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Old 1st September 2007, 06:27 PM   #11
oshifis is offline oshifis  Hungary
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Hm, that makes sense. So this is not a correct measurement. Actually I wanted to measure the internal resistance of the output tubes in the circuit and determine the optimal turns ratio, where the amplifier transfers the highest power onto the load. What about the following method?

Leave the secondary of the output transformer open. Drive the tube amplifier so that is produces some reasonable voltage at the secondary of the output transformer. Then attach a dummy load at the secondary. The internal resistance can be calculated from the readings and from the turns ratio (squared). I know that running the tube amplifier unloaded is dangerous for the OPT primary, so I chose an output voltage of less than 10 V. The amplifier operates in open loop.
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Old 1st September 2007, 07:46 PM   #12
cerrem is offline cerrem  United States
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Normally you don't want to load a transformer durring measurements.... You measure it unloaded....
The main numbers you need are primary inductance... This changes according to flux density, so you graph the inductance vs. power ....
The other key point is the transformers resonance. You get this by driving the secondaries with a power amp and sweep the frequency till you get a peak.... Once you get the peak frequency, you can then determine the capacitance and leakage inductance...
Once you know the leakage inductance, that number stays fixed on a transformer since that is a function of winding geometry...
The problem next is remeasuring the the capacitance at different drive levels, since the capacitance will vary according to drive level, similair how the inductance varies.... This is due to the changing AC voltage gradient on the dielectric material in the transformer...The dielectric constant is actually not constant and has a slope that changes to applied AC voltage gradient...
You will see tha the transformer's resonance peak will drop in frequency as the power level is increased...
Once you have these numbers you can then mathamatically apply the plate resistance of the tubes used to determine the frequency response of the transformer at various power levels...

Chris
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Old 1st September 2007, 07:59 PM   #13
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by oshifis
Hm, that makes sense. So this is not a correct measurement. Actually I wanted to measure the internal resistance of the output tubes in the circuit and determine the optimal turns ratio, where the amplifier transfers the highest power onto the load. What about the following method?

Leave the secondary of the output transformer open. Drive the tube amplifier so that is produces some reasonable voltage at the secondary of the output transformer. Then attach a dummy load at the secondary. The internal resistance can be calculated from the readings and from the turns ratio (squared). I know that running the tube amplifier unloaded is dangerous for the OPT primary, so I chose an output voltage of less than 10 V. The amplifier operates in open loop.
To determine the source impedance of the amplifier this is a technique that will work, I would keep the voltages very moderate. (Just a couple of volts with an accurate volt meter should be more than sufficient.) Note that there is an excellent note on this measurement technique on the Audio Precision website. Generally used to determine source impedance in SS feedback amplifiers, it will work just as well with a tube amplifier whether or not it uses global feedback. Of course you can calculate it as cerrem has indicated as well, but doing the measurement will be more accurate and is actually pretty easy.

The equation Rout = (Vout - Vload) x Rload/Vload where Rout is the amplifier source impedance at the speaker terminals, Vout is the unloaded output voltage, Vload is the loaded output voltage, Rload is your resistor load.. Note that this includes the dc resistance of the secondary which you should subtract from the measurement before multiplying by the impedance transformation ratio (turns^2) to determine the approximate source impedance of the amplifier. Frankly the secondary number is usually the one of most interest. I've not tried this out in practice so I can't say how well it will actually work, I may have missed something obvious along the way.

In normal applications this would be done at 10Vrms, but given the high source impedance here much lower voltages can be used with good accuracy.
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