Filament Transformer for 13E1 Amp

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FILAMENT TRANSFORMER FOR 13E1 POWER AMP

I need a custom transformer to power the filaments of the glorious 13E1 power tubes + the filaments of the EL84 and 6SN7 driver tubes.

The 4 13E1's are power-hungry and need 1.3A at 26V each, the 4 EL84's need .76A at 6.3V, and the 4 6SN7's each need .3A at 6.3V. The basic info entered into my excel spreadsheet is below. The laminations are M6 GOSS laminations which are very high quality. The flux density B = 1.29 was arrived at by trial and error to ensure that the number of windings for the primary was 240, which I can fit into 4 neatly wound layers.

Here are the results from the spreadsheet. I am using a bobbin with a center-leg-width of 38mm and a stack height of 38mm. The calculations give a TPV (turns-per-volt) of 2.09. There is a predicted temperature rise of about 18C. I'll be including a small fan in the power supply enclosure so I think this will be OK.

This is the primary winding after soaking in electric varnish. There are exactly 240 turns. Note the amazingly non-Mike-like neatness of the winding.

After a few turns of kapton tape and a layer of Nomex 410 paper, the ES shield is applied. This is simply a single layer of carefully wound 28ga wire. One end will be unused, and the other end will go to ground.

After the screen is wound, four more layers are wound, each separated by Nomex and Kapton tape. The first 2 are 26V secondaries with 56 turns each of 16AWG wire. The next two are 6.3V windings with 14 turns each, also 16AWG. All the terminations are brought out and soldered to appropriately sized wires.

Finally, the laminations are inserted and hammered into submission. The screws are loosely applied, so that varnish can get between the laminations and onto the wires. The whole transformer is shown below, ready to be vacuum impregnated with varnish.

Below is the finished transformer powering the filaments of 1/2 of the complement of tubes set up on my test rig. It's a little dirty looking - dried varnish, but it doesn't hum or rattle and delivers the proper voltages to the filaments.