Output Transformer load lead identification?

[olddawg]

Is there a simple way to ID the 4,8, and 16 ohm output leads on an output transformer in a working amp? I have an amp with the green OT secondary wire in use and was told that this was the 8 ohm tap, which is pretty standard I guess. The other leads are capped. Can I just measure the DC resistance to ground with a good digital meter? Would the highest or the lowest resistance be the 16 or 4 ohm tap? Could I input a calibrated 1Khz tone and measure the output wave form on a givin load with my scope. If I use an 8 ohm speaker load will the output appear higher or lower on the 4 or the 16 ohm tap? Is there a coventional color code for these taps in most transformers?

[Diablo]

I don't think there is a standard wire color code for output transformers. It's difficult to measure the resistance difference of the different taps - I've tried and it's very low resistance on all of them. I think the only way to determine what's what with certainty is to apply a small AC voltage to the secondary tap and measure the primary AC voltage to determine the winding ratio. Watch out for high voltage!

[octal]

Well, my lazy/cowboy method is the following. Apply a VERY LOW LEVEL constant input signal to the amp (like a sinewave) and measure the AC voltages on the various output taps with no load connected. I advise setting it up with the speaker connected and adjusting the volume so you can just barely hear the signal. Shut the amp off, leaving the volume at the same setting, disconnect the speaker (defeating the shorting jack if there is one) turn it back on and take your measurements. The 16 ohm tap will have the highest voltage and the 4 ohm tap will have the least. (I'll let you figure out where the 8ohm tap will fall!)

I would not advise this method if your amp is some high gain, unstable type which might burst into oscillation with no load applied.

You could also connect a high value resistor, like 100 ohms or so to the taps during this test to make things a bit safer.

Your method of measuring winding DC resistance might work, but the difference could be .1 ohm or so- very hard to measure accurately. The 16 ohm tap will have the highest resistance, four ohm tap the lowest.

Be safe & good luck.

Nathan

[olddawg]

Well, my lazy/cowboy method is the following. Apply a VERY LOW LEVEL constant input signal to the amp (like a sinewave) and measure the AC voltages on the various output taps with no load connected. I advise setting it up with the speaker connected and adjusting the volume so you can just barely hear the signal. Shut the amp off, leaving the volume at the same setting, disconnect the speaker (defeating the shorting jack if there is one) turn it back on and take your measurements. The 16 ohm tap will have the highest voltage and the 4 ohm tap will have the least. (I'll let you figure out where the 8ohm tap will fall!)

I would not advise this method if your amp is some high gain, unstable type which might burst into oscillation with no load applied.

You could also connect a high value resistor, like 100 ohms or so to the taps during this test to make things a bit safer.

Your method of measuring winding DC resistance might work, but the difference could be .1 ohm or so- very hard to measure accurately. The 16 ohm tap will have the highest resistance, four ohm tap the lowest.

Be safe & good luck.

Nathan

Thanks Nathan....I have a good sine wave generator so I will put a very low 1khz tone in the input and clip lead a 100 ohm resistor across the taps to ground one at a time and measure the output. I hope it works. Removing the transformer and figuring out the turns ratio by putting a tone into the secondary is a PIA.
Chears,
Dawg

[Steve Conner]

You don't need to remove the transformer to do that tone test. With the power off and the heaters cold, the tubes just look like open circuits, so you don't really even need to pull them.

In any case, the 16 tap should give twice the output voltage of the 4, and the 8 should give 1.4 (ie, square root of 2) times the voltage of the 4.

If you're backfeeding the secondary and measuring the primary, this is of course reversed.

[olddawg]

You don't need to remove the transformer to do that tone test. With the power off and the heaters cold, the tubes just look like open circuits, so you don't really even need to pull them.

In any case, the 16 tap should give twice the output voltage of the 4, and the 8 should give 1.4 (ie, square root of 2) times the voltage of the 4.

If you're backfeeding the secondary and measuring the primary, this is of course reversed.

Thanks Steve,
I'm still not used to thinking in tube mode. The other day I used a speaker in an old Ampeg combo I have to check out another amp I built. The Ampeg had two jacks mounted in a wood panel on the back paralleled to the OT. I hadn't had my coffee yet and just unplugged the combo speaker from one of those jacks, plugged the other amp in, and started playing. At first I thought I had something wrong with my build or a blown speaker but quickly realized that I was back feeding the OT in the Ampeg. Since the Ampeg was unplugged, I guess I didn't damage the old 6V6s in the Ampeg for the same reason you stated above. But there must of been a hell of a large signal on the primary of that OT. Oh well.

[martin manning]

You don't need to remove the transformer to do that tone test. With the power off and the heaters cold, the tubes just look like open circuits, so you don't really even need to pull them.

In any case, the 16 tap should give twice the output voltage of the 4, and the 8 should give 1.4 (ie, square root of 2) times the voltage of the 4.

If you're backfeeding the secondary and measuring the primary, this is of course reversed.

A negative feedback loop may be connected to one of the taps and provide another path to ground... maybe the voltage ratios would still be close, though(?).

MPM

[Enzo]

The NFB is a high resistance, and shouldn't cloud the issue.

I would just stuff a signal into the primary - as Steve said, tubes and all when the amp is cold - and read the secondary voltages. The output voltages would be in order 4-8-16 with increasing voltage. If you don;t know which is the common lead, then use those ratios to determine that.