OT Taps - Can they be identified?

[atmars]

I am converting an old amp into something usable for guitar.

Here is my issue . The speaker output jacks are not identified on the chassis of the amp. I have the schematic. I know one tap is 16 ohms the other 8 ohms. The schematic identifies the 16 ohm as the green wire and the 8 ohm as the grey wire. Here's is my problem: 50 years ago the the two wires were green and grey, now they are both faded to a similar yellow color. Is there a simple way to tell them apart?

thanks

[Enzo]

You COULD take the end bell off the transformer and look at the wires where they have been protected. SOmetimes you can scrape the insulation and expose color.

But mainly you can apply a steady signal to the primary, and measure AC voltage at each tap. The higher the voltage you find, the higher impedance the tap.

or you can take a steady voltage and aply it to the seondary taps and measure what comes out the primary. The lower the primary voltage, the higher impedance the tap.

[atmars]

Would checking impedance reveal anything? Would the 16 ohm tap have a higher impedance?

[black_labb]

if you know one is 8 and one is 16, then you could use a multimeter to test the resistance. the 16 will have the higher resistance as it will have twice the turns.

[Enzo]

You cannot measure the impedance of the transformer. You can only measure the resistance of its wires.

Yes, the higher impedance winding will have a higher wire resistance. But often the wiresd are so heavy they have little resistance and you find yourself trying to determine if .2 ohms and .3 ohms are accurate readings or not. If one tap is consistently higher resistance than another then it is likely higher impedance as well, as BL said.

[atmars]

Thanks, gents.

[Satamax]

http://www.radioremembered.org/outimp.htm

[Mars Amp Repair]

Well,
You can also just connect the transformer to the amp then drive it into a 4ohm ohm load then determining which tap gives you the highest power output...which would be the 4ohm tap...
next tap down in power (given the mismatch) would be the 8 ohm...etc. glen

[atmars]

You can also just connect the transformer to the amp then drive it into a 4ohm ohm load then determining which tap gives you the highest power output

Determine with the naked ear, or is there some procedure for measurement?

[tubeswell]

The best way is to disconnect all the OT leads and hook up the OT secondary leads in various combinations to a variac (you can use a model railway power supply that has something like a 12VAC to 16VAC output terminal) and then (knowing the VAC you are inputting into the secondary winding) measure the VAC across the primary in each case. This will give you a voltage ratio of primary to secondary in each case. The impedance ratio in each case will be the square of the respective voltage ratio. The secondary that results in the highest impedance ratio (e.g. 1000:1 and opposed to 825:1) will deliver the highest reflected load (in my example - 8k as opposed to 6k6 into an 8R speaker, or conversely, 4k as opposed to 3k3 into a 4R speaker) . (All this is on the site that Satamax posted). Be careful not to electrocute yourself when you take the measurements off the primary, as the voltage coming off the primary winding will be high (like 380VAC - 506VAC in my example - if you use a 12VAC to 16VAC source into the secondary).

[Mars Amp Repair]

Measure the output with a scope while driving the amp with a 1Khz signal. You can also use a DVM on the AC scale to get a comparison measurement if you don't have a scope. g

[Alex R]

Yes, run it into a dummy load at full output, and the wattage is the AC voltage across the load squared, divided by the resistance of the load. Leastways ar think it is.

[Morris Slutsky]

I've been known to just plug a 120V line into the primary, then measure the secondary voltages. Strongly suggest that people who do this use a half-amp fuse or similar gadget. And I generally measure the line voltage because it's never really 120. 120 hurts but not nearly as bad as 400, after all.