as
RF input.
The 4X150a deck can produce 650 watts of audio at 2000 volts and 500ma.
I have done lots of expeiments over the years, with different mod iron,
modified hysing,
and since both the mod and rf deck are variac'd, lots of different power
levels
and voltages.
Things always seem to work better reducing the RF current, and increasing
the mod
voltage.
Its my theory that the extra mod voltage allows the tubes to run in the
low
distortion range, and the RF tubes have reserve emission for modulation
peaks.
Brett
N2DTS
----- Original Message -----
From: "Ben Dover" <[EMAIL PROTECTED]>
To: "Brett Gazdzinski" <[EMAIL PROTECTED]>; "Discussion of AM
Radio in the Amateur Service" <amradio@mailman.qth.net>; "Discussion of AM
Radio in the Amateur Service" <amradio@mailman.qth.net>
Sent: Monday, November 12, 2007 2:00 PM
Subject: Re: [AMRadio] Re:Reactor Wanted
-----Original Message-----
>From: Brett Gazdzinski <[EMAIL PROTECTED]>
>Sent: Nov 12, 2007 12:14 PM
>To: Discussion of AM Radio in the Amateur Service
<amradio@mailman.qth.net>
>Subject: Re: [AMRadio] Re:Reactor Wanted
>
>I also run 2000 volts on the plates of the modulator, 4x150a's, and
usualy
>1500
>volts on the pair of 813's in the final.
>
>It seems to like it that way, i have done experiments as both supplies
>have variacs, things always seem to modulate better, and sound better
>with more mod voltage.
>
>Brett
>N2DTS
>
Howdy, Brett!
Interesting indeed.
That approach flies in the face of decades of design convention; as a
rule, AM
rigs either use the same supply for mods and final, or as the rig gets
bigger,
the mod has it's own supply which is usually a lower voltage than the PA
plate
supply.
The only reason I can see for improved performance using this approach
would be
that when used conventionally the modulator operating parameters were
incorrectly
chosen for low distortion AT THE POWER LEVEL REQUIRED. I've noted over
the
years
that most ham designers tend to be very optimistic when it comes to
estimating the
power that they'll get out of a modulator, and tend to not recognize what
they'll
lose in the transformer, especially those losses that come from a
significant
impedance mismatch between the mod plates and the PA load impedances.
The usual result is that they can't get to 100% modulation, or if they're
using a
pair of pentodes or tetrodes as modulators, they find it necessary to run
the tubes
well into the nonlinear area of the curves caused by the action of the
screen grid
in order to get enough power.
Jacking up the plate voltage to the modulator is ONE way to get the
additional watts
needed without correcting the design defects that caused the problems in
the first
place. In addition to giving more audio power, raising mod plate voltage
will also
increase the amount of low distortion headroom the modulator has
available.
Other than that, I can't see ANY reason, either in terms of physics or
intuition,
for raising mod plate voltage above the PA causing an inherent increase
in
audio
quality.
On the other hand... if the modulator parameters have been properly
chosen in the
design, AND losses in the mod transformer and due to the (almost
inevitable) minor
impedance mismatch are taken into account (so that it's not necessary to
drive the
mod tubes into nonlinearity) when the mod is run at the same B+ voltage
as
the PA
or less, jacking up modulator plate voltage shouldn't make ANY difference
whatever
in resulting signal quality... except INCREASING the distortion, caused
by the
change in mod tube plate to plate impedance creating a mismatch with the
PA load!
What it boils down to is this...
It's GOOD to design a modulator that's capable of producing MORE than 50%
of the PA's
input (i.e., a mod that has some headroom) no matter WHAT the plate
voltage of the
modulator is relative to the PA. If it's got headroom, it's gonna distort
less.
It's BAD to underestimate mod transformer losses (figure 20% or more of
the audio you
put into it ain't gonna come back out). It's even WORSE to pull another
common ham
stunt, driving your mod transformer core toward saturation (face it...
mod
transformers
are getting harder and harder to find, and they AIN'T CHEAP anymore.
Using
too small
a transformer for the design is VERY tempting sometimes).
73's,
Mr. T., W9LBB
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