OPT Impedance calculation, please check.

[paggerman]

Hi all. I know that this is covered in other posts, but this has left me scratching my head. I have picked up an old (date stamp June 1947) SE OPT. There are no impedances marked, so I set up a rig using a 6V rated mains transformer. When I hook up the OPT primary to the the mains secondary, my DMM reads 7.83VAC on the primary and 0.092VAC on the OPT secondary. Using the maths described elsewhere, this gives me a Vp:Vs ratio of 85.1.

85.1^2 = 7243.5

this means that for an 8ohm impedance speaker, the primary impedance would be near enough 58k.

This seems to be very high. I have briefly tried this OPT with a 6BW6 output and it worked. Is my methodology correct, and is such an impedance ratio feasible? What sort of application would an OPT of such a ratio have?

Many thanks as always.

[R.G.]

When I hook up the OPT primary to the the mains secondary, my DMM reads 7.83VAC on the primary and 0.092VAC on the OPT secondary. Using the maths described elsewhere, this gives me a Vp:Vs ratio of 85.1.
...
This seems to be very high. I have briefly tried this OPT with a 6BW6 output and it worked. Is my methodology correct,

Pretty much. It allows the possibility of quite a bit of measurement error, though. It's best to stay out of the millivolt AC ranges with most DMMs that are not of high quality.

I would be tempted to try putting 3.15Vac (half of a 6.3Vct heater winding) onto the secondary, then measure the primary. This gets the AC voltage up out of the fractional-volt region and into a more common range for most DMMs. It's easy to get small errors when measuring millivolts. This will put (if your first measurement was right) 268Vac on the primary, so DO NOT DO THIS UNLESS YOU ALREADY KNOW HOW TO DO IT SAFELY.

That's likely to be nearer the working range of the transformer anyway.

and is such an impedance ratio feasible?

It is. It's done quite often.

What sort of application would an OPT of such a ratio have?

... in the reverb driver transformer of Fender amps with reverb. The output impedance of a 12AX7 is about 67K. And it could have been intended for a lower impedance speaker; some of them, even way back when, were 3.2 ohms.

A pertinent up front question is - how do you know it was an SE output transformer, or for audio?

[paggerman]

Thanks RG. How do I know it's an SE OPT? It looks like one! It has the red & blue plastic coated multistranded primary leads and the solid wire with black and (what was once) green cloth covered secondary leads. I have seen other old OPTs with these features and thus deduced this would be an OPT. And it worked when I tried it very briefly. Another clue is that it is in a can with what would appear to be a curved mounting bracket - perhaps to mount on a speaker?

[Wilder Amplification]

Me personally I usually hook the primary side up to a 120VAC source (they see LOTS more voltage than what comes out of the wall so this will not hurt anything), then measure the output at the secondary.

[paggerman]

Me personally I usually hook the primary side up to a 120VAC source (they see LOTS more voltage than what comes out of the wall so this will not hurt anything), then measure the output at the secondary.

We normally get 230VAC out ot the wall around my way, which gives a peak of 325V - not dissimilar to the DC HT that runs through a primary. I'll give it a go and post the results here. Thank you.

[J M Fahey]

Just in case put *something* in series with those 230V, preferably a series lamp limiter but at least, say, a 470 ohm , 1/2 or 1/4W resistor, which will work as a sort of fuse, vaporizing on any short.
And let me unnecessarily repeat that you will be directly working with 230V MAINS VOLTAGE.
yea, I know you already know, just take care, OK?.