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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