Testing an Output Transformer

[Blindog]

After reading about different ways to check an OT I came up with these findings on a 100W OT.

With 1.6V AC connected to to the 8 Ohm tap, I get 17.5V AC on the primary (plate lead to CT)

This should work out to be about a 1750 ohm primary, correct?
If somone could verify that these numbers look right, and that I did the test correctly I would appreciate it

Mark

[Wayne]

I'm getting 3800 ohms P-P, or 950 per side.

Marshall JCM 800/900?

W

[Blindog]

Wayne, this is from a '83 Hiwatt OL103. I agree with your math (it was very late when I checked it) I was expecting to see a 1750 ohm primary like a Partridge OT. This looks more like a 50 watt impedance, no?

Mine has the Drake trannies that a lot of the Biacrown Hiwatts had, but I would not have expected a higher primary impedance, especially since a lot folks with Drake equiped Hiwatt's say they essentially sound like the older ones. It does make sense to use a JCM800 style OT in my amp (it has the extra gain stage) to keep up with the current competion at the time though. Any thoughts?

Mark

[Blindog]

double

[Blindog]

I did some more accurate readings.

1.7V AC 4 ohm tap, 14.4V P to P = 830 ohms

8 ohm tap, 35V P to P = 3395 ohms

16 ohm tap, 17.6 P to P = 1731 ohms

Using this method, shouldn't the primary impedance be the same on each tap? I'm just trying to make sure this OT is good before I put this thing back together. I'm using this formula: Voltage on primary squared times the tap impedance.

Mark

[Normster]

I typically apply a constant voltage to the primary side and then measure the secondaries to calculate the turns ratio. (Turns ratio squared = impedance, impedance times cab rating = reflected impedance.) Also, many British-style transformers are wound for 15 Ohm cabs, not 16, so the readings may be a bit off.

Anyway, try applying about 25VAC to the primaries and then measure secondaries. My guess is that you'll see approx. 1.2V on the 4 Ohm tap, 1.7V on the 8, and 2.3V on the 15/16 Ohm tap. Doing the calcs, that would be a 1,730 Ohm OPT. Of course, I've never seen a transformer that came out exactly right yet.

[Blindog]

OK, I think I've learned that the primary impedance changes with the output impedance. But my readings for the 8 ohm tap don't look right. The ratio should go down about half for each tap correct?

Mark

EDIT: Thanks Normster..you were replying while I was typing

I'll give that a try.

[Blindog]

double

[Blindog]

Anyway, try applying about 25VAC to the primaries and then measure secondaries. My guess is that you'll see approx. 1.2V on the 4 Ohm tap, 1.7V on the 8, and 2.3V on the 15/16 Ohm tap. Doing the calcs, that would be a 1,730 Ohm OPT. Of course, I've never seen a transformer that came out exactly right yet.

Normster, if I put 22V AC on the primary, I get 1.5V on the 4 Ohm, 1V on the 8 Ohm, and 2.1V on the 16 Ohm. Do these look right? The 8 ohm ratio still does not look right to me.

Mark

[Normster]

Yep, looks about right. I would guess that your 4 Ohm tap IS the 8 Ohm and vice versa.

At 22VAC:

2.1 = 1756 for the 16 Ohm tap
1.5 = 1721 for the 8 Ohm tap
1.0 = 1936 for the 4 Ohm tap

Pretty close, I'd say.

[Blindog]

double

[Blindog]

I would guess that your 4 Ohm tap IS the 8 Ohm and vice versa

That's exactly what I was thinking as well.....but the impedance selector is wired (looking from the rear, left to right) brown-green-yellow. Every pic I've seen is set up 4, 8, 16 left to right from rear. I know the yellow is the 16 tap from the negative feedback wire. The green wire is the one with the funky voltage ratio.

So it's brown 1.5V, green 1V, yellow 2.1.

I'm trying to rule out any OT issues before I reassemble this amp. The power board got scorched where a 470r 10W resistor from the screen supply overheated. The previous owner took it all apart and was going to part it out.

Mark[/quote]