Just for the heck of it the other day I measured an output transformer at 60 hz and it showed 8.8k but when I ran 1Khz out of my HP 200c I got 7.2K. It makes perfect sense actually sort of but it's about turns ratio and you wouldn't think that turns ratio would change with frequency now would you?
It could be the 200C isn't holding output, I need to check the output tube and do some load experiments but all in all:
Is there a reason for OT impedence to change with frequency??
Transformer impedence vs frequency
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skyboltone
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Transformer impedence vs frequency
The Last of the World's Great Human Beings
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Andy Le Blanc
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but it's about turns ratio
I have a HP 202A and I use it for the same purpose.... I also cheat... or so it may seem.....I use the freq. to tweak the math to make it work out with less strain on the one math cell in my brain.....its still in the ballpark.... I think there are two important concepts here...... Your measuring impeadence and reactance a given freq. to find the effective load .... and what is the standard procedure that must be applied so that every one is on the same page?....
lazymaryamps
Re: Transformer impedence vs frequency
Inductive reactance
(Xl) = 2 * Pi * F * L
(Xl) = 2 * Pi * F * L
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Transformer impedence vs frequency
There's other issues with measuring due to the j/i (imaginary) component. For example, the AC voltages across an inductance and resistance in series do not directly add up to the applied voltage!
This is because the current and voltage in the circuit are actually out of phase with each other. The total phase angle depends on the XL to R ratio and is always between zero and 90 degrees. You need to apply the old Pythagorean theorem:
Ztotal^2 = R^2 + XL^2
so
XL = sqrt(Zt^2 - R^2) where Zt = Vtotal(applied)/I(measured).
Note that a transformer primary also has some resistance, which should be included in the 'R' component above (as a good approximation).
Also, this is probably obvious, but any impedance connected to the secondary will reflect back into the primary, so that is important too.
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Same thing happens with caps, but the angle is reversed.
Things get really crazy when you have a cap and inductor in the same circuit.
--mark
This is because the current and voltage in the circuit are actually out of phase with each other. The total phase angle depends on the XL to R ratio and is always between zero and 90 degrees. You need to apply the old Pythagorean theorem:
Ztotal^2 = R^2 + XL^2
so
XL = sqrt(Zt^2 - R^2) where Zt = Vtotal(applied)/I(measured).
Note that a transformer primary also has some resistance, which should be included in the 'R' component above (as a good approximation).
Also, this is probably obvious, but any impedance connected to the secondary will reflect back into the primary, so that is important too.
-----
Same thing happens with caps, but the angle is reversed.
Things get really crazy when you have a cap and inductor in the same circuit.
--mark
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Andy Le Blanc
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but when I ran 1Khz
I did some digging and found a much better explanation
You've run into the "Primary Inductance" of the transformer.
When a current is applied to a transformer, it induces a voltage
that opposes the applied voltage, this is called "back emf".
The magnitude of the back emf is dependant on the rate
current change (frequency). This also pertains to core saturation
and air gapping..... sometimes the right question can lead to new ground.
I've built amps for decades but never botherd to study trannies....
just another part to source....
You've run into the "Primary Inductance" of the transformer.
When a current is applied to a transformer, it induces a voltage
that opposes the applied voltage, this is called "back emf".
The magnitude of the back emf is dependant on the rate
current change (frequency). This also pertains to core saturation
and air gapping..... sometimes the right question can lead to new ground.
I've built amps for decades but never botherd to study trannies....
just another part to source....
lazymaryamps
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skyboltone
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- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: Transformer impedence vs frequency
I proposed the question with the proviso that it sorta maybe made sense. The problem with using the Turns ratio squared times the output load impedence to arrive at the input impedence is that turns ratio seems to be dependent on frequency. Which is silly. The turns ratio is a mechanical certainty. Mark, you are talking about ELI the ICE man right?
It's clear that the inductive reactance of the transformer will change with frequency, as pointed out. Don't we need an equation that includes inductive and capacitive reactance? Or do we just need to terminate the transformer under test with a pure R load? If we do terminate in a load, will we get a better idea of what the turns ratio really is, or is that equation a guesstimate at best anyway?
Dan
It's clear that the inductive reactance of the transformer will change with frequency, as pointed out. Don't we need an equation that includes inductive and capacitive reactance? Or do we just need to terminate the transformer under test with a pure R load? If we do terminate in a load, will we get a better idea of what the turns ratio really is, or is that equation a guesstimate at best anyway?
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
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Andy Le Blanc
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The problem with using the Turns ratio
The problem is the model that the math is based on...
its a rule of thumb based on an "ideal transformer".
It leaves out all of the real..
proper evaluation uses a open-circuit and short-circuit test as a
"general practice" to provide data for an "approximate equivalent circuit"
that gives accurate results....
Im still diggin.... Id love to test a tranny like I can test a tube..
its a rule of thumb based on an "ideal transformer".
It leaves out all of the real..
proper evaluation uses a open-circuit and short-circuit test as a
"general practice" to provide data for an "approximate equivalent circuit"
that gives accurate results....
Im still diggin.... Id love to test a tranny like I can test a tube..
lazymaryamps
Re: Transformer impedence vs frequency
I am no expert here, but doesn't the resistance in the load, normally a speaker, change with signal frequency? This changes around what, 3-5 x from the speakers rated impedance? Thus the reflected primary changes all over the place right? Does a transformer behave similarly?
Even if you can measure the turns ratio and estimate primary impedance for a given load, isn't it still just a good guestimate at best as to what the true impedance is?
Even if you can measure the turns ratio and estimate primary impedance for a given load, isn't it still just a good guestimate at best as to what the true impedance is?
Eardrums!!! We don't need no stinkin' eardrums!
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skyboltone
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Re: Transformer impedence vs frequency
Absoulutely, the reflected load changes with frequency. Most speaker curves include and impedence curve along with frquency response. there's a huge scoop at resonance. Music is composed, thank God, of a great number of frequencies all at once. So the tube is being fed a signal full of frequencies at various amplitutes and is feeding that mix into a load that is highly reactive as well. It's more like a wall of impedence interacting in very complex ways with the electron cloud in the tube. I think all of this must be what separates the men from the boys in OT design. I just would like to know what the parameters are for testing. There is a chapter in The Radiotron Handbook on speaker/amplifier interaction. I guess it's time to read it.dartanion wrote:I am no expert here, but doesn't the resistance in the load, normally a speaker, change with signal frequency? This changes around what, 3-5 x from the speakers rated impedance? Thus the reflected primary changes all over the place right? Does a transformer behave similarly?
Even if you can measure the turns ratio and estimate primary impedance for a given load, isn't it still just a good guestimate at best as to what the true impedance is?
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: Transformer impedence vs frequency
The transformer impedance actually varies with frequency,and add "colour" to the sound that came out from the speaker.The impedance of the primary,for example 1.7kohm ecc ecc is a math calculation that consider only turn ratio and sometimes some other few things.But luckily (not for all the people...) in some tube amp there is the negative feedback that tend to oppose to the varying impedance and damp the source.Actually with a minimum of negative feedback the amplifier output impedance (that is reflected to primary impedance) vary less,and more feedback -> less sound coloration. Disconnecting the NFB from a typical class AB amp cause the output impedance (for example the 16 ohm tap) rise peaks until 30-40 ohms.... is the secret of the mesa rectifier modern mode sound.... 
About laws and calculations to test a transformer...?? I don't know
I test output transformer only with turn ratio (with a good multimeter and a AC source) and if the primary impedance has to be a bit more for my use,I tend to use some filter in the NFB circuits (as Marshall does in the JCM900 EL34 to 5881 conversion...) for example to simulate a 2K ra-a load from a standard 1.7K ra-a transformer...
About laws and calculations to test a transformer...?? I don't know
I test output transformer only with turn ratio (with a good multimeter and a AC source) and if the primary impedance has to be a bit more for my use,I tend to use some filter in the NFB circuits (as Marshall does in the JCM900 EL34 to 5881 conversion...) for example to simulate a 2K ra-a load from a standard 1.7K ra-a transformer...
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
a transformer...?? I don't know
after some more digging... there is a serious gap in the available information
about transformers... we assemble... we dress the cicuit..... we design.....
amps
not the parts that we do all that with...the information about transformers we
need to do that is as a matching device.....turns ration and effective impedance..... most usable info is only that.....
where is the information?
we use the tubes but dont make them.. but we have a wealth of information about them.....
about transformers... we assemble... we dress the cicuit..... we design.....
amps
not the parts that we do all that with...the information about transformers we
need to do that is as a matching device.....turns ration and effective impedance..... most usable info is only that.....
where is the information?
we use the tubes but dont make them.. but we have a wealth of information about them.....
lazymaryamps
Re: Transformer impedence vs frequency
There are some good text books out there about transformer design, but I think a lot of them are out of print. You'll have to find them used somewhere. I'll have to dig this info up, but I know I have it somewhere. Don't have the books, just some names.
Eardrums!!! We don't need no stinkin' eardrums!
Re: Transformer impedence vs frequency
"ELI the ICE man" -- that's one I haven't heard in a decade or two! 
A transformer is just two inductors, magnetically coupled together. All transformers definitely have some self-inductance as well as whatever coupling goes on.
Check out Pete Millet's tech book pages.
http://www.pmillett.com/tecnical_books_online.htm
in particular, 'Electronic Transformers and Circuits' by Reuben Lee
It's not light reading, but very interesting.
--mark
A transformer is just two inductors, magnetically coupled together. All transformers definitely have some self-inductance as well as whatever coupling goes on.
Check out Pete Millet's tech book pages.
http://www.pmillett.com/tecnical_books_online.htm
in particular, 'Electronic Transformers and Circuits' by Reuben Lee
It's not light reading, but very interesting.
--mark