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

[David Stinson]

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."