Stupid question - how to measure OPT?

[DerekVa]

I want to figure out what the primary resistance value is on an output transformer (non-Dyna). How do I measure it? I've tried with a DMM but haven't seen anything that comes close to 8k on a Z565, so I'm thinking I'm doing it all wrong...

Thanks!

-Derek

[EWBrown]

One method that I use is to connect the OPT primary to low voltage AC (use a filament or other AC "wall wart" transformer, the voltage doesn't really matter, as long as it's 24VAC or less. The important part is to never connect directly to the AC power mains.

Measure the AC voltage across the OPT's primary, and then across the secondary (8 ohms), and write down the readings.

The primary impedance is (voltage across primary / voltage across secondary)^2 X the secondary impedance.

Example, 16.00 VAC across the primary, and 0.506 VAC across the secondary. (I use a variac and 16VAC transformer and can fine-tune the input voltage)

16.00 / 0.506 = 31.62

The ratio works out to 31.62 ^2 or 1,000:1 therefore the secondary impedance would be 8,000 Ohms.

Important Caveat:

Never connect the low voltage AC across the OPT's secondary, as the primary will then have very high voltage across it, and this will probably burn out the trannie, as well as create a shock hazard.

/ed B in NH

[DerekVa]

So there isn't a method that doesn't involve threats to life, limb or property? No wonder tubes are cool!

[erichayes]

Hi All,

I've compared results obtained using Ed's method with those measured directly on my General Radio 1650A impedance bridge and they are definitely close enough for DIY work. You'll usually see up to a ±20% deviation from the specified impedance and still have a 'good' transformer. My '565s measure 7.6K.

A couple of things to keep in mind. First, Ed was using canned numbers for demonstration purposes. In real life, stuff like that just doesn't happen.

Second, audio transformers are almost always spec.'d at 1 Kc. Most higher quality transformers will test satisfactorily with Ed's method, which uses 60~, but cheapo strap mounted OTs are going to have a higher impedance at the lower frequency; a fudge factor may need to be applied to get the corret impedance at 1Kc.