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RE: What tubes to use with this Interstage Transformers?

I think you are confusing AC permeability with DC magnetization.

If you have only ac signal and also a small signal signal then you have the possibility that you have to low permeability.

yes... but at no point will adding a DC to a small AC signal increase the permeability.

A good se transformer is designed in between 0,8 and 1,1 T in the area where permeability is high. But only with dc!

The typical numbers you state are for a transformer that carried DC current and it simply states to use about 1/2 the available flux for DC magnetization leaving the remaining half for ac excitation. It has nothing to do with permeability.

You can easely measure that with pp transformers or se without a dc that with small, smallers and the smallest signals the low frequencies become weaker and weaker.

yes... as you reduce excitation perm goes down, but adding DC will not take a low perm and increase it in value as you suggest and biasing a core to the maximum permeability level with DC and applying a small AC signal will not give you the same results as exciting the core with that value of AC flux.

to put this in the form of some real measured behavior I took a 11hy 200ma choke designed for choke input duties and measured it with 0.5V @100hz and got 11.6hy with no DC present. I then placed 100ma of unbalanced DC on the core and measured 11.4hy and at 200ma of unbalanced DC I got 11.1hy.

The 1/2 a volt @ 100hz represents an AC flux of 3 gauss and 100ma a Bdc of 3700 gauss and 200ma a Bdc of 7400 gauss. Applying 400Vac @ 100hz gives a Bac of 2500 gauss and increased the inductance to 12.8hy. All of this was on a 0.1mm grain oriented C-core.

dave



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