[ARC5] BC-375: STABLE AM QSO on 7160 KC

[robinson at tuberadio.com]

Hi Dave,

Thank you,
I appreciate your work on BC-375 drift.

Regards
Ray

On 2015-02-02 06:22, David Stinson wrote:
> Well.... very close to it :-D
> 
> Still using the 211s for now.
> Just finished a pleasent AM QSO on 7160 KC with
> Bill, K5GCW using the BC-375 at 50 Watts on the dynamotor.   I didn't
> allow the rig 30 mins to warm-up, so did have to "chase" the freq for
> about a KC during
> the QSO, but tests yesterday say it would have remained "still" with
> proper warm-up.
> Only very slight FMing.  Unless someone is committing
> the unrightousness of using an unholy product detector in an AM QSO,
> they wouldn't know.  I pay no heed to such blasphemers.
> 
> The non-destructive change I made may provide a key
> insight into why the stock 375 TU-8 tuning unit drifts so badly on 40
> meters.  It may NOT be thermal changes
> in the structure of the 211 tubes.  It may be thermal changes in the
> Osc. tank coil components, because removing the DC current from the MO
> tank
> settled it right down.  The tuning caps are already
> thermo-stabilized and don't carry DC current in any event.
> 
> Leveraging the experiance stablizing the BC-230 on
> 40 Meters,  I changed the 211 MO circuit from Series-Feed to
> Shunt-Feed.  This requires a couple of simple, reversable changes in
> the TU-8 tuning
> drawer.  No changes in the transmitter main chassis are needed.  One
> does need a good, hot
> soldering iron or small propane hand torch. The big buss wires take a
> lot of heat to move them.
> I used this little Bernzomatic "micro torch."
> http://www.bernzomatic.com/item.html?id=43
> 
> Here is the original BC-375 Master Oscillator
> circuit, showing the series-fed B+.  Note that the
> connection point on the OSC tank coil (PVF), while at 1000V DC, is at
> RF ground by the plate bypass cap, BP.
> http://home.netcom.com/~arc5/SCR-287/BC375A.jpg
> 
> I eliminated about 100 mA of DC current through the
> MO tank circuit by changing the plate circuit to shunt-feed.  I
> removed the B+ from the tank coil,
> bypassed this point to RF ground and feed B+ to the OSC tube through
> a plate choke.
> http://home.netcom.com/~arc5/SCR-287/BC375B.jpg
> 
> The big problem was finding something good to use
> for a plate choke, since I don't have a "normal" one.
> This point can carry as much as 100 mA, so one of
> the chokes like those used in the grid circuit will cook-off
> in a hurry.  I tried winding an RF choke on a ferrite "noise" toroid
> core, but the core material overheated
> quickly.  I finally used a big Detector coil salvaged
> from a dead RAK receiver and shoe-horned it into
> the room in the rear of the tuning drawer.
> It works fine.
> Can you suggest a better plate choke?
> 
> I don't know for sure why this is so much more stable than the
> "stock" version, which will drift off and away after key-down.  It
> just is.  Maybe IR losses in
> the coil cause heat?  A fraction of a degree will
> expand the wire and the core enough to send
> the osc. wandering.  Thoughts?
> 
> Some important things for getting the BC-375/-191
> stable on 40 meters:
> 1. Solid power supply that will not sag at 350 mils.
> 2. Don't overmodulate.
> 3.  Neutralization is even more "touchy" than on 80.
>    Get your favorite beverage, gather some patience
>    and you will eventually get it "right."  Even a couple of
> millimeters off makes a difference.
> 4.  Keep the power down. 50 Watts is better-behaved    than 75 Watts
> and makes no difference that matters
>    on the receiving end.
> 
> More work to be done later.   GL OM ES 73 DE Dave AB5S
> 
> P.S. Please let me know if the BC-375 work is of interest to you.  It
> takes some time to do the writing,
> editing and graphics for these and I don't want to waste it if I'm
> just "preaching to empty pews."
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