So, here's the scenario:
I found a stash of transformers scavanvged from audio equipment but they have no useful markings or identifiers.
Is there any way to determine what if any good these transformers would be as output transformers? (besides building a tube amp and listening to the result)
I have access to pretty much any electrical test equipment you can imagine from handheld multimeters to spectrum analysers and VNAs but I just don't know and can't find how I'd go about testing this. Every method I have seen on the web requires you know at least one piece of the puzzle.
Thanks in advance, would be fun to not just design OTL
I found a stash of transformers scavanvged from audio equipment but they have no useful markings or identifiers.
Is there any way to determine what if any good these transformers would be as output transformers? (besides building a tube amp and listening to the result)
I have access to pretty much any electrical test equipment you can imagine from handheld multimeters to spectrum analysers and VNAs but I just don't know and can't find how I'd go about testing this. Every method I have seen on the web requires you know at least one piece of the puzzle.
Thanks in advance, would be fun to not just design OTL
I would measure the winding resistances to get an idea of primary-secondary taps. For a valve amplifier step-down (output) transformer the primary will be much larger resistance than the secondary.
Then I would put in a 1kHz sine into the primary and measure the amplitude on the secondary (pick sensible input amplitude, take care for any high voltage). The amplitude ratio of the input and output will give you the winding ratio of the transformer.
Then I would put in a 1kHz sine into the primary and measure the amplitude on the secondary (pick sensible input amplitude, take care for any high voltage). The amplitude ratio of the input and output will give you the winding ratio of the transformer.
Forgot to say, the square of the winding ratio gives the impedance step-down - this is what you actually want to know. So then you can know if it matches the impedances in your case. So if you are using this with an 8 ohm speaker, then you use this ratio to estimate the primary side design impedance.
Also the DC winding resistances should be much smaller than these impedances, i.e. losses in the transformer should be small when used properly.
Also the DC winding resistances should be much smaller than these impedances, i.e. losses in the transformer should be small when used properly.
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Alright I think this pretty much clears it up.
Find the primary and secondaries with a multimeter, then put a small 1KHz sine in the primary to get the impedance ratio and calculate if it suits my application.
Seems pretty simple then actually, not sure why I couldn't find this clearly stated online...
Ya. Start with an Ohm meter to map the windings. Then you need an AC stimulus, best into the high-Z windings so that you don't ~short your source and/or generate any high voltages. And best to use a middle frequency (~1KHz), avoiding inductance loading at the low end and ringing at the high end. Remember the impedance ration is the square of the voltage ratio. Note the inductance of the winding will be about the rated impedance at the bottom of the frequency range, ie L=2πFZ, or Z=L/2πF, (F~=20Hz) Better transformers have more inductance, ie the low frequency limit is lower.
No doubt there are other methods but here's how I measure output transformers. I don't have a lot of test equipment so no sine waves / square waves, etc. This just requires a multimeter and a variac.So, here's the scenario:
I found a stash of transformers scavanvged from audio equipment but they have no useful markings or identifiers.
Is there any way to determine what if any good these transformers would be as output transformers?
Using a 12v filament transformer and alligator clips (preferably not clip leads):
1. Connect primary to variac and increase voltage until secondary reads 10v
2. Leave variac set and turn it off at power strip
3. Connect secondary of filament transformer to secondary of OT with clips, leave CT open
4. Connect primary of OT to meter, set meter to AC voltage, if PP connect to plate leads and leave CT open, turn on variac at power strip
5. Read AC voltage on meter and divide by 10 to get turns ratio
6. Calculate: turns ratios squared multiplied by ohms of speaker tap (or the nominal impedance of the speaker you want to use) equals primary impedance
Step 1 may burn a small low power OPT, 10v may require too high voltage on the primary.
I have written a little article about this, see: https://tubes.njunis.net/?p=288&lang=en
All I know is that they are scavenged from audio equipment and would not have been snatched if they were not in the signal path, meaning they should be suitable for audio frequencies.
That is all I know. They are definitely not power trafos at least seeing as they are fairly small. I would not expect to push any sort of power through these, less than 10W even for the largest ones.
Perhaps I wasn't clear. In Step 1 the output of the variac is connected to the primary of a 12v filament transformer, not to an output transformer.
The "12v" rating of the filament transformer means that with 120v (or whatever it's rating) on the primary that you would expect to see 12v on the secondary if the filament transformer is connected to a specified load.
In my case, I use a surplus filament transformer that are has 117v primaries and 12v 3A secondaries. Since there is no load (such as from a tube heater) it takes less than 117v on the primary to produce 12v on the secondary. The input voltage from the variac is not important. It's adjusted so that it's putting out 10v on the secondary. Having 10v just makes it more convenient to do the math. You could actually use any voltage you like.
You could actually do this without the filament transformer and connect the variac directly to the secondary of the mystery OT. The only reason to use a filament transformer is that it makes it much easier to adjust the voltage to get precisely 10v.
You then send 10v into the secondary of the mystery OT. The mystery OT is not put under any stress because there is no load on its primary. OTs are rated for a particular wattage but watts is voltage multiplied by the current load. Since there is no current load, the wattage rating is not being exceeded.
As an example, I tested a small OT taken from some Philco something or other. Its turns ratio was calculated as 25:1. So with 10v on the secondary I got a reading of 250v on the primary. Doing the math indicates that the primary impedance is 5k with an 8 ohm load or 2.5k with a 4 ohm load.
I suppose you could exceed the rating of the insulation if you fed it with thousands of volts or something crazy. But putting 10v AC on the OT secondary won't cause any problems.
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Please be aware that unless a special model, a regular Variac is an auto-transformer,so secondary is directly connected to live mains, so anything from dangerous to deadly.
Be very careful,or better yet, use a standard (isolating) 12V transformer.
I understand your point, but I do not see the reason to take any risk using 10V. Just 5V or even less is equally right. The procedure itself is fine, but I prefer to do it the other way, I have a 55V transformer and I apply that to the primary, and measure the voltage in the secondary.
See the discussions here make me wonder why certain transformers are marked stuff like 10k:600 or similar if all that matters is the actual turns ratio(and obvious that it's ment for audio frequencies). Is it all marketing? I've definitely seen transformers from the same seller with the same ratio but different numbers, those should be identical then?
Impedance matching criteria, it is not marketing. For an output stage it guarantees maximum power transfer to the load.
However, you don’t always want to match impedance, for example, in a line out application you want to transfer the most voltage (signal information) to a load (the next stage) without voltage drops. This is called bridging impedance, you want the apparent output impedance to be much lower than the input impedance of the next stage.
So your example, 10k:600 would be a bridging impedance. You amplifier has output impedance of 10k which you want to lower to 600 so that the stage it connects to is transferred most signal. These line level transformers also provide isolation which negates ground loops.
Edit; forgot to answer the final part.
I think what you are saying is 10k:600 has the same ratio as 100k:6000 so what’s the point of the actual numbers?
The actual windings have a parasitic resistance and the certain magnetising inductance (low frequency) and leakage inductance (high frequency). The winding resistances cause insertion loss, the more turns the higher the inductances, which will start to shift your frequency response.
A lot of effort is put in to optimising all the various values to get the best frequency response possible for an audio transformer. As a part of this process you need to know what impedance will be connected on the primary and secondary. That’s why the actual values are specified.
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