Hi Pusch! that 4% number is stuck in my head because that is what i used to chop up at the transformer factory, might have been called Microsil, it has been a while, we made big three phase transformers so core loss was a factor in the type of steel used as you don't have to use as big an oil tank if there is less heat.

so you are right, for small transformers for radios and tv's they probably use less expensive steel.

Lets try the RDH4 method for transformer design and see what they say for core size,

Page 235: Cross Section Area **A = (VA)^.5 / 5.58 **

VA = .250 x 17 V-ac = 4.25

A = 4.25^.5 / 5.58 = 2.06 / 5.58 = 0.3695 This is our cross sectional area in inches

take the root for a sq stack = **0.608"** so RDH4 says use 625 EI (5/8" tongue)

they have an easy mehod for turns also>

Turns: **N = E x 10^8 / 4.44 f B A ** change 4.44 to 4 as stacking is 0.9 x 4.44

watch out! in RDH4 they use Lines per Cm^ for flux and Sq Inches for core area,

most folks swap those> lines of force in Gauss and core area in centimeters, it all works out he same,

N = 220 x 100,000,000 / f B A punch in frequency , core cross area (use 50 EI = and flux

N = 2.2^10 / 4 x 50 Hz x 12 ,000 Gauss x 1.61 cm^2

N = 2.2^10 / 3,864,000 = **5,693 turns** ,

so you are running that core at a pretty high flux level, if using a transformer in a preamp you might want to keep the flux down to like 7 KG to keep the magnetic power inside the core.

here is a real easy way to compute primary turns from RDH4, for area: Page 235 for 60 Hz and 240 VAC:

N = 1860 / Core Area you could de-rate for voltage and freq,

for 50 Hz: N = 2232 / A

for 220 VAC and 50 Hz: N = 2046 / A