[Boatanchors] Smart People: BC-223/BC-375 Osc. Again

David Stinson arc5 at ix.netcom.com
Sat May 3 06:59:42 EDT 2014


As most of you know, I've been working on substitutes 
for the expensive tubes in transmitters like the BC-375 
and BC-223, hoping to hear more of them on the air.
Also trying to address expensive crystals in sets like
the BC-223. Yes; I'm still working on it.  Like a lot of you, 
I get an hour here, 20 minutes there to "play radio" 
before something or someone drags me out by the hair
(both of them), so it takes me awhile to get results.
I may have a "break-through" on the "crystal replacement"
side of the project which might also solve the MO problem,
but a little more work to do before I'm ready to say that.

(Note:  The "turn it into a grounded grid amplifier" suggestion
is a good one and I can apply it using a shorter tube and
a tube socket extender, for which I am gathering parts.  
Making such changes under the chassis would violate
my "non-destructive" principle.)

If I may, I'd like to discuss something that didn't work,
because it's got me bum-fussled.

Please look again at this simplified diagram of the 
BC-223 crystal oscillator stage:

http://home.netcom.com/~arc5/BC-223/223osc.JPG

The tube is an 801.  I sometimes sub a "long plate" 10Y,
which is rumored to be an 801 "in disguise."  
Both seem to work about the same in the circuit.
I built a simple MOSFET amplifier with step-up transformer
output to drive the grid. The scope says I can develop 
as much as 60 volts P-T-P across that 25K grid resistor
(100 pFd in or out of the circuit makes no difference).
The curves in the tube manual say that, when I hit it 
that hot, I should be kicking the heck out of that tube, 
over-driving it to 100 mils on the positive swing 
and deep into cut-off on the negative.

I get almost no output from the stage.  
I don't understand it. 
What's the obvious thing that isn't making it
through my thick skull??

73 DE Dave AB5S



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