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.

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.


