Odd output transformer impedances?

[zhyla]

I bought this old 6L6 organ amplifier for parts for my bassman project and started looking at the transformers today. The output transformer probably isn't going to work for me, it's got 260 ohm primary and what looks like 3 secondary taps that all read about 0.5 ohm.

I don't know what speaker this originally drove but I was expecting the secondary to be something closer to 4 - 8 ohms and the primary in the thousands of ohms. Or are DC resistances not good indicators of transformer impedances?

[Tom Phillips]

I bought this old 6L6 organ amplifier for parts for my bassman project and started looking at the transformers today. The output transformer probably isn't going to work for me, it's got 260 ohm primary and what looks like 3 secondary taps that all read about 0.5 ohm.

How many wires does the primary have? Could be 2 (Single ended amp with 1 6L6), 3 (used with two or four 6L6s in Push-Pull like a Bassman) or 5 (Ultra linear configuration. Could still be useful)
If there were two 6L6s then those transformers probably will work for a Bassman project if it was a decent organ amp.

? Or are DC resistances not good indicators of transformer impedances?

Now your getting closer.
DC resistance is not the same as impedance. There is much more to how an impedance matching transformer does it?s job. If you google around you should be able to find the explanation better than I have time to explain.
Here is one site to get you started Output transformers explained

Regards,
Tom

[zhyla]

Thanks for the info Tom. The amp I pulled the OT from is a 2 6L6 push-pull output stage. There are 3 wires on one side (2 primaries and a center tap) and 4 wires on the secondary side (no idea which is which). I'm not sure what the speaker impedance was in the amp, that's why I was measuring the secondary coils. I want to drive an 8 ohm speaker.

So I knew that impedance can differ from the DC resistance but didn't know it would be that much. I can't measure the actual impedance and the DC resistance tells me nothing. I know it's the right OT for a push-pull 6L6 output stage to drive *some* speaker, how do I know it will work for an 8 ohm speaker and which of the wires to use?

The secondary wires are black, gray, green, and yellow, if that means anything. I know I can just build the amp and try it, but I'm not sure I'll know whether I've mismatched the speaker or not.

[BJosephs]

Primary impedance is the square of the turns ratio multiplied by the secondary impedance. You have some guarantee that the transformer is of sufficient power handling (not very measurable or testable) so here is a way to determine the impedance.

Hook up a signal genaerator across two leads of the secondary, set it for 1 Vrms, and measure the voltage across the primary leads. The ratio of the voltages = the ration of turns. Square this and you have the impedance ratio for the transformer across thise poarticular windings. Multiply by 8 ohms. If the product is not correct for two 6L6s (maybe 4k?) try another two secondary wires. It would be nice if you could first isolate which wire is ground before testing to prevent measuring across two "taps" rather than tap to ground but you may just figure it out when the numbers show up.

Draw a diagram and make some little spreadsheets to write down all of your measurements, this will help you analyze the data.

I have used this method before so, if you have any questions, I can try to help.

[Enzo]

The DC resistance has no relation whatever to the impedance. Lower impedance windings do tend to have lower reistance since the wires are heavier mort cases, but that is not related to impedance, but to current.

Transformers have no inherent impedance, just as a set of gears has no inherent speed. A transformer "tranforms" the impedance of the load back to the driving circuit. It is the electrical equivalent of gears. It makes the load compatible with the impedance the driving circuit expects. The primary impedance will be determined by the load impedance and the turns ratio. When a transformer shows a spec of 4k for the primary, that wil ONLY be the case if the load is the one specified in the same specs.

[zhyla]

Ok, so I'm going to borrow a function generator from a friend and measure my "gear ratio". Enzo, I get what you're saying but I think a more accurate way of saying that is that the primary impedance is dependent on the secondary's impedance and load, and vice versa.

Is there any place to find old data sheets for the random transformers? Mine have part numbers but I couldn't find anything with google.

[Bruce / Mission Amps]

Very simple to do with nearly no tools

Ok, so I'm going to borrow a function generator from a friend and measure my "gear ratio". Enzo, I get what you're saying but I think a more accurate way of saying that is that the primary impedance is dependent on the secondary's impedance and load, and vice versa.

Is there any place to find old data sheets for the random transformers? Mine have part numbers but I couldn't find anything with google.

Nah you don't need a bunch of testing junk for this... it's simple with crap from your junk box.
Get your DVMM out and an old wall wart that can muster 3vac to 12vac at a few hundred ma of current.
See if you can find the common ground wire of the secondary/load/speaker side first and always use that one with one of the other wires... usually black.
If you use an ohmmeter for this you'll notice that with one wire (usually black) there will be a pattern of DC resistance from something very low, like .5 ohms to .65 ohms, to .8 ohms... etc.
That usually means 4 ohms to 8 ohms to 16 ohms.
If it is really high then it could be a 70v tap which is unusable for a speaker.


Connect the "ground" or "common" wire (usually black) and one of the other secondary wires to the 3vac to 12vac source and then measure the voltage on those two secondary wires of your unknown output transformer... remember, with the low 3vac to 12vac connected to the OT... it will load the wall wart down so you need to measure the actual AC voltage under load to do this.
Now measure the high voltage of the primary side.. it will be very high so be careful!!
Now divide the small voltage into the large one and you have the voltage ratio.
Square the voltage ratio and multiply it by the speaker load and you now have the impedance ratio.
Example:
Say you have a 6vac 350ma wall wart.
You hook it up and measure 6vac input on two of the secondary wires and 150vac out across the whole primary, usually brown and blue in guitar amps.
Don't measure from the outside to the center tap.

You get....
150vac/6vac = 25:1
25^2 = 625
625 x 8 ohms = around 5000 ohms. (Typical for Hi-Fi 6L6s)
If it only works out to 2500 ohms, you are probably on the 16 ohm tap with an 8 ohm load... etc.

From there you should be able to figure out what speaker load on what two pairs of wires makes what impedance.
Since we have 60Hz here in the USA, your a low voltage AC output wall wart will be generating a 60Hz signal... no signal generator needed... others in different countries might be 50Hz.
Regardless, any decent output tranny can operate at 50Hz to 60Hz so this is fairly accurate.

By the way, if your OT has a 4 ohm tap..... while 6vac at 4 ohms is only about 9 watts, 12vac at 4 ohms is around 36 watts so make sure your wall wart can handle the current or it will get loaded down badly and overheated.
If your wall wart is 6vac and can only make 3vac while connected, it has too low of a current rating and will get hot but if you can do VERY short timed tests and use the actual measured AC voltage delivered to the OT's secondaries you should be OK... if not it will burn out.

[zhyla]

Thanks Bruce, I was thinking something along those lines would be possible. I guess for this sort of thing measuring the impedance at 60 Hz is good enough.

[zhyla]

I measured my output transformer's impedance ratio this morning using the function generator. It looks like I've got suitable 8 ohm and 16 ohm taps and also maybe a 32 ohm tap? I won't be using it so I am ignoring it.

I couldn't get my spreadsheet to plot the way I wanted but a picture of the data is attached. At the high end it seems to spike up a bit. I'm not as confident in the measurements at 8 kHz because my DMM seems to be inaccurate at higher frequencies. It may be correct but I have no way of knowing. In any case, looks like this OT will work perfectly.

It also looks like the lower-tech 60 Hz wallwart is sufficient for sorting out taps, this transformer seems linear over most of the lower audio range.

Thanks for the instruction everyone, it was very helpful.