If you know the turns ratio and the load impedance, the primary impedance is trivial- the load multiplied by the square of the turns ratio.
If you don't know the turns ratio, it's easy to measure: apply a known AC voltage to the primary, measure the voltage induced in the secondary. The voltage ratio is conveniently the same as the turns ratio.
If you don't know the turns ratio, it's easy to measure: apply a known AC voltage to the primary, measure the voltage induced in the secondary. The voltage ratio is conveniently the same as the turns ratio.
Hello there:
I have used this approach to identify mystery output transformers from Vintage amps where the Raa is not written on any manual or schematic or the amp is long gone.....
Recommended Test Equipment:
>-Oscilloscope (a good AC Digital Voltmeter can also work)
>-Sine Wave Signal Generator (or CD and RCA connector w/leads)
>-Alligator Clip Jumper set (at least 10 jumpers)
>-Calculator with SQRT and Squared function
>-Pencil/paper
>-Transformer Formulas
1) Identify Primary and Secondary of OPT under test
2) Connect Transformer "Primary" speaker side to a 1 KHz Sine Wave from Signal generator at 1.0 Volt AC RMS from your CD player or Signal Generator. On the Oscilloscope this shows up on the CRT as 2.828 Volts Peak on the CRT screen, or 2.828 vertical divisions set to 1volt / division.
"Make sure that you identify what tap you are using and note that tap down (regardless of impedance) mark them with colored tape. You can probable use a good Ohmmeter to identify the secondary taps. The lowest resistance is usually the lowest impedance tap (i.e. 4 Ohms) and progressively the larger Ohms correspond to the 8 and 16 Ohm taps respectively"
3) Connect the Primary to the Oscilloscope and get a steady 1 KHz waveform trace at 0.5 Volts Peak to Peak (or 1 Volt RMS) on the DVM AC Meter.
4) Disconnect the Oscilloscope from the OPT primary and connect it bridged to the secondary in parallel with the signal generator.
5) From the Oscilloscope CRT measure the Peak to Peak AC voltage and write this value down in Volts.
6) re-Connect the Oscilloscope to the Primary and measure the Sine Wave peak to peak voltage and write this down.
From the transformer formulas:
Vsec/Vpri = Sqrt (Zs/Zp)
If you know that the tap you are injecting the signal into the secondary is an 8 Ohm tap then you already have the value of Zs in the formula. Knowing Vsec and Vpri from your Oscilloscope (or you can also use a RMS VOM that is accurate.
Solve for Zp as:
Zp = Zs / ((Vpri/Vsec)^2)
This is a good method for solving for Zp when you at least know what the Secondary Impedance is. If you don't know then you must use the Turns Ratio formula and work from there by assuming a value of speaker load (4, 8, 16) and solving for the respective Primary impedance.
Usually Transformers have 4, 8, or 16 Ohm secondaries. Newer OPT's are usually accompanied by the specs on a label or a manual page.
Example:
Secondary AC Voltage = 0.5 Volts Peak to Peak
Primary AC Voltage = 8.5 Volts Peak to Peak
Secondary Impedance is = 8 Ohms
Zp = Zs / ((Vpri/Vsec)^2)
Raa = Zp = 8 / ((.5/8.5)^2)
Raa = Zp = 8 / ((.05882)^2)
Raa = Zp = 8 / (0.00346)
Raa = Zp = 2.312 K = 2312 Ohms
You can substitute Vsec and Vpri in RMS, Peak to Peak, etc. just as long as both numbers are in the same units so the Ratio calculation is consistent. A good AC voltmeter with capability to measure 1 KHz also can works well if an Oscilloscope is not available.
If you use the 4 or 16 Ohm tap then just substitute this value
into the formula and re-measure the AC voltages that you inject a measure on the other side.
Hope this helps you to determine Raa (Zp) on mystery transformers.
Here are the conversions of AC Voltage to go from RMS to what you see on the Oscilloscope CRT which is usually Peak or to Peak to Peak measuring the humps of the sine wave. Peak to Peak is the complete vertical difference of the sine wave. Peak is one half of the sine wave, but you must center the sine wave vertically along the Zero X axis, or simply divide the Peak to Peak measurement by 2 to get Peak Voltage. RMS is not easy to measure from an Oscilloscope trace unless you have a fancy Digital Scope that provides RMS voltages on the display
1 Volt RMS x 0.9 = "Average AC Voltage"
1 Volt RMS x 1.414 = "Peak AC Voltage"
1 Volt RMS x 2.828 = "Peak to Peak AC Voltage"
The nice thing about RMS voltages is that they provide a value that can be used directly into DC formulas to determine Current and Power (plus resistance as well).
10 Volts RMS is equivalent in a resitive circuit to 10 Volts DC.
So a 10 Ohm Resistor with 10 Volts DC or RMS AC draws
I = 10/10 = 1 ampere DC or AC RMS
Power Dissipated would be P = 10V * 1 amp (DC or RMS) = 10 Watts
I have used this approach to identify mystery output transformers from Vintage amps where the Raa is not written on any manual or schematic or the amp is long gone.....
Recommended Test Equipment:
>-Oscilloscope (a good AC Digital Voltmeter can also work)
>-Sine Wave Signal Generator (or CD and RCA connector w/leads)
>-Alligator Clip Jumper set (at least 10 jumpers)
>-Calculator with SQRT and Squared function
>-Pencil/paper
>-Transformer Formulas
1) Identify Primary and Secondary of OPT under test
2) Connect Transformer "Primary" speaker side to a 1 KHz Sine Wave from Signal generator at 1.0 Volt AC RMS from your CD player or Signal Generator. On the Oscilloscope this shows up on the CRT as 2.828 Volts Peak on the CRT screen, or 2.828 vertical divisions set to 1volt / division.
"Make sure that you identify what tap you are using and note that tap down (regardless of impedance) mark them with colored tape. You can probable use a good Ohmmeter to identify the secondary taps. The lowest resistance is usually the lowest impedance tap (i.e. 4 Ohms) and progressively the larger Ohms correspond to the 8 and 16 Ohm taps respectively"
3) Connect the Primary to the Oscilloscope and get a steady 1 KHz waveform trace at 0.5 Volts Peak to Peak (or 1 Volt RMS) on the DVM AC Meter.
4) Disconnect the Oscilloscope from the OPT primary and connect it bridged to the secondary in parallel with the signal generator.
5) From the Oscilloscope CRT measure the Peak to Peak AC voltage and write this value down in Volts.
6) re-Connect the Oscilloscope to the Primary and measure the Sine Wave peak to peak voltage and write this down.
From the transformer formulas:
Vsec/Vpri = Sqrt (Zs/Zp)
If you know that the tap you are injecting the signal into the secondary is an 8 Ohm tap then you already have the value of Zs in the formula. Knowing Vsec and Vpri from your Oscilloscope (or you can also use a RMS VOM that is accurate.
Solve for Zp as:
Zp = Zs / ((Vpri/Vsec)^2)
This is a good method for solving for Zp when you at least know what the Secondary Impedance is. If you don't know then you must use the Turns Ratio formula and work from there by assuming a value of speaker load (4, 8, 16) and solving for the respective Primary impedance.
Usually Transformers have 4, 8, or 16 Ohm secondaries. Newer OPT's are usually accompanied by the specs on a label or a manual page.
Example:
Secondary AC Voltage = 0.5 Volts Peak to Peak
Primary AC Voltage = 8.5 Volts Peak to Peak
Secondary Impedance is = 8 Ohms
Zp = Zs / ((Vpri/Vsec)^2)
Raa = Zp = 8 / ((.5/8.5)^2)
Raa = Zp = 8 / ((.05882)^2)
Raa = Zp = 8 / (0.00346)
Raa = Zp = 2.312 K = 2312 Ohms
You can substitute Vsec and Vpri in RMS, Peak to Peak, etc. just as long as both numbers are in the same units so the Ratio calculation is consistent. A good AC voltmeter with capability to measure 1 KHz also can works well if an Oscilloscope is not available.
If you use the 4 or 16 Ohm tap then just substitute this value
into the formula and re-measure the AC voltages that you inject a measure on the other side.
Hope this helps you to determine Raa (Zp) on mystery transformers.
Here are the conversions of AC Voltage to go from RMS to what you see on the Oscilloscope CRT which is usually Peak or to Peak to Peak measuring the humps of the sine wave. Peak to Peak is the complete vertical difference of the sine wave. Peak is one half of the sine wave, but you must center the sine wave vertically along the Zero X axis, or simply divide the Peak to Peak measurement by 2 to get Peak Voltage. RMS is not easy to measure from an Oscilloscope trace unless you have a fancy Digital Scope that provides RMS voltages on the display
1 Volt RMS x 0.9 = "Average AC Voltage"
1 Volt RMS x 1.414 = "Peak AC Voltage"
1 Volt RMS x 2.828 = "Peak to Peak AC Voltage"
The nice thing about RMS voltages is that they provide a value that can be used directly into DC formulas to determine Current and Power (plus resistance as well).
10 Volts RMS is equivalent in a resitive circuit to 10 Volts DC.
So a 10 Ohm Resistor with 10 Volts DC or RMS AC draws
I = 10/10 = 1 ampere DC or AC RMS
Power Dissipated would be P = 10V * 1 amp (DC or RMS) = 10 Watts
That sounds like an awfully complicated way to work it out, especially since many people (including me) don't have a scope at home...
Why do you think you need to use 1KHz? I thought most OPTs should be able to pass mains frequency reasonably well.
Lets say you have a 20V RMS source (at 50/60Hz, from a small mains transformer) applied across the primary, and you measure 1V across the secondary taps you want to use (lets say the 8 ohm taps). That means the turns ratio is 20:1, hence the impedance ratio is the square of the turns ratio, i.e. 400:1. So the primary impedance is 3K2 for an 8 ohm load on that tap (calculated as 8 x 400 = 3200). Each valve in push-pull will see 1/2 the turns, or 1/4 the impedance of the transformer, so you'll have to plot an 800 ohm loadline.
Voltages measured with a DMM on "RMS" mode - yes, I know many don't measure true RMS. And of course you'd have to have some way to identify the taps.
I have heard of people measuring the primary impedance by applying the mains power to the primary (i.e. 240V RMS here), but I'm not sure about how safe that is (though these transformers were designed to handle rather large voltage swings on their primaries, hmm....)
Why do you think you need to use 1KHz? I thought most OPTs should be able to pass mains frequency reasonably well.
Lets say you have a 20V RMS source (at 50/60Hz, from a small mains transformer) applied across the primary, and you measure 1V across the secondary taps you want to use (lets say the 8 ohm taps). That means the turns ratio is 20:1, hence the impedance ratio is the square of the turns ratio, i.e. 400:1. So the primary impedance is 3K2 for an 8 ohm load on that tap (calculated as 8 x 400 = 3200). Each valve in push-pull will see 1/2 the turns, or 1/4 the impedance of the transformer, so you'll have to plot an 800 ohm loadline.
Voltages measured with a DMM on "RMS" mode - yes, I know many don't measure true RMS. And of course you'd have to have some way to identify the taps.
I have heard of people measuring the primary impedance by applying the mains power to the primary (i.e. 240V RMS here), but I'm not sure about how safe that is (though these transformers were designed to handle rather large voltage swings on their primaries, hmm....)
Using a step-down transformer to inject 60 Hz at 20 VAC into the OPT sounds reasonable in lieu of a Test Tone CD and Player or a Signal Generator. Yet another creative way to home-brew test equipment.
My intention was to describe mathematically the complete process like a cookbook, it only looks complicated when explained in detail I believe (just an opinion.)
If you notice my post is just an expanded version of a prior posting. One can express things in few words and assume a reader will be able to expand into a real working solution.
I don't see any reason why 60 Hz would be different than 1000 Hz, unless the OPT is not designed to pass 60 Hz, unlikely for an OPT.
Center tapped primaries carry 1/2 the impedance on each side, but most OPT transformer specs are provided as the complete winding, not from center tap.
For instance 6600 Ohms, 20 Watt, 4-8-16 Ohms means 6600 Ohms from Plate to Plate (Raa). The actual Push-Pull Raa formula divides by 4 for the Power Tube combination from the previous computation of Raa.
Yet most Push Pull OPT's are specified with their primaries from Plate to Plate which is not excatly the same as when one calculates the Raa for the particular Power Tube combination.
I will peruse my books and post the formulas if you would like to see them.
Sorry if it appeared complicated..... If one knows the Turns Ratio then the whole thing is much simpler as it avoids the need to measure Voltages.
My intention was to describe mathematically the complete process like a cookbook, it only looks complicated when explained in detail I believe (just an opinion.)
If you notice my post is just an expanded version of a prior posting. One can express things in few words and assume a reader will be able to expand into a real working solution.
I don't see any reason why 60 Hz would be different than 1000 Hz, unless the OPT is not designed to pass 60 Hz, unlikely for an OPT.
Center tapped primaries carry 1/2 the impedance on each side, but most OPT transformer specs are provided as the complete winding, not from center tap.
For instance 6600 Ohms, 20 Watt, 4-8-16 Ohms means 6600 Ohms from Plate to Plate (Raa). The actual Push-Pull Raa formula divides by 4 for the Power Tube combination from the previous computation of Raa.
Yet most Push Pull OPT's are specified with their primaries from Plate to Plate which is not excatly the same as when one calculates the Raa for the particular Power Tube combination.
I will peruse my books and post the formulas if you would like to see them.
Sorry if it appeared complicated..... If one knows the Turns Ratio then the whole thing is much simpler as it avoids the need to measure Voltages.
jueic said:
What do you mean?
Find the turns ratio, square it to find the impedance ratio and then you can find the plate-to-plate impedance for a given secondary load.
Each valve sees half the turns, or one quarter the impedance of the primary. So when plotting loadlines on composite characteristics, plot 1/4 the plate-to-plate impedance.
So if you have a PP OPT with an 8K primary impedance (plate-to-plate), each valve sees 8/4 = 2K.
At least that's how I think it's done.... damn, I always get confused when trying to explain things.....
audiousername said:
What do you mean?
Find the turns ratio, square it to find the impedance ratio and then you can find the plate-to-plate impedance for a given secondary load.
Each valve sees half the turns, or one quarter the impedance of the primary. So when plotting loadlines on composite characteristics, plot 1/4 the plate-to-plate impedance.
So if you have a PP OPT with an 8K primary impedance (plate-to-plate), each valve sees 8/4 = 2K.
At least that's how I think it's done.... damn, I always get confused when trying to explain things.....
Hi, Audiosername
That is the spot-on answer for me... I was wondering eihter the primary impedance is 8/2 K or 8/4 K..
Cheers,
JueiC
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