|[AMRadio] High level modulation of tetrodes|
John E. Coleman (ARS WA5BXO)
wa5bxo2005 at pctechref.com
Tue Sep 27 20:54:28 EDT 2005
I changed the subject title here to somewhat match the subject.
When a choke or a high value resistor is used in series with the
screen supply line the screen grid will self modulate following the voltage
on the plate to some degree. This happens because if the plate voltage is
increased the plate will attract more electrons to it leaving less for the
screen. The reduced screen current will cause the drop across the resistor
or screen choke to be less and therefore more voltage on the screen. The
opposite thing happens if the plate voltage is reduced as the audio take it
down less plate current is drawn and more is available for screen current.
The increase in screen current causes a larger drop across the choke or
screen resistor resulting in less screen voltage
These changes will not be 100% proportionate. But that is generally
good because it has been found that when the plate modulated at 100 percent
the screen should be modulated at approximately 40 percent.
Many times I have found 6146 rigs will have a premature drop in RF
voltage at about 85-90 percent on the negative cycle. This is caused by too
much audio on the screens. It occurs when the screen dropping resistor
supply side is connected to the high side of the modulation XFMR.
Connecting the screen resistor to the low side of the modulation XFMR will
result in too little audio on the screen.
The screen choke with a fixed screen supply works well but there is another
fix for small rigs.
One fix would be to have a separate winding on the modulation XFMR
and a fixed screen supply. This scenario means hard to find parts and
complex circuits. I have found that a good fix is to leave the original
screen resistor in place and find a value of resistor in series with a audio
bypass capacitor that when placed from screen to ground will reduce the
audio on the screen to 40% while leaving the DC voltage on the screen at the
level it needs to be.
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