[ARC5] BC-230 Transmitter w/ Inexpensive Tubes

[David Stinson]

Thanks, everyone, for the replies.

There are a lot of BC-230/-430 transmitters sitting in boxes
because the "audiophool" tubes are gone.   It makes no
sense to spend $400 on tubes to fire-up a QRP transmitter.

That's a shame, because the BC-230 / -430 is an
interesting and well-engineered set with a long and
successful service history.  I decided to find a way to
hear a few more of these on-the-air.

I've written several posts previous to this on other aspects
of getting these running.  This one will focus on substituting
inexpensive tubes for those which have disappeared into
Audio Pergatory.  Please refer to my earlier posts for
more information about neutralizing, tuning, etc.

I'm indebted to Dennis DeVall and Jack Antonio
for the clues that made the "bias" connection in my old head
and to Hue Miller for generously providing an excellent
"test bed" for the work, as well as the many others
smarter than me on this site for help and guidance.
Jack gets the credit for the "use a pot" idea.
Please forgive me if I forgot someone.  I'm old now.

The original design work on the -183 set was done in the
late 1920s and early 1930s.  The first production models
were issued in 1932 and the design changed little
thereafter in the U.S. Army Air Corps sets.
It uses two VT-52 filament-type triodes in parallel as
Modulators, one VT-25 filament-type triode for OSC
and one VT-25 for PA.
The filaments of 1920s and early 1930s transmitting tubes
were, by later standards, only modest producers of electrons.
Power was developed by high plate voltages at relatively
low current.   Later tubes, which used a heater inside
a cathode heavily coated with oxides produced
bucket-loads of electrons, so power could be developed
with lower voltages at higher currents.
Subbing inexpensive, triode-connected modern
heater/cathode tubes requires some changes to biasing.

The first task was to choose which tubes to try.
I did not consult thick tomes of tube lore, nor fill a chalk
board with Einsteinian calculations.  I did listen to
many people smarter than me, though.

I picked 6AQ5 and 5763 because they were used in
low-power transmitters in the ARC-type-12 and Jack
had already done some preliminary work with them.
 A quick look in the RCA tube manuals said
I wasn't going to melt them down (probably).
Most important: they are cheap and plentiful,
and that's the whole point.

The cathodes of the "modern" tubes must be connected
to the filaments at the 4-pin socket, pin 1.
If you look at the diagram, connecting them to pin 4
will result in Modulator cathodes with differing potentials
(thanks again, Jack).
Plate and screen are connected together and go
to the 4-pin socket, pin 2.

Filament side A connects to Fil, Cathode
and Suppressor Grid, then wired to 4-pin socket, pin 1.
Connecting the 5763 Suppressor to the Plate caused the
OSC to be unstable.  I don't know why.

Filament side B goes to 4-pin, pin 4.

Signal grid goes to 4-pin, pin 3.

With the 6AQ5s, I mounted 7-pin sockets in old 4-pin bases.
http://home.netcom.com/~arc5/SCR-183/6AQ5s.JPG
yeah, yeah... it's sloppy.  "Prototyping," guys.
That's my story and I'm stickin' to it!
The "leaning" tube is the OSC. stage and is "stuck" that way
on purpose.  It must be mechanically solid, since even tiny
vibrations will wibble-wobble the frequency.
I didn't have four more old 4-pin bases for the 5763s
and besides- I didn't like the "sloppy" of using them,
so I wired the 9-pin 5763 sockets with "pigtails"
which I fed through the 4-pin socket holes and soldered
directly onto the connection points:
http://home.netcom.com/~arc5/SCR-183/5763s.JPG

If you wire the "modern" tube socket carefully, you can
end-up with the pigtails just where you need to feed them
through the 4-pin socket holes.
For instance- here's how to wire the 5763 sockets:
http://home.netcom.com/~arc5/SCR-183/sockets.jpg
and, once the pigtails are fed-through the 4-pin socket holes
and settled tight to the deck, here's how they're soldered:
http://home.netcom.com/~arc5/SCR-183/soldered.JPG

Next step is the bias changes.
Tune the little rig up "as is" to start.  You will get power out.
Look at the diagram at:
http://home.netcom.com/~arc5/SCR-183/BC230TX.jpg
The biasing circuit is shown in green.  When the PA is driven,
bias voltage developes at the junction of 20K resistor 104
and bypass cap 113.  This bias voltage is also fed to the MOD
grids via the 100K isolation resistor marked 100.
In order to use different triode-connected tubes,
these values and the ratio of the bias fed back to the MOD
stage must change.

Now, I didn't break-out the Smith Charts, load lines, scientific
calculator or magical alchemy potions.  I'm a "number klutz,"
guys.  I can barely balance my check book.
I disconnected the MOD plates and resistor 100 to isolate
the RF stages and subbed a 25 K-ohm pot for resistor 104.
I keyed the rig and twisted the pot for "maximum smoke."
The 6AQ5s wanted about 11 K to make a maximum
CW carrier of 5 watts out.  The 5763s wanted about 3.3 K
to do about 7 watts out,  but they get a little unstable and
hard to neutralize at that level, so I went with 11K like
the 6AQ5s for about 5 watts out max on CW.
This is not the "optimum" value for the 5763, but I haven't
tweaked them further because I like the 6AQ5s better
as I'll cover later.

I want these to work on AM as well as CW.
To set the best bias point for the MOD stages,
I substituted a pot for resistor 104 and connected the
MOD grid isolation resistor 100 to the wiper.
I moved the ANT coil tap down near the base of
the coil and retuned for 2-3 W out as the manual
says to do (the MOD tranny is at risk if you try to
go far beyond the original specs).
I modulated the transmitter while watching the scope
and tweaked for nice, full modulation.
My peak-reading Watt meter went smartly up-scale
as one would expect on peaks.

In both cases, about 1/3rd of the way up from Ground
end of the bias resistance was the "sweet spot"
for the MOD stage.  If you get it too low, the MOD
stages will draw heavy current, load down the B+
and reduce power.  Turn the pot "down" until you
start to see the power drop, then back up into the
"stable" area.
Look at the scope and tweak for "niceness."
Here's how I installed the pot but you might want
to bring some wires out, since there's like 300 volts B+
in there.  I'm probably lucky Kay didn't find me sizzling
in my own juices:
http://home.netcom.com/~arc5/SCR-183/pot.jpg

Once I got the "right" values, I removed the pot and
"tacked-in" fixed resistors.
http://home.netcom.com/~arc5/SCR-183/Rebias.JPG
The original old "bar" resistors in this set were over 50% high.
I preserved them and stored them in the set.

Here's a diagram of the change.
Everything is 100% restoreable to original:
http://home.netcom.com/~arc5/SCR-183/BC230Mod.JPG

The 6AQ5s settled-down easily and were no trouble to
neutralize.  The best setting with them to eliminate FMing
was right on the neutralization point.
The 5763s will run more power out with a reduction in
the total bias resistance to something in the 4 K range,
but the more power out with them, the more "touchy"
the neutralization and the harder to get rid of the FMing.
Not a big problem if you're just running CW,
but I needed AM to work as well.
With 5763, find the neutralization minimum, then
tweak the Neut. cap just a couple of degrees clockwise.
That seemed the best for minimum FMing.

One thing I just "don't get:"  I hate to sound like an
audio-nutter, but the 5763s have a "tinny" sound
I don't hear on the 6AQ5s.  No clue why, but
it's quite noticable.  Ideas?

So that's if for now.  I think I'll consolidate all my previous
posts on this little rig and do my first article for ER.
It's just too sweet a rig to leave cold and dark.
If you have any suggestions or do some "tweaking"
to make things better, please let us know.

GL OM ES 73 DE Dave AB5S