[AMRadio] RE: Using a Ranger as a driver


doxemf at aol.com doxemf at aol.com
Tue Feb 12 23:53:29 EST 2008


Gary,

That is exactly what I meant by the sweet spot in the final current. By 
dropping the screen voltage only, it would allow better modulation and 
also reduce saturation. I found that my VKII had the best audio and mod 
% when run at about 90 watts on the TenTec tuner meter. Reference only 
really.Plate current was reading no more than 190-200ma and mod current 
swinging to about the same on peaks. Not sure just how accurate the 
stock meter is. The meters had different dynamics in the 2 VKs that I 
used to run.  Been about 5 years ago and the 807s were driven by a UTC 
line to grid directly from the audio rack.

This has been a great thread !
Thanks,
Bill, KB3DKS in 1 land

-----Original Message-----
From: Gary Schafer Subject: RE: [AMRadio] RE: Using a Ranger as a driver

>SNIP

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




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