here is a worksheet for the V72 plate choke,

that weird lam with the built in gap has no match in the Mag met catalog,

so we just use the tongue width which was 1/2 inch and find a EE lam has a 1/2 inch tongue.

this would be the 50 EI, 50 means 1/2 inch, so we use those specs for L , A and MPL.

since the gap is on the tongue only, we divide it by 2 since the legs are not gapped.

this lam has a built in gap of about 16 mils so we use 8 as our gap.

we use mills since gaps rarely go over 30 thousandths, so 1 thru 30 is a good notation system for gap length.

notice the dramatic drop in perm with the gap, as well as the drop in Henries.

this takes core perm variations out of the equation which is desirable.

also notice that will all those turns, flux is very low, less than 3 kg on a core material that will take 18 kg, so distortion is very low.

tubes like the EF804 want about 400 Henries when operated as a pentode,

this is because the plate resistance is high and we do not want to drag the tube down.

now with a gapped inductor or transformer, you have to take into account the DC flux as well as AC flux, this DC flux is caused by the plate current, so we add the flux caused by the signal voltage and the flux created plate current.

there are also calculations for different wire sizes just in case we can not squeeze on all the turns and have to jump to a thinner gauge wire.

some V72 circuit show 300 H, the V76 uses a 400 H choke, so we go right in between with 350 H which gives us a nice round figure of 18,000 turns which divides nicely by 3 into 6000 turns per bobbin chamber, which should be a piece of cake with #44 wire.

to get MTL, or mean turn length of wire, we just take a piece of stiff wire and bend it around the bobbin about 2/3 rds of the way out , straighten it out and measure it,

then we can get wire length by turns multiplication which can then be extrapolated the DCR from wire tables. there is a wire table link listed at the bottom which is the only one on the net that has #48 thru #60 specs.

wire tables will have turns per sq inch, so multiply the bobbin width by the height, this will give you your winding area in sq inches, then simply multiply the two numbers to get the

*theoretical* amount of turns on that bobbin section.

EF804s only uses 0.6 ma plate current, it has the coldest running cathode of just about any pentode out there at 680 degrees C, that is why you can barely see them glow,

voltage drop across even a choke with 12,000 ohms will only be .6 times 12 = 7 volts.

note that on some Supersonic V76 prints, the plate voltage on V2 is shown at 175, when in fact the B+ is 245. this would mean a DCR of 100,000 ohms on that choke which is obviously in error, so watch out for that, plate voltage will actually be about 245 - 7 = 238 VDC, not 175 VDC.

this means most of the 245 B+ volts can be used for amplification.