[ARC5] Input impedance - BC-453-B - un-un

[Kenneth G. Gordon]

On 30 Jan 2015 at 10:03, Ian Wilson wrote:

> Reposting my approximate calculations. I have added a note ([1] below).
> 
> Robert is correct about the input trimmer: this is in parallel with the tuned
> circuit, and the antenna connects to that through a small fixed capacitor. My
> bad. The impedance of the input at resonance is not determined by the grid leak
> resistor of the first tube, but by the effective parallel resistance of
> the coil, namely 2pi * L * Q [1]. Guessing that the tuning capacitor is around
> 300pF at 500kHz, then the coil inductance will be on the order of 350uH. With a
> Q of 50 the equivalent parallel resistance will be around 55k ohm. 
> 
> So the effective input circuit will look like 8.5pF in series with about 55k ohm
> (to ground).

OK. I was off by at least a factor of 10: 4K vs ~40K. My memory is defective.
 
> [1] at the frequency of operation of the BC-453, we can ignore the real
> component of the complex input admittance of the RF amplifier pentode. At
> higher frequencies, various losses and effects such as transit time and lead
> reduce the effective input resistance of tubes radically.

Yes. That all makes perfect sense. I firmly believe in ignoring those factors 
which cannot influence data beyond a few percent. It makes calculations 
easier, although not more accurate. We really don't NEED accuracy beyond 
10, or even 20% in the circuits with which we are dealing.

Now, my next question:

As far as the input impedance of our receivers is concerned, in my opinion, 
any thing between 40K and 60K ohms is close enough.

So, my next question: if we routinely want to connect our receivers to 50 ohm 
systems, what sort of un-un would we have to make?

7 turns on the 50 ohm side to ~200+ turns on the 40K ohm side...but on what 
sort of form? Toroid? What "mix"? What size wire?

Expectations of improvement? Any? None?

Ken W7EKB