[Johnson] A question from A New BA operator.

Thu, 21 Mar 2002 13:08:08 -0500

Thanks, as I indicated, I hadn't given any thought to this before
mentioning it.

Never to old to learn.......

73, Ray  W2EC

Gary Schafer wrote:
> 
> 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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> > > > >
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