Usually I don't bother measuring phase plots, but here is one on my regular 300B SE transformers. The scale is 36deg / division. Here's how it should look like with the correct phase.
Here's how it performs with an unloaded secondary, where the HF phase is smoother, which indicates a leakage inductance dominant roll-off. Which is the way I usually design my transformers.
Here's how it performs with an unloaded secondary, where the HF phase is smoother, which indicates a leakage inductance dominant roll-off. Which is the way I usually design my transformers.
50AE,
Your 9 Ohm tap . . .
Reminded me of some output transformers I had.
4 secondary windings, each with equal turns.
Could be wired for 2, 4, 9, and 16 Ohms.
It has been 22 years since I had access to the R&S vector network analyzer.
I was reminded of how I tested for the turns ratio.
Use a low impedance drive at 1 kHz, and unload the secondary.
The turns ratio is the same as the voltage ratio, because at mid frequencies, the DCR of the primary and DCR of the secondary essentially drop out of the equation when the secondary is unloaded.
For turns ratio testing, low frequency and high frequency can not be used . . .
At low frequencies, the inductance "gets in the way"; and at high frequencies, the distributed capacitance of the primary "gets in the way".
I seem to remember that the exact 0 degrees phase of all the output transformers I ever tested was between 500Hz and 2kHz.
On the R&S VNA, there is a graphical convenience that shows the + to - phase wrap at either side of zero degrees.
Your 9 Ohm tap . . .
Reminded me of some output transformers I had.
4 secondary windings, each with equal turns.
Could be wired for 2, 4, 9, and 16 Ohms.
It has been 22 years since I had access to the R&S vector network analyzer.
I was reminded of how I tested for the turns ratio.
Use a low impedance drive at 1 kHz, and unload the secondary.
The turns ratio is the same as the voltage ratio, because at mid frequencies, the DCR of the primary and DCR of the secondary essentially drop out of the equation when the secondary is unloaded.
For turns ratio testing, low frequency and high frequency can not be used . . .
At low frequencies, the inductance "gets in the way"; and at high frequencies, the distributed capacitance of the primary "gets in the way".
I seem to remember that the exact 0 degrees phase of all the output transformers I ever tested was between 500Hz and 2kHz.
On the R&S VNA, there is a graphical convenience that shows the + to - phase wrap at either side of zero degrees.
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6A3, I usually test turn ratio with my 50Hz low impedance generator, then I love to double check when doing the oscilloscope/frequency response measurements. Usually, 1kHz is the trusted frequency. I run such sinewave.
Phase plots are good for simulation of NFB. Never sure how to trim leakage inductance and primary capacitance at HF. They don't match the real transformer as are lumped models. Nevertheless if you can match the phase plot you do get accurate value for the FB compensation. For accuracy the plots need to go down to 1Hz and up to 200KHz. as the unity gain points are needed.
Recreating models of transformers is complex, because capacitances and leakage inductances are all over the place in different values and connection poins. As a rule of thumb, the more the interleaving, further complicated the model gets.
Btw, I'm wondering if AI could recreate a model somehow by inverse-engineering a frequency response under the stated conditions?
For transformer to be used with NFB, I believe it is better to aim for a shunt capacitance dominant roll-off, where such transformer exhibits a more stable frequency response with different load variety.
Btw, I'm wondering if AI could recreate a model somehow by inverse-engineering a frequency response under the stated conditions?
For transformer to be used with NFB, I believe it is better to aim for a shunt capacitance dominant roll-off, where such transformer exhibits a more stable frequency response with different load variety.
On a old thread opened by me about the OT tests in backward mode there are a lot of graph with different situation of tests; there is also a plot done by a sw from Fabrizio, tech director of Audioreview, that help to understand the quality of a trafo ( in general inductors) and this tests are done with secondary open
Very interesting; unfortunately the sw is not available, I ask him to put it online without success!
About phase I think that it is important to check but in pair with a THD vs Frequency at different ouput power on a live circuit.
The shape of the curve give the sonic result we will get
Normally is similar to a U ; more is wide and costant more the sonic results are well balanced.
This because the power tube tend to have distorion in the zone where the Z is going low (low freq and high freq)
In attach a graph about a good s.e. 300B Jap, well considered in audio, 1 w at 8 ohm
This another 300B, done by me years ago with a good trafo, 1 w at 8 ohm
Walter
Very interesting; unfortunately the sw is not available, I ask him to put it online without success!
About phase I think that it is important to check but in pair with a THD vs Frequency at different ouput power on a live circuit.
The shape of the curve give the sonic result we will get
Normally is similar to a U ; more is wide and costant more the sonic results are well balanced.
This because the power tube tend to have distorion in the zone where the Z is going low (low freq and high freq)
In attach a graph about a good s.e. 300B Jap, well considered in audio, 1 w at 8 ohm
This another 300B, done by me years ago with a good trafo, 1 w at 8 ohm
Walter
It is also true but if the mix between the tube and trafo give good shape of thd the balance of sound is good
Then there are the personal sensibilty
Anyway, to be a bit back on topic. This Japanese OPT shows a tremendous amount of distortion in the HF region. I would conclude we're talking about too much shunt capacitance for the driving impedance it has been tested with. It is a reason I like design my transformer for leakage inductance roll-off instead of capacitance roll-off. Leakage increases reflected primary impedance with frequency, which presents a higher load to a triode stage, this lower distortion. The trick is controlling the resonances, as higher leakage transformers are more picky towards interlayer capacitance distribution.
I'm curious about the measurements of your transformers. What is the difference in terms of conditions between the red and blue graphs? What could be the reason behind the dip of the blue graph located at 50Hz? A speaker load with an impedance peak perhaps?
I'm curious about the measurements of your transformers. What is the difference in terms of conditions between the red and blue graphs? What could be the reason behind the dip of the blue graph located at 50Hz? A speaker load with an impedance peak perhaps?
The red is 700 ohm as source and blue is 1 kohm
The load is 7,5 ohm
Next week I will start with stronger test
Are coming a p-p for 300B and KT150
And s.e. for 845.
But with a slow timing!
Walter
That's plenty high for the resonance. If you bandlimit the input signal to 20kHz or so, then you'll never severely excite that.
Maybe a look at this tests at the European Triode Festival 2009
extension://bfdogplmndidlpjfhoijckpakkdjkkil/pdf/viewer.html?file=https%3A%2F%2Fwww.diyparadiso.com%2Flundahl%2Fetf%2520transformer%2520test.pdf
Many doubts if this is a good test but non of them show a distortion so horribel as Walter shows.
In attach an old graph alredy published here in the old thread about reverse test of OT
The quality of image is not so clear.
Test done with a sw of my friend Fabrizio , a little box to switch inout/out of sound card and a R as reference.
Secondary open
Three different OT, Fiat ( same people who made for me the trafo), Hammond s.e. and Sowter SF potted type
The red line is the impedance module and the blu is the fase.
The peak is different for each one and for all it is possible to see that until the resonance the andament is inductive after became capacitive.
For Fiat the resonace is at 2 kHz, Hammond is at 2 kHz (around) and Sowter is 3 kHz
( I try to ge the original image to read better the numbers
Walter
The quality of image is not so clear.
Test done with a sw of my friend Fabrizio , a little box to switch inout/out of sound card and a R as reference.
Secondary open
Three different OT, Fiat ( same people who made for me the trafo), Hammond s.e. and Sowter SF potted type
The red line is the impedance module and the blu is the fase.
The peak is different for each one and for all it is possible to see that until the resonance the andament is inductive after became capacitive.
For Fiat the resonace is at 2 kHz, Hammond is at 2 kHz (around) and Sowter is 3 kHz
( I try to ge the original image to read better the numbers
Walter
Without a knowing how you tested this it doesn’t mean anything to me. What kind of probe did you use?
It is the same as testing a coil with a proper hw and sw
A sound card and switch box
I think that similar test can be done with Limp of Arta
( I don’t have the proper knowledge)
I will send the graph of test of the three trafos where it is possible to see the results that match with the indication of the previous graph
( already published here)
Walter
A sound card and switch box
I think that similar test can be done with Limp of Arta
( I don’t have the proper knowledge)
I will send the graph of test of the three trafos where it is possible to see the results that match with the indication of the previous graph
( already published here)
Walter
Not important, but as you are usually quite accurate, you had four 1 ohm secondaries which can be wired for 1, 2, 9 and 16 ohms.