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

[Hubert Miller]

Make that me three.   
I understand with the loop so close to the radio, there's RF all over the thing. So i'm open to the 500 pF somehow working for compensation, but right now i'll bet money against it.
I have to wonder how a nice standard  figure of 500 pF was indicated, and not say, 463 or 532 pF.  
I do not think 500 pF is large enough to have any effect on keying characteristics whatsoever.
I notice with curiosity that the 500 pF is across only the 120 supply, not to ground; altho you would expect the to-ground impedance of the 4 volt battery to be about nil. 
Eagerly waiting for the next chapter in this story.
Is there something about the VT-1 tubes that would have different characteristics from modern tubes? The RF plate current figures do not seem out of line compared to more recent
low-mu battery triodes. 
-Hue 

-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of Kenneth G. Gordon
Sent: Tuesday, February 14, 2017 5:06 PM
To: Mike Morrow <kk5f at earthlink.net>
Cc: Milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] BC-9: Answer One Question, Get Another.

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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