The
modifications below are what I call Level One Mods. These will improve
the audio and overall efficiency of the Valiant transmitter. Set up is
for D 10-4 or other high impedance peizoelectric microphonium.
EF Johnson produced the Valiant in mass quantities. It seemed
impressive with three 6146s in the PA stage, a pair in the modulator
and a big-assed silver plated tank coil for all the world to see on top
of the chassis. The reality is a transmitter fraught with reliability
issues in the PA section with underrated tank components that tend to
fail.The anemic modulator has all it can do to reach 100% modulation.
The audio quality is that of two dixie cups and a string. There is also
an audio clipper circuit that just adds more distortion and less talk
power when engaged. One might as well not use it.
The choice of using the 6146 in modulator service is a mistake. While
the 6146 is a great RF tube it is a poor audio tube. After solid
stating all of the power supplies and re -arranging the rocks, it is
possible to stuff in some rather large modulation components and other
heavy duty items to make the Valiant into a real performer. It is
possible to stuff a pair of 811s into the modulator or big TV sweep
tubes. The combinations of tubes and components are almost endless. It
becomes dependent on the stash of parts as to which way the wind is
going to blow here.
The following modifications are for those who want to make the best of
what is there in the first place. While the modulator will still strain
to make 100% it will have much better audio quality than it originally
did.
Read carefully and make sure you have ALL of the components before
starting. DO NOT shortchange the modification process by eliminating
steps with the thought of finishing it later. Later sometimes never
comes.
There are many other Valiant transmitter modifications by others. Most
involve changes to transformers and other components. The only real
change is, perhaps, the use of the 12B4 as an audio driver . You can
still use the 12AU7. It will work harder in the circuit due to the
lower cathode resistor value. The 12BH7 will give somewhat better
results and have better life expectancy. With the changes to the plate
and grid resistors in the speech amp stages there will a noticeable
increase of gain, probably too much. If so, simply change the first and
second speech amp stage (V12) from a 12AX7 to a 12AU7. The 12AT7 has a
gain figure somewhere in between the two.
I did not cover solid stating the power supplies in this article. You
may if you want. There will be no increase in the high voltage B+ when
changing the 866s to solid state. Mercury vapor tubes have a forward
drop of only 15 volts regardless of load. The 3B28 works just as well.
Solid stating the low voltage rectifier raises the B+ voltage to 400
volts.
A few changes will have to be made to accommodate the increase on the
low voltage.The drive control pot is a weak link in the system. Place a
5k, 5-10 watt wire wound resistor in series with the bottom of the pot
as well as another 5k resistor in series with the B+ to the top of the
pot. This will reduce the dissipation of power in the pot and improve
reliability. One may also go with choke input which will drop the low
voltage to 250 volts. This will greatly reduce the load burden on the
LV and filament transformer. The downside is that there will not be
enough drive on 10 meters without some changes.
Here are the audio mods.
1. Change R-25
(1Meg) to 10-22 meg ohm (first stage grid resistor)
2. Reverse places in circuit for R26 (47k plate resistor) with R53
(220) decoupling resistor. R26 becomes 220K and R 53 becomes 47k.
3. Add a 10-40 uF/450 volt electrolytic in parallel with C 76 (0.1 uF
decoupling cap)
4. Replace C 79 (0.01 disc ceramic audio coupling cap) with a
0.01-0.05/600V orange drop or other good quality tubular cap.
5. Remove C 60 (200 pF cap)
6. Disconnect Phone Patch input from pin 8 of V12-B and reconnect it to
top of R 28 (audio gain). This changes the phone patch to a line level
input that is useful for connection of an external preamp/EQ
using a high quality microphonium or play back from a tape
recorder or computer sound card through the transmitter.
7. Change R 29 (680 ohm cathode resistor from pin 8 of V 12-B ) to
2.7-3.3k. Place a 25-100 uF/10-25 volt electrolytic cap across the
cathode resistor. This becomes a cathode bypass cap added to the
circuit.
8. Parallel a 10-40 uF/450 volt electrolytic cap across C82 (0.1MFD
decoupling cap)
9. Change R 30 (47k plate load to pin 6 of V12-B )
to a 150-220k resistor.
10. Bypass the clipper stage entirely by disconnecting C 83 from pin 5
of V13 (6AL5 clipper stage) to pin 6 of V 14 (6C4 third audio stage).
While you are at it replace C 83 with a 0.01-0.05/600V orange drop or
other good quality tubular cap,( See step 4).
11. Change R-37 (grid resistor from pin6 to ground on V 14) with
330-680k resistor.
12. Change C -90 (0.01 disc ceramic cap from pin 1 o r5 from V 14 6C4
to pins 2&7 on V 15,12AU7) to a 0.01-0.05/600V orange drop or
other good quality tubular cap. (See steps 4 and 10)
13. Change R43(150 K grid resistor from pins2 and 7 of V15 to
ground with a 330-680K resistor. (see step 11)
14. Remove C 85 (0.1MFD decoupling cap)
15. Connect a 82-100k resistor from pins 1 or 5 on V14 (plate of 6C4)
to plates of V 15 (12AU7) pins 1&6. This added step adds
negative feedback around the audio driver. This is an important step.
16. Replace R45 (820 ohm 1/2 watt cathode resistor from pins
3&8 of V15,12AU7 to ground) with a 470-560 ohm 2 watt resistor.
17. Replace C 99 (10 uF cathode bypass cap from pins 3&8 of V
15 (12AU7) with a 50-100 uF/25 -50 volt electrolytic cap.
18. Connect a 47k 1 watt resistor across secondary of T4 (audio driver
transformer) from pin 5 (Grid) of V16 (6146) to pin 5(Grid) of V 17
(6146). This provides a constant impedance swamping load on T4 . This
is an important step.
19. Replace V15,12AU7 with a 12BH7. The 12BH7 has the same pin out and
is a much more rugged replacement for the 12AU7. The best tube to use
would be the 12B4. This tube is a low Mu triode. Its use will require
rewiring the socket and changing the cathode resistor (R 45) to a 1800
ohm 10 watt wire wound resistor and upping the voltage rating of C 99
to 100 volts minimum. The 12B4 is a superior tube for audio driver
service. These tubes are commonly used as series pass regulators in
various pieces of test equipment . Between the use of negative feedback
around the driver stage and the low plate resistance of the 12B4, the
lack of inductance of T4, which kills the low end frequency response,
is overcome. The use of the 12BH7 is a step in the right direction
though.
20. Connect a 100-250 uF/100-150 volt electrolytic cap from the center
tap of T4 (audio driver transformer) to ground. Remember we are dealing
with a negative voltage here, so place + end of cap to ground, - end to
the center tap of secondary of T4. This provides for dynamic bias
stabilization as well as providing a low audio source impedance for the
bias source.
21. Add a 220-330 ohm resistor (2 total) in series with the screen
voltage source to each modulator tube screen (pin 3 of V16 &
V17). This provides a degree of current feedback around the 6146
modulators.
22. Connect a 10-25 uF/350 volt electrolytic capacitor from pin 2 of V
9 (VR105 Screen regulator tube) to pin 5 of V 10 (VR105 screen
regulator tube. Remove jumper wire between pin 2 of V10 and pin 5 of V
9. Insert a 330-500 ohm 1 watt resistor in place of jumper wire. This
resistor breaks up any possibility of the relaxation oscillator effect
that can happen when a capacitor is placed across a gaseous regulator
tube. The purpose of this capacitor is to provide a low source
impedance to audio frequencies that are present at the screen.
Bypassing this step will allow a noticeable audio distortion to be
present when modulation exceeds 70%.
The basic audio mods are now complete. We must move on to the PA stage.
The reasoning here is that the 6146 PA tubes are not necessarily
operating into good hard class C operation. The low value of the screen
dropping resistor could be responsible for over dissipation of the
screen grid when grid current is brought to 10 Ma or better. The grid
bias is now a combination of grid leak and fixed bias. It is preferable
to have total grid leak bias for best modulation linearity.
Due to the protective bias scheme in the Valiant, I have come up with a
compromise usubf part fixed bias and part grid leak bias.
Here are the PA stage mods.
1. Replace
capacitors C 47 & C 48 (0.01/1.5 kV disc ceramic caps) with
0.001-0.003/3kV disc ceramic caps. This improves high audio frequency
response performance.
2. Add a 5k 10 watt wire wound resistor in series with R 16 (12k/50
watt screen dropping resistor) The screen dropping resistance for three
6146s should be 18-20k total, NOT 12k.
3. Insert a 4700-5000 Ohm 2 watt resistor from the wiper arm of R 61
(RF Bias pot) to R 57 (Grid current meter shunt).
4, Replace C 37 (500 pF TV doorknob cap., Plate blocking cap) with a
1000 pF 5Kv Centralab or equivalent RF-rated door knob capacitor. This
improves efficiency on 160 meters.
In order for the modifications to be effective ALL of the changes
outlined MUST be done. No skimping out and saying, "well its good
enough for now." It will be noticed that there are a number of
component values over wide range that may be used with good results
either way. If you do not have a 0.01 uF coupling capacitor for the
mods and you have a stash of 0.033 uF caps, use them.
The following is an addendum to the modifications to resolve various
ills of the Valiant transmitter. These SHOULD be considered.
There are a number of weak links in the RF section of the Valiant. For
starters there is an 18k/2 watt resistor in the VFO compartment (R3).
This resistor often fails with disastrous results. It is a pain in the
scrote to get at. The value of the resistor is too low in the first
place and causes an undue amount of heat to build up in the VFO. The
resistor SHOULD be replaced with a 25-30k/10 watt wire wound resistor.
This is a permanent fix. I also took a chassis punch and punched a one
inch hole in the top plate of the VFO compartment above the 6AU6 VFO
tube and the 0A2 VR tube. This allows for easy replacement of the tubes
when the time comes. The holes can be covered with hole plugs used for
electrical work.
Other issues with the Valiant are the band switch and the associated
padding capacitors. Ameritron replacement parts are very usable here.
The plate band switch section of the Ameritron AL 572 and the Ameritron
AL 80 works very well and the contacts can be doubled up. It will be
necessary to make use of one of the other wafers that is used to select
padding capacitors in the AL 572 amp to select the 100 pF and 300 pF
padding caps used in the PA tank circuit. The switch wafer will have to
be modified by adding additional contacts to engage the 300 pF padder
on 80 as well as 160 meters. You may parallel up both sections of the
dual air variable used for plate tuning. This will eliminate the need
for the 100 pF padding cap used for 40 meters. The padding capacitors
tend to crap out. You can use a combination of series/parallel mica
capacitors. Ameritron uses 170 pF transmitting doorknob capacitors as
padders in the AL 572 amplifier. Two of these caps paralleled up will
be 340 pF, a good replacement for the 350 pF tank padder. A
single 170 pF doorknob cap may be used for the 40 meter padder.
Ameritron also uses 210 uF/450 volt computer grade capacitors in their
power supplies in many of their linear amplifiers. These make ideal
replacements for the two 80 uF caps used in the HV supply in
the Valiant.
The fixed loading padding capacitors are also known for failure. There
is a stud type assembly of four mica capacitors The values are: 300 pF,
600 pF, 900 pF and 1200 pF. Some of these values are not necessarily
common place. A 500 pF transmitting cap may be used in place of the 600
pF cap. A 1000 pF cap my be used for the 900 pF cap. Check
the internet for other sources of RF rated transmitting capacitors.
Modification
Addendum: There is the icing on the cake - negative feedback around the
modulator. It is not a necessary modification but it does greatly
improve the overall audio response and distortion figures.
Negative feedback may be added by connecting an appropriate resistor
from the top of the secondary of the mod transformer (T3) to the
cathode of V14 (6C4 3rd audio) Pin 7. It is best to start out with a
high value of resistance to see if the feedback is positive or
negative, about 2 Meg ohms for starters. This should be done with two 1
Meg ohm 1 watt resistors in series with a 0.1-0.5,/1000 volt capacitor
in series with the resistors to block the high voltage DC which is
present on the modulator output. If the overall gain goes up or the
modulator breaks out in a squeal oscillation the feedback is positive
and not negative. To correct this condition simply reverse the plate
caps on the 6146 modulator tubes. The gain should decrease.
The objective is to have the gain decrease by 6-10 dB as long as the
modulator is stable and not breaking out into ultrasonic
parasitic oscillations. A scope should be used for this set up
procedure. The amount of negative feed back is increased by reducing
the value of the feedback resistors from 2 Meg ohms to a value like
maybe 600-800k. When the value has been determined use a number of
100-150k 2 watt resistors to make up the total value.
While negative feedback is not necessary to this basic modification,
the addition of negative feedback is a very good addition to the
overall improvement of audio performance of the Viking Valiant
transmitter.
In closing, one of the problems with most of the vintage transmitters
is getting rid of heat. Heat shortens component life and
increases the likely hood of crap out. The VFO stability is also
affected by the large change in internal temperature rise within the
cabinet. The perforated metal cabinet of the Valiant tends to
hold the heat in. Simply cut an appropriate sized hole in the rear of
the cabinet to accommodate a 4-inch muffin fan. Operating a 240 volt AC
muffin fan at 120 volts to exhaust the hot air from the cabinet will
reduce the cabinet temperature to that of the ambient air temperature
of the environment it is placed in. The 240 volt fan will run very
quietly and not have the annoying whine of the typical muffin fan. The
AC connection are made from an additional 2 pin plug mounted on the
rear of the chassis. The connector should be wired in parallel with the
primary of the low voltage and filament transformer.
Good luck. I am looking forward to hearing many improved signals on
the bands emanating from various Valiants throughout the land.
Tim WA1HnyLR |