Conn Organ K-58459 Ouput Transformer Questions

[Charlie Wilson]

Hello, I purchased a couple old Conn Organ transformers K-58459. I know that some of the guys have liked them in Tweed Deluxe circuits but I am a bit confused trying to spec these things. I compared them to a stock MC Tweed Deluxe transformer and there are similarities and differences. First of all I measured the resistance across the primaries and got 327.9 ohms for the Conn and 662 ohms for the MC. I then put 4 volts 400 Hz across the primaries and got .127v across the 8 ohm secondary of the Conn and .126v across the MC. Which after the math makes the Conn have 7936 ohm primary impedance and the MC have 8062 ohm primary impedance. So far so good. Now going back to the turns ratio which for the Conn is 31.496 and MC 31.74. Which means 31.496 to 1 right? I put the 4 volts on the secondary expecting to see 31.496 x 4 volts across the primary of the Conn and instead get 13.78 v. On the MC I get 31.74 volts. So how can both these transformers have similar primary impedance and way different voltages on the secondarys? I would like to add that the stock MC transformer has more core material than the Conn. I notice that the voltage on the Conn primary goes up pretty quickly as I go up in frequency on the signal generator.
CW

[Ten Over]

The impedance of the K-58459 secondary at 400Hz is so low that I could only get 1.35V out of my function generator giving me 43V on the primary. At 1300Hz I could get 4V on the secondary and I got 130V on the primary. Most likely you weren't actually putting 4V on the secondary at 400Hz.

[sluckey]

The impedance of the K-58459 secondary at 400Hz is so low that I could only get 1.35V out of my function generator giving me 43V on the primary. At 1300Hz I could get 4V on the secondary and I got 130V on the primary. Most likely you weren't actually putting 4V on the secondary at 400Hz.

That's probably the case with any FG that has a 600Ω or 50Ω output. I don't recall ever seeing a sig gen or FG with a 4Ω or 8Ω output. Nevertheless, 43/1.35=31.85 and 130/4=32.5. Well within human or meter error tolerance. I'd say they are the same.

[Charlie Wilson]

So Ten Over, are you saying that even though I measure 4v at 400Hz across the leads of the signal generator unloaded, that when I put them across the secondary that voltage drops because of the low impedance of the secondary winding? I have suspected that signal generator is probably not the best way to spec transformers. I have been advised to use a variac at a low voltage.
CW

[sluckey]

I like to use a variac and filament transformer connected to the secondary. Very easy to set the voltage to exactly 1V. Makes the math easy.

[martin manning]

That's a good idea, sluckey, that isolates the supply too!

CW, repeat that test again and measure the voltage across the signal generator when it's connected to the OT. It should work fine if you get the loaded voltage for the calc.

[Ten Over]

So Ten Over, are you saying that even though I measure 4v at 400Hz across the leads of the signal generator unloaded, that when I put them across the secondary that voltage drops because of the low impedance of the secondary winding?

Yes.

[Charlie Wilson]

You guys are right. I put the signal generator across the secondary and then measured the voltage, and yes I had to almost max the thing at 400 Hz to get 1 volt. So I get about 32 volts. That makes it pretty much an 8K primary at 8 ohms.
CW

[Ronno25]

I got my hands on a few of these transformers and am trying to figure out what secondary lead is what impedance. There is a green and a yellow. Does anyone know? TIA! Charlie Wilson mentions it's an 8k at 8ohm. But I'm not sure which lead he is speaking about.

[Stevem]

Figuring out the secondary taps can 1) be done by resistance testing and since the resistances are so low I like to use a cap tester that can read out the ESR. Or 2) just do it empirically.

Hook the transformer up to your amp and drive the amp with low test signal , like .035 volts, then measure the voltage on the secondary side between its black lead and the others.
Your highest voltage will be the highest impedance winding with lower voltages seen in progression as the other lower impeadences.

[Phil_S]

Read this for unknown output transformer impedance. http://www.radioremembered.org/outimp.htm or http://www.sarris.info/main/calculating ... -impedance

[Ronno25]

Read this for unknown output transformer impedance. http://www.radioremembered.org/outimp.htm or http://www.sarris.info/main/calculating ... -impedance

This was super helpful, thank you! The Sarris.info method was really easy and quick. Did it with a function generator. My results: Green turns ratio 31/1. Yellow Turns ratio 42.5/1. Green: 7.688K at 8ohm. Yellow 7.224K at 4ohm.