vintage transformers inductances

[rafafredd]

So, as I said on the other thread about recaping, I just got a inducatnce and capacitance meter, and I have been measuring inductance on some of my audio transformers.

The strange thing is that those old UTC stuff measures very high inductance compared to modern transformers.

While modern transformer has an inductance of 2.5H to 5H for 600ohms windings, the UTCs measures 15H to 20H for the 600ohms windings.

So, I think the UTCs would work better with weaker gear that delivers less current/power and maybe could be used as a higher impedance transfomer than 600ohms...

I just think it´s very interesting and I would like to here any comments about this. Why those vintage iron has always higher inductances for the same recommended impedances?

CJ? PRR? Anyone?

 

[NewYorkDave]

Are these "modern" transformers rated for full audio bandwidth, or are they for telephone service (300Hz-4kHz bandwidth)?

You can get away with a lower primary inductance if your source impedance is very low. In the old days, amplifier source impedances tended to be rather high compared to now.

...and they may have just made 'em better back then. UTCs were made in New York, after all.

 

[PRR]

> While modern transformer has an inductance of 2.5H to 5H for 600ohms windings, the UTCs measures 15H to 20H for the 600ohms windings. ... Why those vintage iron has always higher inductances for the same recommended impedances?

I bet this isn't true in all cases.

However there was a time when most outputs really were 600 ohms. These days, most outputs are really under 100 ohms.

Don't trust impedance bridges on iron core coils. The inductance will vary with signal level, sometimes a lot.

The older iron may be mu-metal, which has fairly high inductance at low flux but falls off badly at high flux.

I do not know what "modern" iron you are looking at, but if it uses the silicon steel cores often used for output and power transformers, it can have very low inductance at zero flux and much higher inductance at medium flux.

Put your signal generator, a 6K resistor, and the winding in series. Measure the voltage across the winding, at several signal levels like 10mV, 100mV, 1V, 10V. The voltage should be quite flat from about 500Hz to about 2KHz, but drop at lower frequency. The place it drops 3dB or 0.07 of the midband level is the frequency where the inductance equals the 6K test resistor. Simple math or reactance chart tells you the inductance. Some transformers will show very different -3dB points depending on drive level.

I suggest 6K for 600 ohm windings to shift the frequency up 10 times in frequency (I hate measuring below 20Hz). If you see a 350Hz drop with 6K, then with 600 ohm source it will be good to 35Hz, and even lower with modern low-impedance outputs.

 

[rafafredd]

Thanks PRR!

Yes, The vintage transfoprmers are mumetal and the modern ones are not. I didn´t know that inductance would vary that much with diferent levels, and yes, the meter I´m using put a really weak signal for testing. It´s mainly intended to measure inductors.

I´ll try this out.

 

[CJ]

Inductances started out lower in the really old UTC's. The black ones for example. As core science went along, improvements were made. Grain orientation for instance. As well as going from 49 to 80 percent nickel. The black UTC HA-100X has about half the inductance of the dark and light gray models. The lams are the same size and the lam count is about the same.
The balck ones are probably 49 Ni and later models 80. There might be advantages to having the lower inductance models in some situations, as the might tend to saturate at higher levels. It's all about a balancing act from one parameter to the next when it comes to transformers.

Inductance bridges will vary a lot. The Sencore I use has a different way of measuring inductance. It looks like it puts out a spike and then measures ring or some werid deal. The Gen Rad 1650-B uses a bridge circuit that you have to null manually. You can inject your own oscillator frequency into the Gen Rad which is nice as inductance varies with frequency. The Sencore only goes up to about 8 henries where as the Gen Rad goes up to 1100 henries. This is needed for high inductance secondaries on a 1:10 for example. The Sencore might give me a reading of 6 henries where as the Gen Rad might only measur three henries. This is because they do not use the same method to measure henries.
The Gen Rad lets you see the nulling action of the bridge via the meter on the front. This can very a lot from transformer to transformer. SOmetimes you can dial in a null so that if you barely move the bridge knob, the needle goes wild, whereas on other transformers, the null is very dull and sluggish.
I also have access to a HP inductance bridge down the street at Zambre Transformers. The Indian man who own's it is very nice and has done a lot of free work for me, like processing my V76 chokes, etc. Zambre did some stuff for Bruce Moore who put out the Paragon Model 12 Preamp, the Precision Fidelity preamp, the Audible Illusions preamp and MFA Luminescence products, so he is a nice guy to have down the street!
It is nice tohave the Q reading also, which the Sencore does not have.