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In Reply to: RE: What tubes to use with this Interstage Transformers? posted by Lenin on August 28, 2016 at 12:52:20
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
Follow Ups:
Dave your measurements are way to rough to see any differances.I mean low frequencies much lower then 100Hz and not only 1 medium large signals as 500mV.
See this picture measure with only ac signal. The low frequency response depends very large on signal level ( i used different probes to get all in 1 picture).
This behave is not present with a SE transformer with dc (at least a lot less).
Edits: 08/29/16 08/29/16 08/29/16
There is nothing crude about my measurements.
See this picture measure with only ac signal. The low frequency response depends very large on signal level ( i used different probes to get all in 1 picture).
was the transformer you measured airgapped to handle DC?
This behave is not present with a SE transformer with dc (at least a lot less).
I would expect that to some degree because in order to handle the DC offset an airgap is needed and the introduction of the airgap will temper any variation of permeability with applied signal. A proper test would be to measure the same gapped transformer at various excitation levels with and without DC current and then compare. I predict the transformer with the DC bias will never show more inductance (unless the DC is pulling shut an unstable gap)
TO clarify my statement I am saying it is normal for a gapped SE transformer to show a much smaller range of variation on inductance with applied signal and as signal is increased the inductance will also increase, however this behavior is not a function of applying DC but of the airgap needed to deal with the DC reducing the range of variability. The increase perm seen in all cases is simply due to the increased AC excitiation.
I am also saying that it is impossible for a DC bias on a core to increase the permeability of a core. It can in some cases increase the effective permeability of a core by manipulating the airgap but that is a simple mechanical issue.
dave
I don't know if your measurements are correct Dave.
I measured today a real output transformer (Audionote Se EI-core) and not a choke but i can't confirm your measurements. At least a factor 2 differance in induction. And because i can't measure with low signel levels wih this transformer i can't confirm my own statements at the moment. Damn!
can you please give me the measured results with and without DC current at two different levels of AC excitation.
For example... 1V and 100V at some frequency well below 1Khz (50-120hz) with no DC bias and then repeat the same two AC measurements with like 75% of the rated DC current in addition to the AC.
I have done this 1000 times or more and have yet to find a case where adding DC increases the permeability. The image below is a set of plots of inductance vs. current for a fixed AC excitation. As you can see that the addition of DC doesn't increase the measured inductance and if it did you would see the inductance increase as DC current is added.
dave
?
Edits: 08/30/16 08/30/16
For the plots I attached, the measurement setup is a Genrad 1633 setup sitting 10 feet behind me.
dave
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