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Amplitude Modulation
Heising Modulation
After extensive
search, review, and testing,
I wound up doing
minimal
modifications to my
Valiant. I found many
mods did
nothing, and a few
that actually hurt
performance.
I added negative
feedback from the
secondary of the
modulation
transformer to the
cathode of the 6C4.
I did this through a
.01uF 2kV
capacitor in
series with a 2 megohm HV resistor.
Reversing the
modulator plate caps
is sometimes
necessary to get
feedback phase correct.
The value of the
resistance controls
the level of the
feedback. Just be
sure to use high
voltage rated
resistors, or
several lower
voltage lower
resistance resistors
in series.
The diode clipper
sometimes placed in
the secondary of the
mod transformer does
no good at all. It’s
a total waste of
time. Don’t bother.
If non-symmetrical
clipping is desired,
to enhance positive
peaks without going
beyond 100% on
negative peaks,
the 6AL5 clipper can
be modified. I disabled
one diode in the
6AL5 by putting a
.022 1kV disk
capacitor across
6AL5 one
diode section from
anode-to-cathode.
Watch on a scope,
and pick the diode
that limits NEGATIVE
RF peaks.
I decreased the
impedance change at
the grids of the
6146 tubes by
swamping the
interstage
transformer
secondary with a
100K 1-watt
resistor.
I used a higher
transconductance
dual-triode as a
driver, setting bias
at the maximum
dissipation of the
tube. This made very
little change, but
it did help.
I increased
coupling caps
(C79,83, 84 and 90)
by adding .05uf in
parallel with the
.01uF caps. I did
not bother removing
the old caps in case
I wanted to restore
back to original
sound. Ceramic disks
work perfectly. It is
NOT necessary to use
paper or Mylar
capacitors. There is
absolutely no reason
to use anything else
in audio circuits,
piezo-electric
effects of ceramic
are inconsequential
except in high-Q
circuits like
filters or
oscillators.
I increased bias
on the 6146 PA tubes
by adding a small 1
watt 10K resistor in
series with the grid
return path. This
new resistor should
go between the lead
from the grid meter
shunt resistor and
the final bias tap
on R22, the large 5K
power resistor for
bias adjustment.
This resistor adds
about 10 volts of
bias for every 1 mA
of PA grid current,
making the 6146’s
move
further into class C
without increasing
grid current. This
reduces conduction
angle, and reduced
conduction angle increases
modulation linearity.
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