The "Mystery Parts" in the Knight Broadcaster
Updated 4/12/08 with observations made on actual T1 and L1.

There are three parts in the Broadcaster that are largely undocumented: L1, the oscillator coil, C9, the tuning capacitor, and T1, the transformer.

With no promises as to the accuracy of the following information, this is what we know so far:

C9 is about 470pf, variable.

T1 is a low power plate to voice coil audio transformer. Recent tests made on an actual T1 sample showed it had a turns ration of approximately 1:28, an estimated primary Z of 6.8K with a secondary load of 8 ohms. This is a fairly good match with the 50C5 plate Z of 10K, and the tube has about a 2W capability. The closest match would be from Antique Electronic Supply, their part #P-T125A

Updated 3/4/09 Thanks Cris!

L1 was a custom Knight Kit part. An actual L1 sample shows two windings. One of about 155 to 160 turns of #35 wire around a 0.625" carboard coil form. That should be about 225uH to 235uH. The other is coverd by the first winding and a layer of white fabric tape. Resistnace measurements are 5.7 ohms for the outer coil (the one we can see) and about 1.1 ohms for the inner coil. Both coils seem to be wound of the same wire gage, but with different laquer colors, the outer is green, the inner is red. If you want to wind your own, and you scale the inner coil based in its resistance ratio with the outer, you'll probably end up fairly close. The impact of an error here would be the tuning range.

We still plan to remove our sample L1 and put it on a network analyzer some day soon. Until that data is in, this is the best we have.

One of our fans has suggested the following substitution: Antique Electronic Supply part number P-C70-RF (www.tubesandmore.com). Pins 1 and 2 are the grid circuit, pins 3 and 4 are the plae circuit. It is also suggested that if this part is used, C9 will need to be lower, like 150pf, to put the broadcaster in the AM Radio band.

Most vintage parts can easily be found at Antique Electronic Supply.

That's all we know for now. "Have fun storming the castle!"