OT impedance test

[stingray_65]

I just picked up a Staco variable transformer off the junk pile at work.

I've been wanting to make a test jig for determining OT impedances.

The way I've used to check them was with a wall wart transformer (12VAC)

I want to use the variable transformer on the wall wart and dial down the voltage to 10VAC for easier math

Ocationally, the supplied voltage will drop when connected to the OT.

My question is should I use the unloaded or loaded voltage for figuring out my ratio?

[tubeswell]

A ratio is a ratio. So figure it out by reading the primary VAC with your 10VAC source on the secondary, then square the result of the VAC ratio to get the impedance ratio. In reality there will be some leakage on any transformer under load. But you don't need pinpoint accuracy IHMO.  That's just part of the mojo ain't it.

[stingray_65]

 
okey dokey!

[PRR]

> should I use the unloaded or loaded voltage for figuring out my ratio?

Loaded.

Technically there are several 5%-10% corrections both ways. And your "8 ohm speaker" is everything except 8.0 ohms. This is shotgun work, not sniper rifle accuracy.

> Ocationally, the supplied voltage will drop when connected to the OT.

Transformer 12.6VAC 300mA is 12.3/0.3= 4.1 ohms. Many OT secondaries are nominal 4 ohms. And that is at "audio", which for gitar amps may be "above 80 or 100Hz" (accepting "poor" impedance at lower freqs; more mojo). You are testing with 50/60Hz.

> Ocationally, the supplied voltage will drop when connected to the OT.

You want the source impedance much-much lower than the load impedance. The 12.6VAC 300mA can feed a 4.1 ohm impedance but it sags. 10%? On a cheap wart, maybe 20%! So internally it is 0.4 or 0.8 ohms impedance.

The Variac has other good uses. Get a "real" 10VAC source, not a lame wall-wart. Yet it needs to have some impedance, so a dead-short OT won't cause a cascade of smoke.

And note that 12V AC into an 8 ohm impedance is 18 Watts. If you are testing little OTs, they will be in overload, especially since they are likely to be soft below 100Hz or even 150Hz and you are testing with 50/60Hz.

Why doncha scale for 1V? Get a 6.3V 1A CT transformer. Use one side: 3.15V. Put three 1 ohm 2 watt resistors in series across that 3.15V. The voltage across one resistor is a third of the winding, a bit over 1V. The source impedance facing the OT will be about 0.66 ohms, "much-less" than 4 ohms. A big drop from 1V when you connect an OT tells you that 60Hz response is not perfect. On most non-lame gitar OTs, the voltage will be close enough to 1.0V to run your math without messy extra digits. If the OT is lame at 60Hz, everything is approximate anyway, you only want to know your 2K from your 8K, you can't say "this is a 6.78K OT" cuz its a cheap sloppy part (extra mojo).

[stingray_65]

I just might try that!

I've been gathering parts for the jig, biding posts for meter test points, fancy oak board to mount it all to and some Fahnstock clips to clip tranny leads in ( just love to say Fahnstock clips)

seems every time i end up with a bunch of clips I end up making a razor blade radio :)