Does this make sense?

[jutsin]

I just got a new OT for my 5150. I ordered a 3K primary, but when I check it with 1VAC across the secondary on each tap, I've come up with about 5.3k primary. I asked the manufacturer if this was correct, and he said to do the following:
"Primary is Brown - Red - Blue
Secondary:

Common Black
4 ohm Yellow
8 ohm Green
16 Ohm Orange

If you take primary impedance of 3000 X 120 watts = 600 volts
4 ohm X 120 watts = 21.9 volts 16 ohms = 43.8 volts and 8 ohms = 31.0 volts

put 120 volts across red & brown or red & blue
Secondary should be
common to yellow 8.7 volts
common to green 12.4 volts
common to orange 17.6 volts

If these voltages check out it is correct"

Does this make sense? It's just another way to find the primary? Thanks!

[R.G.]

I just got a new OT for my 5150. I ordered a 3K primary, but when I check it with 1VAC across the secondary on each tap, I've come up with about 5.3k primary. I asked the manufacturer if this was correct, and he said to do the following:
"Primary is Brown - Red - Blue
Secondary:

Common Black
4 ohm Yellow
8 ohm Green
16 Ohm Orange

If you take primary impedance of 3000 X 120 watts = 600 volts
4 ohm X 120 watts = 21.9 volts 16 ohms = 43.8 volts and 8 ohms = 31.0 volts

put 120 volts across red & brown or red & blue
Secondary should be
common to yellow 8.7 volts
common to green 12.4 volts
common to orange 17.6 volts

If these voltages check out it is correct"

Does this make sense? It's just another way to find the primary? Thanks!

The impedance ratio of a transformer is the square of the voltage ratio.

From what they say, with 120Vac on red and (blue or brown), you get ratios of 120/8.7 = 13.793, 120/12.4 = 9.6774, and 120/17.6 = 6.818. The squares are 190, 93.65, and 46.5. That's plate loads of 190*4 =760, 12.4*8 = 749.2, and 46.48*16 = 743.8. The plate-to-plate load is four times that, or
3040, 2996.8, and 2975. That's if their numbers are correct, so their advice is consistent.

Checking it backwards, with 1Vac across the common to 4 ohm tap, the red-to-(brown or blue voltages would be: 13.79, 9.7, and 6.82. The blue-to-brown voltages would be twice that.

[jutsin]

Alright, so I did what he said and put 120VAC across the red and blue primary side. Here are the voltages I got:
Yellow=8.3
Green=12.0
Orange=7.017

So, if I find the ratios by dividing 120 by those, I get:

Yellow= 14.457 squared=204.504 times 4 (ohms tap)= 818.016
Green=10 squared=100 times 8 (ohms tap)= 800
Orange= 17.1 squared=49.238 times 16 (ohms tap)= 787.812

If I multiply each of those by four for the plate to plate resistance I get 3272, 3200 and 3151. Closer to the 3K primary that I wanted.

BUT- if this all seems correct, then why is it that you are supposed to measure across the entire primary (brown to blue) to find impedance when using the 1VAC on the secondary method? Using that method I got around 5.3K primary.

Thank you very much for the help!

[MWJB]

"If you take primary impedance of 3000 X 120 watts = 600 volts
4 ohm X 120 watts = 21.9 volts 16 ohms = 43.8 volts and 8 ohms = 31.0 volts"

1 VAC on the secondary (with a 3K primary) should give 27VAC (3000/4 = 750, 27 is the sq. root of 750) accross the primary on the 4 ohm tap, 19VAC on the 8 ohm tap, 13.69VAC on the 16ohm tap...basically what RG has already said. Every time you switch secondary taps you must recalibrate the voltage applied, use 2 meters one on the primary, one on the secondary. Applying the voltage at the secondary works fine, I do it all the time, it's much more practical when checking out complete amps. In short, you did something wrong.

[jutsin]

Thanks for the reply. I'll double check all of my work again. My question though still remains on the 1VAC on the secondary method. When you do this, are you measuring across the whole primary (blue to brown) or are measuring from brown/blue to red?



http://music-electronics-forum.com/t16022/
"You place an AC voltage across the secondary and measure the voltage on the primary from end to end, omitting the center tap. "

[R.G.]

My question though still remains on the 1VAC on the secondary method. When you do this, are you measuring across the whole primary (blue to brown) or are measuring from brown/blue to red?

Here's the real skinny.

The impedance transformation ratio is the square of the voltage ratio.

1Vac across any secondary gives X volts across whatever winding you are measuring, and that gives a voltage ratio of X because you used 1Vac (ideally 1.000Vac!) to measure it, and X/1 = X.

The impedance ratio is then X squared.

You can measure X across either a half-primary (red to either brown or blue) or across the whole primary. If you measure it across red to either other one, then you get a number. Converting this to impedance gives you X squared, which is the ratio from a secondary to a half-primary.

The ratio across the *whole* primary is four times this, because the voltage across the whole primary is twice the voltage across a half-primary, and then the plate-to-plate impedance across the whole primary is four times the half-primary impedance.

A nominally 3000 ohm plate-to-plate transformer (to a given secondary impedance) has a nominal 750 ohm half-primary impedance. Same transformer.

The more accurately you can measure the 1.000Vac input, the more accurate your calculations will be, obviously. It only needs to be 1Vac to make the calculation easier. Any AC voltage that will not saturate the secondaries (while they're used as primaries!) will work, the numbers are just not as easy to calculate.

[jutsin]

Okay, I was out of town over the weekend for work, but I've done some more measurements on the OT. I fed 1VAC into the secondary on each tap and measured what came out on the primary. If you don't mind using these numbers and telling me what you get so that I can compare and see if I got this right or not that would be great. I'll give you all the measurements I took.
I measured both half primary (red to blue or brown) and full (brown to blue).
For the half primary these are the measurements that I got:
From Black/Orange: 9.0 VAC
Black/Green: 12.9
Black/Yellow: 18.6

Then measuring the full primary (brown to blue) the numbers were:
Black/Orange: 18.5 VAC
Black/Green: 26.3
Black/Yellow: 37.6

I made sure every time that I was sending in 1.0VAC. Any help is appreciated!