current it "unclamps" or stops conducting. That
allows the screen voltage to rise to its normal level. If you reduce drive the
clamp tube starts to conduct and shut down the screen voltage.

If you try to operate AM with the clamp tube conducting it will inter fear
with the modulation voltage of the screen. It is also not good for the finals
to be run with low grid current.

The drive control on the Johnson transmitters is a pot that controls the
screen voltage of the DRIVER tube. That controls the power out of the driver
tube and thus controls the grid drive to the final.

Tuning the final by lightly loading it to reduce power out causes the final
screen current to go way high. Although the screens are somewhat protected
because they are supplied through a dropping resistor so as the current goes
up the voltage drops. However even though the screen voltage drops with
increased current and helps limit the maximum screen current you can still
damage the screens this way.
That is why you should tune up quickly and get the final loaded without
leaving it lightly loaded for very long.

If you want to reduce the power of the final the only real way to do it is to
switch in a larger screen resistor in the final screen circuit. Lowering the
final screen voltage will drop the screen current and the power out safely.
Then the only thing that you have to worry about is the amount of screen
current that your modulator tubes will draw. Because you have lowered the
power of the final you will be running less final plate current. Less plate
current means that the modulator is going to be seeing a higher load impedance
than before. That unloads the modulator plates and causes the modulator screen
current to go up.
It is the same thing as unloading the final plate circuit. If you do not load
it heavy enough the screen current goes way high.  With the modulator you may
not have a problem though as the demand for audio will be lower than it was
before so you will have to turn the mike gain down and not drive the modulator
as hard.

73
Gary  K4FMX

"Ray, W2EC" wrote:

> That's interesting. As I said, I hadn't tried it on the Valiant. I
> mentioned it only because that is the method indicated by Collins to run
> the KWS-1. According to the Collins manual, you load the KWS-1 (pair of
> 4X250B's) to full output, then reduce the grid drive (watching the plate
> meter and a power output meter) until you get to a power output level
> you want. For example, I load the KWS-1 to 500ma which gives about 575
> watts CW out. Then I reduce the grid drive to a plate reading of about
> 275ma, which gives about 140 watts CW out. This is the exact procedure
> as depcited in the Collins Manual for reducing power and is how I run AM
> with the KWS-1.
>
> Without thought or investigation I just took it that dropping the grid
> drive a bit in the Valiant would provide the same power reduction. What
> would be the different forces at play that would make it OK to do it on
> the KWS-1 but not on the Valiant?
>
> 73, Ray  W2EC
>
> [email protected] wrote:
> >
> > Always run the 6146's at their rated grid drive voltage. Loading up the
> > Valiant to full power and then using the grid drive control to reduce
> > power is wrong. It will cause AM distortion and drastically reduce the
> > life of the tubes. If you want to reduce output power, put a 3, 6, 8, or
> > 10db pad between the transmitter output and your antenna. A less
> > attractive way, and not normally recommended, would be to put a variable
> > pot for the screen resister of the driver tube. Always run the output
> > tubes at their rated grid voltage.
> > Pete,WA2CWA
> >
> > On Thu, 21 Mar 2002 09:12:55 -0500 "Ray, W2EC" <[email protected]>
> > writes:
> > )........
> > >
> > > Another thought, and I haven't tried this, is to load the Valiant up
> > > to
> > > full power (330ma for AM, 450ma for CW), peak the grid then back off
> > > the
> > > grid drive (normally 8ma) until you get the power level you want.
> > >
> > > 73, Ray  W2EC
> > >
> > > "Guy Olinger, K2AV" wrote:
> > > >
> > > > Correct. Run it at specs.
> > > >
> > > > The plate voltage is fixed, the screen voltage is fixed, the L/C
> > > > ratio's in the tank circuit are only partly variable since the L
> > > being
> > > > used on a given band is fixed. The whole shebang is at maximum
> > > > efficiency at the specified settings.
> > > >
> > > > I've seen guys operate a Valiant AM until the cows came home.
> > > Just
> > > > don't drop a book on the mike switch and get up and leave. When
> > > your
> > > > voice starts to hurt let the other guy transmit :>)
> > > >
> > > > ----- Original Message -----
> > > > From: "Dr. Clifford L. Coleman" <[email protected]>
> > > > To: <[email protected]>
> > > > Sent: Wednesday, March 20, 2002 4:23 PM
> > > > Subject: Re: [Johnson] A question from A New BA operator.
> > > >
> > > > > While restoring my valiant, I found(as the manual suggests )
> > > that
> > > > less
> > > > > loaded causes high modulator current-so I tune it up to full
> > > power.
> > > > cliff
> > > > > k9jts
> > > > > ----- Original Message -----
> > > > > From: "Mullin, Edward J." <[email protected]>
> > > > > To: <[email protected]>
> > > > > Sent: Wednesday, March 20, 2002 1:40 PM
> > > > > Subject: [Johnson] A question from A New BA operator.
> > > > >
> > > > >
> > > > > > Hello All,
> > > > > >
> > > > > > I've been lurking on this mail list for a while, as I've been
> > > > > > working on my Valiant.  So far I have gotten it working
> > > again,
> > > > (recapping
> > > > > > and replacing a couple of tubes was all it needed:)   Now I
> > > want
> > > > to use it
> > > > > > on the air.  (I've been testing into a dummy load during
> > > > restoration). I'm
> > > > > > new to BA's (and Ham operations) in general, and  I've been
> > > > reading the
> > > > > > manuals alot.  What I want to know is: What is the correct
> > > method
> > > > for
> > > > > > transmitting at less than full power. The manual is good at
> > > > describing the
> > > > > > tuning process, but it doesn't really cover this
> > > information.(at
> > > > least I
> > > > > > didn't figure out where it was.)
> > > > > >
> > > > > >
> > > > > > 73 de KB1HYS
> > > > > >
> > > > > >
> > > > > > > Edward Mullin
> > > > > > >
> > > > > > >
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> > > > >
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