[Milsurplus] BC-9: Answer One Question, Get Another.

[Kenneth G. Gordon]

On 14 Feb 2017 at 3:28, Mike Morrow wrote:

> Dave wrote:
> 
> > The mystery of the frequency shift appears to be solved:
> > https://goo.gl/photos/Cp3hyLwEJmX3Ua3M7
> 
> I hope so, Dave, but I would not bet on it

Me either....

> Referring to the full schematic (fig. 8) of the manual cited above,
> all those interconnections are misleading because they reflect
> connections to what is both the signal common and DC common bus...the
> "ground" bus.  It runs from the junction of the two C1s, connects to
> the bottom of the 1000 mmF capacitors in both filters, the bottom of
> the filament resistors of all three VT-1 tubes, terminal 3 on both
> T1s, etc.  All the stuff that needs a common bus connection. 
> 
> That means there is nothing sneaky going on with the bottom of the
> 1000 mmF plate filter capacitor or its connection to the bottom of the
> RF VT-1 filament resistor.  Those are all connections to the circuit's
> ground bus. 
> 
> The voltage existing at the plate current meter, with respect to the
> ground bus, is 4 vdc from the filament battery plus 120 vdc from the
> plate battery, 124 vdc total.  The 500 mmF compensating capacitor C6
> is in parallel with the 120 vdc plate battery and can have no *direct*
> effect on RF currents where it is because C6 is exposed only to DC.  I
> think it is intended to cause very short and transient effects on the
> *DC* voltage that exists where the key, the 1000 mmF filter capacitor,
> and 100k R5 meet, as the key is opened (voltage there is about 70 vdc)
> and closed (voltage there is about 124 vdc).  I think C6 supplies some
> transient DC energy to the filter reactances as the DC voltage rises
> across the filter capacitor to ground and current increases in the
> filter inductances to the plate, as the circuit is keyed.  Stabilizing
> the voltage there helps stabilize it at the plate.   Perhaps that is
> the rationale and function for C6...or perhaps it is not.  :-) 
> 
> I agree with Ken when he suggests that the benefits of having C6 in
> place was likely empirically determined by experimentation, and not by
> circuit theory.  I doubt that messing with C6 will change performance
> much when tubes that aren't a close substitute for the dynamics of a
> VT-1 are used in the RF stage. 

That is what I am afraid of too. Not having the original tubes makes this an exercise in 
futility...or at least frustration...

> Good luck with your BC-9-A saga, Dave.  I just found my old Army
> Training Manual No. 26 for Radio Operators a short while ago.  It's
> dated 27 December 1923, and issued for General of the Armies J. J.
> Pershing.  It briefly describes the BC-9-A.  It's surprising how
> entertaining a 95 year old "simple" radio set can be.  Thanks for
> starting tge threads. 

Ha ha! Well, **I** am most certainly enjoying this saga! :-)

To me, a CW op, a complete CW-only transceiver with full-QSK which was designed almost 
90 years ago is absolutely fascinating! I'd LOVE to build and use one for 80 and/or 40 
meters. I think I would use sub-minature tubes, though, and NOT a 304TL as Robert 
suggested....or an 833A (which, by the way, I have seen being used as a regenertive 
detector in a receiver)

BTW, low mu triodes make the very best triode-regenerative detectors. They "slide" into and 
out of oscillation and don't suddenly "pop" into and/or out of it. This feature makes 
regennies using low mu triodes more stable, reliable, and easier to use, especially for 
novices.

Ken W7EKB

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