I will be using cathode feedback for the output stage, which means that the grid voltage on the output tubes will need to be considerably higher. This means that the driver tubes (EL84) will need to swing lots of volts. By using choke loading rather than a simple resistor or even a CCS, the output voltages can swing higher than the anode voltage on the EL84. This is a unique property of chokes. So, I'll use an old-fashioned choke loaded design.

Below is the initial info going into my handy-dandy Excel spreadsheet. The tube is a power tube, the EL84. There will be 425V going into the center tap of the choke and into the EL84 anode with Idc = 20ma per tube. Ra is the intrinsic anode resistance of the EL84 and can be determined by datasheet or with my uTracer. RL is the anode resistor between the choke and the EL84 anode.

Va is the voltage forced on the EL84 anode by the I19ma dropped across the winding resistance (estimated here at 200R) and the 8.2K resistor: 425 - 19 *8.2 = 265.
To determine wire size, figure out how much current flows in the event of dead shorted tube. Ishort = 425/8.4 = 51ma. Double that for safety. Using acceptable current density of 3, wire size is determined to be .2mm or AWG 31.8. I'll use AWG32 wire.

Basic Info

The final design of the choke calls for a 29mm core with a stack height of 40mm, with 5000 turns of .2mm wire (32AWG). I am using old iron laminations salvaged from some old transformers that I baked at 500 degrees. This melts the old varnish and allows the laminations to be removed and reused. They are probably pretty low quality, but they are fine for chokes. They are shown below baking in my grill.

Basic Info

Using a hammer and screwdriver, I pry apart the red-hot laminations and am left with a pile of E and I laminations. Glamorous work.

Basic Info

Alright, I built the choke, fully interleaved the laminations, and still got a lousy inductance less than 50H. So time to switch gears. I am now going to use my shiny new GOSS laminations with a tongue width a = 38.1mm and a stack height of 12mm. They will be funny looking pancake chokes, but they should provide a huge inductance.

I did calculations as above with some minor alterations, and came up with N = 12500 turns of 32AWG magnet wire; this will be 6250 turns per half. Predicted inductance is in the thousands.

I had to cut down my 1-1/2"