The "Mystery Parts" in the Knight Broadcaster
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
L1 was a custom Knight Kit part. An actual L1 sample shows two windings, the top layer of which is about 155 to 160 turns of #35 wire around a 0.625" carboard coil form. That should be about 225uH to 235uH. The bottom layer closest t the cardboard is impossible to count without ruining the part, but it looks to be the same diameter wire and perhaps a few more turns, like 5 - 10% more. The layers are separated by a piece of fabric. The inner winding of our sample is red laquered wire, and the outer winding is green, perhaps to make the human-hair sized wires easier to see when identifying terminals.
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!"