Measure impedence of OT?

[bastardbus]

What is the accuracy of this method? I have used it on several OT transformers and I get some very off the mark readings.

I have a NEW OT for a Champ that was given to me from a nice fellow ToneMerc on these forums with supposed 5K with 4 and 8 ohm taps. Here is what I got when checking it.

100vac pr= 3.0vac tap 1 and 2.0vac tap 2
33:1 and 50:1 turns ratio
the 33:1 would be 4435 @ 4ohm but the the 50:1 is way too high for either the 4 or 8 ohm tap ??

Valco Oahu single ended 6V6 amp similar to tweed princeton
should be 5000 @4ohm OT
100vac pr=2.4vac sec
41.6 turns ratio
6944@4ohms

Voice of music 560A push pull 6V6 amp with supposed 6600@4ohm OT
100vac pr= 1.9vac sec
52.63 turns ratio
11079@4ohms

I also have two `50s era Meritt Universal Output transformers a 2900 and 2904 and a chart that tells you the specs for the primary in and output at the different 6 taps. When I put 100vac in the primary and do the figuring the outputs exactly match the chart I have for them, so I am pretty sure I am not making any mistakes in my figuring.

Any ideas on why I am getting great variances from what these OTs? Both the Valco and the VM OTs work GREAT and sound amazing in their amps mind you but seem quite a bit off from what they should be.

[bastardbus]

double post delete

[Phil_S]

bbus: Are you using the same meter to check input voltage and output voltage or two meters that reliably report the same voltage? It is possible you have a variance between meters. Also, it is important to check the input voltage under load, as it will drop. Unless you've got a calibrated Fluke or other very high end meter, I wouldn't take readings under 1V as reliable.

If you are doing that, then I believe the method is accurate. It is just basic physics that the turns ratio determines the voltage step up or down. It is quite possible that your sample transformers are exactly as you found them to be.

[tubeswell]

What is the accuracy of this method?

Its how you measure the voltages that matters. The method is highly accurate for measuring the actual voltage ratio of your OT provided your meters are calibrated the same and you apply the voltage to the secondary and measure the reflected voltage on the primary.

(Obviously if you are putting the signal onto the primary to measure the secondary, you are going to have a smaller voltage on the secondary, so it will be harder to get an accurate secondary reading)

Also for maximum accuracy, you need to be measuring both windings of the transformer at the same time.

[rp]

When I put 100vac in the primary...

Isn't that a lot? BTW what are the risks to the OT in all this? I always go into the secondary with no more than 5V, I'd use less but it's hard to get less than that reasonably stable on the variac. I don't use a load, not sure how to do that going backwards. I always wondered if testing this way could hurt the OT but I do it fast with two meters, and I'm just looking for a general value and to determine the taps.

Also as for accuracy, it's my understanding that an OT's ratings are determined at 1K hz, so when using 60Hz is will be a little off, no?

[bastardbus]

For the measurements I took I used my variac and set it to 100VAC then took my tester and checked it to be 100VAC with leads hooked to the primary. Then used the same tester to check the output at the secondaries. I varied the voltages for the heck of it 60vac 100vac and 120vac and the turns ratio was always basically the same.

I tried the opposite method of voltage into the secondary and out the primary but it kept blowing the fuse in my variac for some reason thus why I did it this method. I could try a spare power transformer with a 5.3 heater tap instead of my variac.

The only thing that I find odd is the two Merrit Transformers I have came out dead on while everything else was pretty much all over the board, weird huh?

Someone mentioned checking the OT under load, can you give me detailed directions on doing that properly as well.

Thanks
T

[Firestorm]

Even tiny measurement errors (like a tenth of a volt) can throw off your results, since your squaring a ratio and then multiplying by it. But, in the case of your first OT, plenty of Champs have OTs in the 10K range: with yours, the 50:1 with 4 ohms is 10K; the 33:1 with 8 ohms is 8K7. That's pretty close, assuming the OT was meant to be 10K, or close. Depends on what the manufacturer wanted (and on how carefully the winder was paying attention).

[bastardbus]

Even tiny measurement errors (like a tenth of a volt) can throw off your results, since your squaring a ratio and then multiplying by it. But, in the case of your first OT, plenty of Champs have OTs in the 10K range: with yours, the 50:1 with 4 ohms is 10K; the 33:1 with 8 ohms is 8K7. That's pretty close, assuming the OT was meant to be 10K, or close. Depends on what the manufacturer wanted (and on how carefully the winder was paying attention).

I thought a champ OT was supposed to be 5000@4ohm ?

[Ian444]

bbus, I think you are measuring correctly. What you see is what you get. I have measured many OPT's and they don't always turn out to be the manufacturer's specs, but some are close. Even between different windings (say 4 ohm and 8 ohm) the reflected primary impedance can differ a fair bit. I have tried measuring at different frequencies up to 1kHz and it doesn't make any difference, and I'm using Fluke meters. I measured a pair of old OPT's wound around 40 years ago, both were around 4k1 primary impedance, but they were advertised as 4k5. The list goes on. The best one was some Chinese OPT's in a PP EL34 hifi amp, 11K primary impedance!

[bastardbus]

I am going to try 5.3V into the secondary just to see what difference I might get.

Also once again, someone mentioned testing OTs under load. What is the proper details of doing this or is it necessary?

T

[Firestorm]

I thought a champ OT was supposed to be 5000@4ohm ?

Late 50s-early Fender Champ trannies seem to be around 10K. I'll bet there was a lot of variation, too. Entry level amp = entry level parts.

[bastardbus]

How about this testing OTs under load comment?

[Cliff Schecht]

Not necessary for testing the voltage/turns ratio of an OT, that's more for testing PT's..

[Phil_S]

How about this testing OTs under load comment?

By "under load" I mean, you've got to check the input voltage when the voltage source is attached to the transformer being tested.

This might seem obvious, almost silly, but it is wrong to assume that it is understood. Let's say you are using a 5V filament transformer. When you plug the source into the wall socket, maybe you get 5.7V. When you hook it up to the transformer being tested, it might drop to 5.2V. If you assume the "unloaded" 5.7V in your calculation, you will get the wrong answer.

It's not complicated, but making the assumption that 5.7V is your input voltage is all wrong. That's all I'm saying.

BTW, if you've got a variac and can set it to exactly 5V input, the math gets a lot easier. Remember, many guitar OT's have turns ratios of 32:1 up to 50:1. Input 5V to the secondary on a 50:1, and you get 250V out. That's why 10V input is potentially too high, as your OT might not be up to the task of taking 300V-500V. Don't intentionally put the OT in harm's way.

[bastardbus]

How about this testing OTs under load comment?

By "under load" I mean, you've got to check the input voltage when the voltage source is attached to the transformer being tested.

This might seem obvious, almost silly, but it is wrong to assume that it is understood. Let's say you are using a 5V filament transformer. When you plug the source into the wall socket, maybe you get 5.7V. When you hook it up to the transformer being tested, it might drop to 5.2V. If you assume the "unloaded" 5.7V in your calculation, you will get the wrong answer.

It's not complicated, but making the assumption that 5.7V is your input voltage is all wrong. That's all I'm saying.

BTW, if you've got a variac and can set it to exactly 5V input, the math gets a lot easier. Remember, many guitar OT's have turns ratios of 32:1 up to 50:1. Input 5V to the secondary on a 50:1, and you get 250V out. That's why 10V input is potentially too high, as your OT might not be up to the task of taking 300V-500V. Don't intentionally put the OT in harm's way.

Gotcha...understand now and yes that is how I was doing it.

I understand the point about the 10V into the secondary being too high and that is one of the reasons I did it the other way..100VAC into the primary I figured I would be safe doing that as I know the secondary would be a fraction of that. When I get some free time in the next couple days I am going to do some more testing with these different methods and tips given here and update you guys.

T