[ARC5] BC-230 Transmitter w/ Inexpensive Tubes

Tue Jul 29 16:26:34 EDT 2014

Great stuff, Dave!

If the 6AQ5 works here then I would expect the 6V6 and 7C5
to work also. The 6V6GT is 3.312" tall and the 7C5 (Loctal) is
3 5/32" tall, so there might just be space for a kludge, er,
conversion to a 4-pin plug.

Also note that the triode 2 section of the 6EM7 (octal) and
6GF7A (Compactron) look similar to a triode-strapped 6V6.

The 6GF7 has 10GF7 and 13GF7 versions - different filament
voltages, but if you are subbing all the tubes then you are free
to use what you have.

Gosh, I do have the hulk of a BC-230 something in the garage,
not sure whether any of the coilsets that I have are correct/useful
though...

73, ian K3IMW

On Tue, Jul 29, 2014 at 12:23 PM, David Stinson <arc5 at ix.netcom.com> wrote:

> 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
>
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