These have been done & redone over the years by myself - WA1HZK,
Tim-WA1HLR, Chuck - K1KW , Brent - W1IA & Mike - N1IW
Some High Points to consider doing:
#1 Replace All of the filter caps. Use higher values, Bigger is better.
#2 Remove C54, C55 & C56 on the accessory plug. (Better Highs)
#3 Change the Plate Choke Bypass Caps to .001 2KV RF types. (Better Highs)
#4 Remove the OC3 tubes, replace the two OC3's with (1) OD3 to lower
the screen voltage to 150 VDC, rewire sockets to use (1) tube. (Timtron
Mod.) I have not done this to mine.
#5 Change the Screen Resistor (R16) from 12K to 20K. (Lower Voltage saves
the tubes) (Timtron Mod) Only necessary if changing the screen voltage.
#6 Change the series dropping resistor in the VFO on the OA2 (R3) to
22K, 10 watt. Mount this resistor outside the VFO to get rid of the heat.
#7 Consider solid stating the rig's power supply. Change all the rectifier
tubes to silicon rectifiers. (Get rid of the heat)
#8 If you go solid state, you may need to add a resistor in the 300 volt
supply after L44 to get your proper 300 volts. The HV & Bias will be
OK (Usually not necessary)
#9 Once you have done all the audio mods in the schematic, you may wish
to add some Negative Feedback to the audio chain.
Instructions for Negative Feedback Installation:
Start with one meg resistance & a .1 mf 800 VDC cap connected in
Make the 1 meg with (2) 500K resistors.
You will connect the .1 mf cap to one of the mod. transformer primaries
& the other end of the string to the cathode of the 6C4
Bypass the cathode of the 6C4 with around 300 pf to get rid of the RF
on the tube.
You are looking for around 10 db of negative feedback. (reduced gain)
On one plate of the mod tubes you will go into feedback & the other
plate will give you the negative feedback you desire.
You should end up with the .1 mf cap & maybe (3) 100K resistors in
Use several 1/2 watt resistors to divide the voltage, it's kinda high
One way I did my own rig was to come off the proper 6146B plate lead
and go through a .1 to .22 mf cap (600 volt or better) and 3 100K _ watt
resistors in series to the top side of the old unused clipping control.
From the wiper I ran a series 2.2K resistor to the cathode of the 6C4 tube
after bypassing the cathode resistor with 300 pF to get rid of RF. Some
may say why bother, I say "why not" the control is there and you
can screw around with the feedback you are using. I find around the 12:00
position to be enough feedback to eliminate the distortion.
On the Audio Mods
There is nothing radical about this. We wanted to get rid of the interstage
transformer in order to pass more lows & highs. We are using Symetrix
528 voice processors that have a 600 ohm output so we modified the first
stages of the 12AX7 to do this.
I have found that most of the noise picked up by the Valiant audio system
gets introduced in that first stage mic. preamp. Getting rid of that stage
and feeding the audio into the "phone patch" input works perfect.
The stage will now require about 1/2 volt P-P audio to function.
To get your front panel audio level control to function you will need
to rewire it to the second stage audio between the plate of the 12AX7 second
stage and the grid of the 6C4. Now the first audio stage works to convert
the 600 ohm audio to the 1 meg. audio required by the gain control and 6C4.
Rip out the entire "Clipper" stage and all of it's components.
You should have a nice empty tube hole at that point. Now, bypassing that
old clipper circuit, go into your next stage that is going to be your phase
splitter. Get rid of the 12AU7 driver and all of it's components. When you
have stripped the tube socket and removed the interstage transformer, get
a 12BY7 & wire in the new circuit as shown in the schematic.
(You can also use the 12AU7 here but the 12BH7 has more power). Pay attention
to the 39K & 50K resistors on the plates of the 12BH7. they determine
the positive & negative peaks on the modulation envelope. You can fudge
the values to get maybe 150% positive modulation with these two parts.
Brent, W1IA has gone one step further and swapped the 6146 modulators
with type 6DQ5 TV sweep tubes. You will need to rewire the tube base to
do this but it will give you much more audio punch from the tubes. You will
probably want to change the meter resistor in the cathode circuit of the
modulators to a value of .202 ohms. Do this by cutting it in half &
butt splicing into it with some copper hook-up wire so you can reconnect
to the tube socket. It's made of nichrome wire & will not solder. Your
meter will now read 500 mA full scale. This is needed to measure all your
new audio punch. Brent, W1IA has also removed the air gap spacing of the
mod, transformer per Tim's (WA1HLR's) instructions. This seemed to boost
the low end response of the transformer. Now figure out the value of the
feedback components as described above & install the negative feedback
If you do not plan to run a 600 ohm input voice processor, just change
the input cap on the first stage to 0.1 mf & the input resistor to 5
megs. You will now be able to drive it with crystal mics. (D104 etc.) as
the transmitter was designed for. If you go this route, do not modify the
gain control, just make sure all the audio coupling caps. are .1 mf. 600
Volt. Rip out the clipper & build the phase splitter. Set up the feedback
the same way. Great Fun and "Turbo" results!
The last thing you may want to try is a Heising modulator. This hook
up idea came from Mike - N1IW. This modification is done very easy in the
Valiant. Remove the accessory plug. Note
the original jumpers go from pin 7 to 8, pin 2 to 6, pin 3 to 5. I mention
this so you can put it back to stock easily. Now connect a 2 mf 2500 Volt
oil cap from pins 2 & 9. Connect a 20-30 Henry 300 mA choke from pins
5 & 6. Connect another 2 mf, 2500 VDC oil cap from pins 5 & 3. This
is a modified Heising modulator using 2 caps instead of one. I did it this
way so I would not have to cut into the transmitter at all to do this experiment.
It worked perfect.
On the Power Supply Mods
Add another full wave rectifier stack for the high voltage. If you have
swapped out the tube rectifier for solid state (4 X 1KV diodes), do it again
but route the diodes to a cap. Input filter (3 X 100 mf., 450 Volt caps
in series). I also put 100 K ohm, 2 watt resistors (1 each, across each
cap in the series filter) to act as bleeders and voltage dividers. This
gives 1100 volts for the modulators. Route this 1100 volts to the mod transformer
primary center tap. Disconnect the center tap original 680 Volt feed &
connect the 1100 volts. I swapped out the 6146's for a new pair of 6146B's.
Re adjust the bias for 50 mils or so. If you run out of bias volts just
put a few K-ohms in the ground return of the modulator bias pot to step
up the volts. I have also added, on the recommendation of other Northeast
AM'ers, a 1K screen resistor in each screen of the modulator tubes and a
15 ohm cathode resistor in each mod tube cathode ckt. This gave me 120%
positive peaks! Don't worry about the 1100 volts on the 6146's, they can
take this no sweat.
On Relaminating the Mod Transformer
The next mod I tried is taking the mod iron completely apart to re-assemble
it in the more normal Hi-Fi Cross-lamination setup. Carefully disassemble
the mod transformer & tap out the "E" laminations with a socket,
etc. I disassembled the laminations with a box cutter & a rubber mallet.
I soaked the laminations in paint thinner to remove any excess crud &
re-assembled the transformer by alternating the "E" and "I"
pieces until it was re-assembled.
The test is 100% perfect. Rig now runs 120% positive peaks & sounds
great. Taking the transformer apart is a labor of love but if you want to
100% revamp the audio, this seems necessary. At this point the radio is
not much of a Johnson Valiant as a HI-FI Broadcast rig. No doubt you can
take things even further if you can get your hands on some broadcast quality
Mod Iron but all the mods listed above seem to work out fine.