[Milsurplus] BC-9: Is It Halloween??

[Mike Morrow]

I'd like to know what the set behavior is with that 3 mH choke added in series with the plate side of F.  If I understand Dave's description, this has not been tried.  It would be educational to see results after RF is effectively removed (with the 3 mH choke, and maybe a bypass capacitor) from the junction of the key and R5 and C5 and F.

It's obvious that RF is all over the place right now.  Choke it before it gets to the key just as an experiment to see what happens, perhaps even adding a bypass capacitor across the 1000 pF unit.  Who knows what the present characteristics are of that 95 year old device, both here and in the other F in the grid bias supply circuit?

I'd like to see yet another experiment conducted without the added RF choke, but with known good 1000 pF capacitors in both F filters.  If that's been tried already, I missed it.

Hue wrote:

>To try to replicate original operating characteristics, you must use batteries for all 4 voltage sources. No external supplies.
>And, you must use a filamentary ( DC ) tube, no tube with a cathode. Because i think a certain amount of RF is coupled 
>back into the circuit via the filament. I think you're kind of chasing your tail, without setup as original as possible.

Yes.  I would include in that originality restoration using the original 4600 kHz operating frequency, before attempting to operate the set at 3570 kHz (a change of 23 percent).

It may be worth noting that the 4000 kHz BC-9 configured filter F differently than it is in the 4600 kHz BC-9-A.  The BC-9 has a capacitor connected to the junction of its two inductors that is omitted in the BC-9-A.  That suggests that there is some importance to the specifics of the design and frequency response of these filters...performance that may be upset by the attempted large departure from set design frequency or by F capacitor degradation or both.

Mike / KK5F