• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

PP OPT Core dimension


2004-08-28 1:38 pm
:) hi there

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???:confused:

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?
exact formula?

Ciao Silurato, I'm pleased to meet You also in diyaudio.
I think that the only 'exact' formula You're seeking for is the usual formula that match (winding area) * (core section) to max flux and frequency.
This formula can be found on several reference You have.

Once determined the product Aw*Ac, there is infinite possibilities to choose the lamination aor cores. You can prefer less iron and more turn or viceversa.

see also previus post discussions:


Hi Silurato !

I'm too lazy to do and redo among infinite iteration to have a workable solution, so I've done that:


Feel free for asking precisions if you need.

" Give 1 hour to a programmer for solving a problem, he will spend 59 minutes in writing a program able to solve that problem in 1 minute "



Paid Member
2003-06-12 7:04 pm
Maine USA
> problem with core dimension. .... i need formulas that invoke other parameters like primary turns etc...

No. Minimum core size is only about Power. And while you can wear-out a slide-rule computing the "right" size, it is easier to look at similar designs (the world is FULL of transformers suitable for P-P 6BQ5) and find a similar size core.

Yes, super-perm might allow a smaller core than plain iron. But most transformer properties improve with size! Space and cost are the only reasons to aim for a minimum-size core. In DIY, iron cost is insignificant next to the value of your labor. Use a plenty-big core, and of super-perm iron if you like.

Now that you found a pretty-big core, compute your winding. You need inductance, but that is not usually the limiting factor in push-pull output iron, especually with super-perm. You must keep the B level far below the maximum saturation B to keep distortion down. Most modern iron does not come with distortion specs. Aim for maximum flux of 1/3rd or less of the flux allowed in power-transformer duty. When you have a trial value for number of turns, find a wire-size that will fit the core and estimate the winding resistance. You usually have to compromise to get low flux and reasonably low resistance loss. (A bigger core needs fewer turns and has more window space: just one example of why bigger is better.)