[ARC5] BC-455 IFT - measurements

[Kenneth G. Gordon]

On 29 Oct 2014 at 8:12, Mike Feher wrote:

> I still have not measured my 7277, and between Les doing it and you
> confirming there is no need, but, have come to the conclusion that the data
> provided by Les is pretty close. I put a grid dip meter to the primary and got a
> nice dip at about 2.8 MHz with the IFT out of circuit.

Yes. I had presumed that it would be resonant at the IF frequency...

> There was no detectable
> resonance in the un-tuned secondary, also out of circuit.

Yes. I found none here either. Just looking at the circuit as shown on the 
schematic, my immediate conclusion was that the untuned secondary was 
really nothing more than an RF choke...but I don't have all the necessary 
data either, so I could be wrong...probably am, in fact.

> The secondary goes
> directly to the grid of the 12SK7 and the input capacitance of the tube is
> closer to about 6 pf or so according to some data sheets. I too calculated that
> the required resonating cap only needed to be about 1.5 pf. I too was scratching
> my head for a bit as to why the secondary was un-tuned, but, realized quickly
> that it really did not need to be. While it would provide more selectivity if it
> was, as it is it is acting more like an impedance matching transformer to the
> grid of the 12SK7 in my opinion.

Well, that makes sense to me. However, I still cannot fathom why they did it 
that way. The result would have been far less selectivity, and maybe that 
was the point. There must have been a good reason, and I hardly think that 
economics had anything whatever to do with it.

Neither can I believe that there was any "hand work" done to each receiver 
to tweak it. That would take too long and cost too much.

Ken W7EKB