[ARC5] BC-455 IFT - measurements

[Mike Feher]

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. There was no
detectable resonance in the un-tuned secondary, also out of circuit. 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. 73 -
Mike 

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-886-5960 

-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Kenneth G.
Gordon
Sent: Wednesday, October 29, 2014 12:30 AM
To: ARC-5
Subject: [ARC5] BC-455 IFT - measurements

I just checked the inductance of the two coils in the BC-455B IFT type
number 7277 I have here:

The pie-wound, larger, coil tests at 2.264 mH. This coil is L-7

The smaller, single-layer coil, tests at 16.14 uH. This coil is L-6

So, my measurements follow Les'.

Now, the capacitor in parallel with L-6, C-12 is 180 pfd.

The pie-wound coil, L-7, has no capacitor shown in parallel with it on the
schematic. In order for it to be resonant at 2.830 MHz, the capacitance
required would be on the order of 1.5 pfd.

I don't understand 1) why L-7 is ~2.3 mH, 2) why it is untuned, and 3) how
does this IFT work?

My instrument must be off a bit. In order for L-6 and C-12 at 180 pfd to
resonate at 2830 Khz, L-6 must be about 17.5 uH.

Ken W7EKB
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