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OT impedances calculations: where's my mistake??

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I've got several OT's from all sorts of radios, vinyl players etc.
I want to see what their impedance ratios are - and seems like I have some mistake in my calculations that gives me wrong results every time. I know I'm an idiot but I still want to know.

Here's what I'm doing - where the mistake is?

1. I feed the primary of the SE OT with 40VAC from a variac.
2. I measure the secondary voltage.
3. I'm not sure already, but since impedance ratio = voltage ratio squared, I calculate it that way.

For example:
a SE OT from a reel to reel. The power tube was ECL82, speaker - 8 ohm, power transformer has 235VAC secondary (around 250V B+). ECL 82's datasheet says 8K load for 3.5W output.

So my measuremenst go like this: 40VAC on the primary, measured 0.6VAC on the secondary.

Z = (40/0.6)squared = 66.6666squared = 4444.44

With 8 ohm speaker it must be 4444 X 8 = 35.5K primary.

Makes sence? NO!!!

Another SE OT, from a radio which had 7C5 tube. must be 5K.

40VAC primary - 0.5VAC secondary.
Z=6400. With the 8ohm speaker it should have been 51.2K primary.

etc. etc.

:bawling: :bawling: :bawling:
Joined 2004
Paid Member
Something wrong with those voltage measurements, and or some of your assumptions. I'd make sure your meter is working correctly on that low voltage range..

A 10K to 8 ohm transformer has an impedance transformation ratio of 1250 and a turns ratio of 35.35 as an example.

You might be wrongly assuming an 8 ohm tap when in fact it is a 4 or even a 3.2 ohm tap - the results you get indicate that that might well be the case, but your secondary voltage meter readings are highly suspect. In the example cited above the voltage on the output winding would be 1.13Vrm..

Some old EL95 Telefunken transformers I have have 3.2 ohm secondaries...
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