practice coil: assume 625 EI twice stack, bobbin has 1/4" flange, we can use about 0.1" of that for wire if equal current density between pri and sec, we need to use some of that space for insulation and other losses, so we can not simply divide that flange dim in half,
old layered paper like they use in the original has margins so we use 3/4" winding length per layer,
if we compute square feet of pri wire we can use some nifty charts in RDH4 (page 1412) to play around with turns(AC and DC flux and Henries) and DCR,
.1" tall by .75" wide = .075" sq ft of wire, RDH4 says we can spin 62,600 turns of #39 wire (ball park for 6 ma DC) per sq inch, so 0.075" x 62600 = 4695 turns, this would yield 5 K Gauss AC and 10 K Gauss DC (lots of amp-turns) Henries would be 40 H for a core perm of 300 (normal for a gapped core)
to get DCR we need cubic inches, just multiply sq inches by 1 average turn length, about 4.25" for a double stack, so .075" x 4.25" = .318 cubic inches, now we get DCR from same chart in RDH4 page 1412,
#39 says 4332 ohms per cubic feet, so .318 x 4332 = 1377, too low for what Doug reads off his coils,
try #40, still reasonable for 6 ma, #40 has 6770 ohms per cube, so .318 x 6770 = 2152 ohms, now we are getting somewhere, what if we bring the turns down to get 2000 ohms? 2000/2152= 93 percent,
.93 x 4695 T = 4363 T. since we are using smaller 40 wire, we will have room for the FB wind and the insulation it needs on both sides.
now we go back and plug those turns into the calculator to get flux and Henries,
looks like 5.7 K Gauss at 100 V-rms and 20 Hz, reasonable.
DC flux is 9.4 K Gauss. It is normal to have more DC flux than AC flux in coils of this type.
Total flux is then 5.7 + 9.4 = 15.1 KG at peak input, so we are close to saturation, (18 KG) but still have 3 KG of headroom.
Henries for 4363 T on 1.25" of 625 EI (5/8" tongue) will be about 40 H (perm = 350 for gapped core)
we had a figure of 35 H mentioned, but that was at 120 Hz, a gapped coil will ramp up a bit in terms of Henries as frequency decreases but not as steep as a non gapped core so 40 H is reasonable.
Now we can go back and get sec turns, 4363 T/ 5.25 = 831 T for the 600 Ohm wind, 150 ohm tap will be 831/2 = 416 T
F/B will be .12 x 831 = 99 turns. lets adjust some numbers, round up the F/B to 100 T, 150/600 = 416/832, so pri = 4368 T.
now we go back and use sec DCR numbers posted to get wire size, 70 Ohms for 832 T,
MLT = 4.25" x 832 = 294 ft., how many ohms if we had 1000 ft of wire?
1000/294 = 3.4 x 70 = 238 Ohms/1K ft. that's probably #34,
help here: http://www.coonerwire.com/magnet-wire/
FB = 100 T x 4.25 = 35 Ft. 1000/35 = 28.57 x 4 ohms = 114 Ohms/1000 ft , probably #31
summary:
Pri: 4368 T #40 AWG
Sec: 416/832 T #34 AWG
F/B: 100 T #31 AWG
Core: 1.25" of 625 EI Butt Stack , .002 gap, 29gaM6 or Cold Rolled.
Structure: Try Sec-Pri-FB-Pri
you could also use a 1" stack of 87 EI (7/8" Tongue) and get about the same flux density.
.625" x 1.25" bobbins for 625 EI are hard to source, 1 " bobbin for 87 EI available at Edcor.