Using 70 volt audio transformers for impedance matching tube radios.

Produced with CyberLink PowerDirector 15

[@Scorpio722]

Great quick review of audio line transformers and the Tenma 72-6948 Audio Impedance Test.

Hey, good work

[@kb8urr]

Great video, very interesting to watch. I was the Product Manager for MCM Electronics (now Newark Element14) that originally added that product to the Tenma Product line. With a great deal of customers in the distributed audio installation business, this product is very popular for installing 70V systems, as it allows the installer to do a final impedance check on the speaker network prior to connecting the amplifier. Great to see this bench application as well.

[@theradiomechanic9625]

Wanted one of these for years, finally decided I was old enough to buy one. :-)

[@W1RMD]

Thanks for explaining about audio transformer matching. Valuable information! The only thing I don't understand is why hasn't anyone done a video using this as an audio tube amp output matching transformer to a speaker. 6V6, Etc.

[@medhawk4885]

Great video with some VERY good information. Thanks again

[@theradiomechanic9625]

Hi MedHawk. Thank you, and thanks for stopping by.

[@quantumleap359]

A load of great info here. Thanks for such a helpful video.

[@theradiomechanic9625]

Your Welcome and thanks for watching.

[@YouAllKnowBob]

Very informative! Thanks.

[@theradiomechanic9625]

Hi Robert, Thanks for watching.

[@wa6gxq]

Thanks!!!

[@theradiomechanic9625]

Your Welcome

[@richardgoebel226]

Very good tutorial. Tenma might be using a square wave instead of a sine wave to inject the odd harmonics as well as part of the tests.

[@theradiomechanic9625]

Good thought. Didn't consider that, but this is a pretty cheap instrument.

[@misterhat5823]

The other meter is measuring the DC resistance and time constant. That way it knows the DCR and inductance. That's why it measures the speaker fine, but fails on the transformer.

[@theradiomechanic9625]

I learn something new every day....thanks..

[@allthegearnoidea6752]

I imported a Genrad impedance bridge from the USA. It is very good but balancing the bridge is some kind of black magic. I like the Tenma units but they are expensive here.

[@mikesradiorepair]

I worked on a radio some time ago where someone used a transformer like this and they used the incorrect tap. Customer complained about low volume. Switched to the correct tap and it worked fine.

[@theradiomechanic9625]

Seems to be a fairly common use for these. Cheap and available.

[@clifffiftytwo]

I wonder what the output impedance of the little Chinese meter is - it should be able to read a resistance through the matching transformer but may not be able to excite the transformer completely. The TENMA puts out a robust signal since it is intended to measure such things. The Chinese meter is designed to test components and semiconductors so it's output signal may not be up to the job but it might work on one of those small Radio Shack matching transformers. What an interesting video - thanks!

[@theradiomechanic9625]

Hi Cliff, The issue is we need to measure impedance, not resistance. The Chinese meter is not designed to measure the actual impedance. Thanks for watching.

[@mattmahoney9815]

I purchased the Tenma 72-6948 meter to measure the actual impedance of the speakers in my car, so I could match a crossover to them. I tried the meter on a couple other speakers first, just to try it out. One speaker I know is 4 ohms, and the meter read 10 ohms. Tried it on another speaker/woofer that is 6 ohms, and it read close to 16 ohms. I calibrated the unit as outlined in the instructions prior to both measurements. Not sure if I am doing something wrong, or misunderstanding what the unit is intended for?? If anyone can offer some insight, I would appreciate it! :)

[@klcbsoft]

Some good info there! Something like the TENMA might come in handy in identifying some old mysterious mic-preamp-transformers here. And yes: as great as the little Chinese tester is, it has it's problems with chokes/inductive elements/ transformers etc. A question: the test signal of the Chinese tester you showed on the scope ... which ports did you use and does the tester fire out a "generalized lead in" and divert into special patterns after that, or is it always the same pattern, regardless whether or not the ports are attached to a component or open? Thanks for sharing this useful infos!

[@theradiomechanic9625]

I haven't tried it with out anything attached, but I suspect that it is always the same routine. The port is not supposed to matter either. I have tested other components in various lead combinations and the result is always the same. Cleaver piece of software.

[@mannyknowles]

Thanks for this video. I have the same tester (Tenma) but I always get very different results from what I'm expecting. For example 6 speakers tapped at 7.5W each. I'm expecting 5000/45 = 111Ohms, but I'm getting a reading that's over 200. Any idea why that might be? Could the wire itself be a factor?

[@Nirky]

"...will give you accurate readings on speakers." Reading of DC resistance yes, because that's what they are designed to measure...AC impedance, NO. Resistance and impedance are two entirely different things.

[@ebones6957]

I have a Dukane 1A753 solid state 50Watt output, 70.7 v public address amplifier. The amplifier has a transformer output (on chassis). On the schematic is states : 70.7v , 50watts, 100 ohm load. This makes sense: 70.7 squared / 50 W =100. I would like to drive a modulation transformer with this. Do I need a Xfmr with 100 ohm primary and a 8000 ohm sec. ( modulate (2) 807 ‘s , AB 1? Is my calculation correct?

[@flurng]

An excellent and informative video, but I'm still a bit confused; certainly, I understand HOW a transformer can be used for impedance matching, I'm just curious as to WHY one would choose to do so - after all, isn't it's primary function that of transforming voltages and currents, hence the name? Also, wouldn't the transformation itself interfere with the circuit's operation? For instance, suppose I've a 1K ohm output, and I wish to drive an 8 ohm speaker. Wouldn't a transformer change my output voltage and/or current by the act of matching the impedance? Couldn't I instead approximate a match, simply by inserting a 1k ohm resistor in series with the speaker?

[@theradiomechanic9625]

First, your confusing resistance with impedance. An 8 ohm speaker does not measure ( usually ) 8 ohms DC. They can be anything from less than an ohm to 20 or more ohms. They need to be measured by special equipment at 1 KHz. The typical primary on lets say a tube radio will carry about 250 volts with an impedance of 5 thousand ohms. Trying to use a resistor will cause all the voltage to drop across the resistor and create large losses and a lot of heat. Transformers "transform" the impedance down to 8 ohms with nearly 99% efficiency to drive the speaker with minimal loss. To fully explain this would take an entire book. I suggest picking up a few books on transformer design. Sorry I could not be more help here.

[@mohammadmoezi3813]

Hi sir thankyou for make this video that was very useful for me..sir I have a question how can I calculate. Zoutpout((I mean cathod to anod impedance)))) on 12au7 vacuum tube..please answer my question..and thankyou for make this beautiful video

[@theradiomechanic9625]

Hello Mohammad, thanks for watching. Go online and look for 12AU7 tube datasheet. It will tell you load impedance for various conditions such as class A or class B and voltages. I also suggest going to ebay and getting some RCA tube manuals. I have several from different years to cover the various changes in tube design. I am presently in Thailand and do not have access to them.

[@theradiomechanic9625]

It may also be listed as plate resistance.

[@mohammadmoezi3813]

@theradiomechanic9625 sir thankyou for lead me..

[@jimcatanzaro7808]

These 70v trans don’t work