I'm tryin to desing a PPOPT for a 6bq5 triode strapped PP. this is my first time n i found a problem with core dimension. Let me explain the steps i followed:

input data are: primary load plate to plate is Raa=10K, internal resistance of the tube is rp=1.5k, expected Pout=4Wrms, minimum operating frequency is fmin=20Hz, and quiescient current is Ibias=60mA.

1)Transformer ratio k= sqrt(Zp/Zs) = 40.8

2)Minimum required inductance (Radiotron rule) L > 2*Req/2*pi*fmin =27H

max laekage inductance Llm < Rb/4*pi*fmax =32mH

3) Core area: i choose grain oriented lamination because even if it had a max allowed B similar to that of the normal silicon steel, relative permeability shows higher values and this helps to keep high value of inductance, following the formula: (is it right???)

L = (3.2 * N^2 * mu * A)/(10^8*lm)

then i found the following rule of thumb:

A=900*sqrt(Pout/fmin)*sqrt(14000/Bmax) or

A=20*sqrt(2*Pout/fmin) or

A=10*sqrt(Pout/fmin) or

A=sqrt(250*Pout/fmin) ...

Here's the question: i need formulas that invoke other parameters like primary turns etc... does anyone know the exact formula???

As a mean value i take approx 1sqin

4) Primary turns N1: radiotron says

N1 = E * 10^8 / (4,44 * fmin * Bmax *A)

where E = sqrt(Pout * Raa) = 200V

so take Bmax e.g. 10000 Gauss = 10000 * 6.25 =62500 lines/sq-in that gives:

N1 = 200 * 10^8 / (4,44 * 20 * 62500 * 1) = 4000 turns

Comments about it?