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

[Mike Morrow]

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

> Just how does this freq. compensation work?
> The manual is available as a PDF at:
> https://drive.google.com/file/d/0B1zXYo8XYQS8MmgtNHVsektvT1k/view?usp=sharing
>
> A simplified schematic is on page 19 (PDF paging).
> A full diagram is on page 20 (PDF paging).
>
> It appears that the compensation cap, C6, when the rig is keyed,
> is connected one side to the Cathode circuit of the RF stage and
> the other, in series with the .001 cap in the Plate circuit
> filter, to the center of the Oscillator tank at the junction of
> the C1 caps.  So the effect is in the cathode circuit?  Anyone
> understand what's going on here?

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.

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.

Mike / KK5F