[ARC5] [Milsurplus] BC-230 Transmitter w/ Inexpensive Tubes

Wed Jul 30 01:19:21 EDT 2014

Dug out my fractional BC-AS-230 this evening. This has a front panel
in nice original condition but is missing other stuff including 2 tube
sockets.

I have coil sets for something else in this family but nothing that is
quite right for the BC-AS-230. If someone has a couple of the
ceramic formers to trade then I could do some sort of restoration
using alternative tubes a la Dave. Failing this, any suggestions for
how to connect non-destructively to the pins where the coil formers
plug onto?

73, ian K3IMW

On Tue, Jul 29, 2014 at 4:49 PM, Howard Holden <holden7471 at msn.com> wrote:

> This is terrific. Now, where are all those BC-230s sitting in boxes?
> inquiring minds want to know!
>
> Thanks so much Dave! This is a keeper.
>
> Howie WB2AWQ
>
> -----Original Message----- From: Ian Wilson
> Sent: Tuesday, July 29, 2014 1:26 PM
> To: David Stinson
> Cc: ARC-5 ; Military Surplus net List
> Subject: Re: [Milsurplus] [ARC5] BC-230 Transmitter w/ Inexpensive Tubes
>
>
> 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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