Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 25, 2018, 12:14:04 pm
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How accurate are the primary impedances of the commercial OTs in general?
I measured recently an OT specified with 8K primary and 4, 8 and 16 Ohm secondary.
I used a signal generator with 500Hz for determining the ratio which was 29.4 on 8 Ohm.
When counting the impedance I get ~6914 Ohm (29.4 x 29.4 x 8 = 6914,9)
I got similar values for other outputs as well.
Is that common or is there something wrong in my methods?
/Leevi
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If you do a search here, there have been tons on this ?
me, I look at the data sheet, send money to the smart guys and have never been disappointed with results
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I have used their products and been very satisfied
/Leevi
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Leevi, what I do to test an OPT is apply 10v to the secondary, take reading on the primary`s, divide that voltage by 10, then square that number ,then multiply by speaker Z. See what you get.
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Ok, thanks. I did it by putting 10v on the primary and measured the secondary.
/Leevi
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I like to put 1V on the secondary. Keeps the primary in a safe zone and makes the math easier.
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My understanding is that good tranny's are within 35% of spec; and testing methods are proprietary. If the testing methodology isn't stated, and it hardly ever is, then we're basically left with guessing, hope & faith.
Also, 1K Hz is the audio industry standard for all testing, unless specified otherwise. Of course that doesn't guarantee that the manufacturer actually used a 1K Hz test frequency, leaving us to assume that they used the industry standard. Anyway, if you use some other test frequency, like 500 Hz, by definition you should expect a different impedance measurement.
Frustratingly, the 1K Hz performance may be useless information when dealing with bass frequencies (below 300 Hz), and makers of trannies & speakers rarely address this crucial info.
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I made some new measurements:
I used 1kHz and 10v on the primary and didn't find any difference what comes to the ratio.
I also noticed that the frequency affects if you put 1v on the secondary and measure on the primary side.
On a frequency a bit more than 1kHz (1200-1300Hz) I got a ratio that corresponds the 8K primary impedance.
It seems that there are a lot of uncertainties in the measurements so it is difficult to see if result is reliable or not.
Maybe it is better to trust on the manufacturer's information;)
/Leevi
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Maybe it is better to trust on the manufacturer's information;)
Yes. Much easier. Save the transformer test for unknown OTs.
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I used 1kHz and 10v on the primary and didn't find any difference what comes to the ratio.
I also noticed that the frequency affects if you put 1v on the secondary and measure on the primary side.
On a frequency a bit more than 1kHz (1200-1300Hz) I got a ratio that corresponds the 8K primary impedance.
The ratios don't change with frequency. If you have a test that gives different ratios at different frequencies, then you have a bad test.
Putting 1V on the secondary and reading the primary can give significantly different results with different probes and whether the probes are set to 1X or 10X. Setting the probes to 1X and adjusting the secondary to 1V every time you change frequencies seems to result in fairly consistent ratios. I have always put a voltage on the primary and read the secondary using the logic (possibly flawed) that the output impedance of the secondary is so low that the probe couldn't possibly affect the results.
Hammond frequently gives the turns ratio for their output transformers. These rarely result mathematically in the exact impedance ratio, but they are pretty close. Sometimes they give primary and secondary voltages that mathematically result in the exact impedance ratio, and sometimes they don't. I believe that the turns ratio is accurate and the transformer will perform accordingly.
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I don't use a scope. I use a Fluke 87V.
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When you drive a signal in to a winding (for testing purposes), the winding terminal voltage is not an accurate 'unloaded' voltage representation - due to the driven signal (magnetising) current.
If you have a PP output transformer, then apply a significant signal voltage from CT to one of the plate terminals, and measure the voltage across the other plate terminal-to-CT winding.
As an alternative, if you have an output transformer with multiple secondary taps, then apply signal to one section of that secondary, and measure secondary voltage from another unrelated section of winding.
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I have not used oscilloscope for my measurement.
I did a test measurement with another OT Hammond 1750F which has 6950 Ohm primary.
I got with 1kHz and 10V (primary) a result which is very close to that.
/Leevi
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I agree that the nominal impedance is simply a turns ratio thing. But if you really want to get into the nitty gritty, trobbins is the man.
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If you have a PP output transformer, then apply a significant signal voltage from CT to one of the plate terminals, and measure the voltage across the other plate terminal-to-CT winding.
Probably apparent to others, but I'm not following. The voltage on the loose end is going to be virtually identical to the voltage on the applied signal end relative to CT (although inverted). So do I double the measured voltage on the loose end and use that number as my primary voltage?
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Unless you KNOW your signal generator and meter impedances, it may be wise to use TWO meters.
You could set-up 1V on secondary, then move meter to primary, and because of meter loading the secondary drops to 0.9V. Which is similar to your numbers. With two meters you can watch for drop.
You also want to beware using general electrician's meters above ~~400Hz. Many old ones dropped-off bad. Some "True RMS" jobs actually had variable frequency response with level. Swap your two meters and see if results change.
The 3.15V side of a center-tapped heater winding is safe for almost any loudspeaker winding, is low-impedance so will not change much with loading, and the 50/60Hz is well within the frequency range of any AC meter. While a cheap/small transformer may not pass 50Hz from a tube, it usually does with a low-Z source like a heater transformer.
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The voltage on the loose end is going to be virtually identical to the voltage on the applied signal end relative to CT (although inverted). So do I double the measured voltage on the loose end and use that number as my primary voltage?
The suggested method for turns ratio is aimed at removing some of the error that may creep in. The measured voltage on the loose end will be a bit different from a measurement of the driven voltage. Yes double that measured voltage to give the effective PP primary voltage being applied. A spreadsheet that may help: http://dalmura.com.au/projects/OT%20calcs.xls (http://dalmura.com.au/projects/OT%20calcs.xls)
Note that the resolution and accuracy of your meter may also cause some error when reading the secondary voltage, as you may only be reading a few hundred mV, and the difference between 0.5V to 0.6V could then give a significant change in turns ratio.
Another concern can be if you apply too much voltage for the particular winding. Eg, a 4 ohm impedance rated winding on a 15W OPT would likely be saturating if you apply a 12Vrms 50Hz level from a mains transformer as a test signal.
If you don't know for certain either the rated primary impedance and the secondary impedance(s), as for an unknown output transformer, then it is a guessing game unless you can gather some clues such as about the original amp, or output stage valves, or the tap ratios of the secondaries, or if it has UL taps, etc.