Related Pages:
Audio
Mod Johnson Ranger
Heising
Modulation
Amplitude Modulation
I became
infatuated with
Valiant when I first
heard W8YAT on 160
meters in 1962. Bob
was using a Valiant
with a D-104. The
big thump when that
carrier came on and
such
audio!
I bought my first
Valiant in 1970.
Chassis View
There are a lot
of Internet
modifications to the
Valiant. My take on
the mods is people
are getting a little
too carried away.
The Valiant isn’t
all that bad. As
time permits I’ll
add my mods here.
All of my mods are
verified with test
gear, including
spectrum and
distortion
analyzers.
HV Power Supply
Some people claim
the power supply is
weak. Factually, the
supply is pretty
well engineered
unless you plan on
tearing the Valiant
all up and making it
into a totally
different
transmitter.
I tested the
supply on CW and AM.
CW is easy. On CW,
strings of dots at
various speeds would
show power supply
defects. CW is also
the worse case
loading for the
supply, since the
dynamic power supply load varies
quite abruptly and to
extreme amounts as
the tubes go into
and out of
conduction between
dots and dashes. The
unkeyed power supply
load current is a
few dozen mA, and
the full load
current is around
500mA.
AM is a little
tricky to test,
since there are both
syllabic and tone
variations. The
supply has to be
reasonably stable
for slow dynamic
variations and not
have excessive
ripple induced at
audio frequencies by
the peak load caused
by peak modulator
current. Let’s look
at just how good or bad a
Valiant is.
CW Test
Running 475mA
(ouch) and adjusting
dot duty cycle and
speed for maximum
ripple, here is what
my stock Valiant
does:
The scope is
scaled for 100v/div,
or 20x scale
reading. 800 volts
is the top line. The
scope trace says
ΔV1=9.20V on the top
left. Multiply
9.2*20 = 184
The power supply has 184
volts p-p dc
fluctuation with
load. This
fluctuation occurs
because the power
supply choke cannot
recover fast enough
to impede current
when load suddenly
stops, and likewise
impedance cannot
fall fast enough to
hold voltage steady when
load starts.
The
mean supply voltage
in my Valiant is 760
volts with no load.
It is 620 volts
under normal CW
load. Most of that
sag is because the
filter choke is
marginal on
inductance for the
bleeder current.
This allows the supply to
move higher than the
.9*RMS of a perfect
choke input supply.
We should not
interpret a no-load supply
voltage increase
from lack of bleeder
current or critical
inductance as “weak
supply”.
Well-designed unregulated
single phase
supplies typically sag 10-15%
from no-load to full-load. The Valiant
sag is less than
20%, and that could
easily be improved
to 12% or better
with an increase in
bleeder current or
filter choke
reactance.
As for the
voltage bounce,
almost any choke
input supply behaves
this way. Even a
commercial BC
transmitter will do
this when the
carrier is keyed at
the right rate. This
is not a sign of a
weak supply. It is
typical of
choke input supply
behavior under CW
keying loads. If we CW
key a 900-pound
kilowatt AM BC
transmitter with a
choke input single
phase power supply,
it will behave the
same way!
While I have not
addressed this issue
because it doesn’t
mean much to me or
the signal, the first
thing I’d try is
resonating the choke with a
non-polarized
capacitor. It is
often possible to
find a “sweet value”
of capacitor that
takes much of the
no-load voltage
increase out. The
capacitor-choke
combination should
be made
resonant slightly
above 120 Hz. There
should be about 400
volts or so peak AC
across the filter
choke. From
appearances and my
experiences with
other choke input
supplies, I would
say the addition of
a single component
(a capacitor across
the choke) would
pull no-load voltage
down to 700 volts or
less without
changing full-load
voltage.
A second option
would be to load the
supply heavier with
a lower resistance
bleeder resistor,
but that would waste
power and make
extra heat.
The no-load
to full-load
bounce would be
greatly reduced.
Maybe someday
I’ll try tuning the
Valiant supply choke
to reduce the
no-load voltage
peak. For now, it
does not appear to
be that big of a
deal. Remember the
scope capture above
shows the WORSE CASE
dynamic load
variation I could
find.
We can see the
effect of the power
supply sag
and overshoot on the
CW envelope. Look at
the hump in the
leading edge of the
CW envelope in the
following picture:
Notice the
leading edge
overshoot. The
envelope overshoot
at the leading edge
is created by HV
supply dynamics.
Since the envelope
is rounded and
occupies 7.5 ms, it
does not affect
bandwidth. I opted
to not do anything
about it. (By the
way, I reshaped the
CW waveform from
stock, more on that
later.)
AM Dynamics
To test AM, I
used a pulsed tone.
I set the tone for
maximum ripple and
maximum dynamic
variation. This
wound up being about
300 Hz AF, and a
pulse rate of 5 Hz.
This test was
running 350 mA and
slamming the
modulator current
against the pin. you
will never have it
this bad in real
life. Here is the
worse case power
supply error on AM:
The 300 Hz audio
ripple is down in
the noise of power
supply hum, and the
total pulsed-audio dynamic
voltage variation is only 64
volts p-p, or 10%.
This pulsed variation is at
a syllabic rate, and
is only this bad
under worse-case
pulse audio
conditions.
Supply voltage
variation is
not measurable as
audio distortion on
the output. Without any
question, the
Valiant does not
need power supply
work for AM
operation. (By the
way, this is with
old original
electrolytics!)
Modifications
Audio
Here is the 3kHz
envelope of my
Valiant after
minor
modification
at 100% modulation:
The photo below
shows modulation at
350Hz. Notice the
rolling off after
the peak. The
Valiant is running
out of iron in the
modulation
transformer (the
grids of the 6146’s
are fine) at low
frequencies.
Comparing both envelopes,
there is very
little amplitude
change. For a fixed
generator input, my
Valiant audio
response is almost flat from
about 300 Hz to 3500
Hz. The addition of
a few capacitors flattened
the audio response. I
did not
remove or change
the value of power
amplifier RF
bypass capacitors. I
did not remove
the low-pass audio
filter that follows
the audio clipper.
As a matter of
fact removing the
audio lowpass filter
is detrimental.
Removing the
low-pass caused the
response to
sag above 2500Hz!
The audio filter,
besides starting
rolloff at 3500Hz or
so, adds a slight upward
modulation gain
increase as an audio
frequency of 3500 Hz is
approached. This
upward slope is just
right to compensate
downward slope
caused by bypass
capacitors. Take
that filter out, and
you’ll also have to
remove bypass caps.
Removing the bypass
caps is bad for RF,
and allows harmonic
and intermodulation distortion
products generated
in the modulator
driver and output
stages to modulate
the PA for 10kHz and
further away. No
need to do that!
related pages:
Amplitude Modulation
page
Ranger and Valiant
audio page
|