# Thread: Mystery OT and impedance help

1. ## Mystery OT and impedance help

Hi, I just spent 3 hours surfing the net and found no complete answers; HELP!!!
I have an output transformer from an old PA amp and need to ID the leads. This is a PP tranny with CT. I found a number of articles about using a test power source and measuring voltages to determine the turns ratio, but no one speaks to the PP CT tranny. I don't know if I should just bypass the CT? Or read using the CT and only 1 end of the primary??? Also I read where a 6L6 tube wants to see 4k to 5k impedance, again is this on half of the OT??? Also I was thinking of using 4 6V6's for power tubes, so how do I calculate the impedance of 1 pair in PP with a second pair??? Every thing I've read so far has confused the issue. Any help would be greatly appreciated

Thanks

D

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2. Originally Posted by DaBreeze
Hi, I just spent 3 hours surfing the net and found no complete answers; HELP!!!
I have an output transformer from an old PA amp and need to ID the leads. This is a PP tranny with CT. I found a number of articles about using a test power source and measuring voltages to determine the turns ratio, but no one speaks to the PP CT tranny. I don't know if I should just bypass the CT? Or read using the CT and only 1 end of the primary???
You place an AC voltage across the secondary and measure the voltage on the primary from end to end, omitting the center tap. Be sure and measure your input voltage while testing because the tranformer inductance and resistance will load down your input signal somewhat. So, if you have, say, 1 VAC measured at the secondary from your source and then measure 29 volts on the primary, divide the voltage on the primary by what you are putting into the secondary. That would be a 29:1 ratio. You can then square the ratio (29 X 29 = 841) and then multiply that number by common impedance loads to learn the primary impedance that OT will reflect on the tubes (841 X 8 = 6728) (841 X 4 = 3364). You can reason from here that since 6600 is a common primary impedance for a pair of 6L6/5881 or 6V6 tubes that the OT was intended for a pair of 6L6's into an 8 ohm load. Or, if the OT seems especially large, it could be for a four 6L6 amp at 4 ohms. If the OT seems too puny for two big bottles it may be for a pair of 6V6's at 8 ohms. With a mystery OT some guess work may be in order. That isn't to say you couldn't use the above tranny with a pair of 6L6's into an 8 ohm load if it's especially large, you could, but if the OT seems too small for an 80 to 100 watt amps you probably wouldn't want to use it for 4 6L6's at 4 ohms... Get it???

Originally Posted by DaBreeze
Also I read where a 6L6 tube wants to see 4k to 5k impedance, again is this on half of the OT???
No, this is from one end of the secondary to the other, omit the center tap (it should be disconnected) for this measurement.

And, FWIW, that 4k to 5k primary is more typical in post 1962 or so guitar amps than any other tube gear. And early guitar amps often used values more typical of other gear. 6600 is likely the most common primary impedance for a pair of 6L6's. But Fender and others found that they could get more power (at elevated harmonic distortion content) by using the lower 4k ish primary impedance. It's a slightly different feel and sound between the two. But just think tweed era vs. BF era Fender and you'll get SOME of the idea. Of course there were many differences between the preamps in either era too.

Originally Posted by DaBreeze
Also I was thinking of using 4 6V6's for power tubes, so how do I calculate the impedance of 1 pair in PP with a second pair??? Every thing I've read so far has confused the issue. Any help would be greatly appreciated
When running 4 tubes vs. 2 you just cut the primary impedance in half. Some guys will get ruffled and start in with the tube charts and all, but that's generally how it's handled. So your , roughly, 6600 ohm primary impedance for a pair of 6L6's could be run at 4 ohms to "reflect" a 3300 ohm impedance on four tubes. So, now...

6V6's like pretty much what 6L6's do. Except that 6L6's handle lower primary impedances with more aplomb. When guitar amp companies started running 6L6's at 4k primaries they seemed to keep higher numbers for the 6V6's. But for most amps up to, again 1962 or so, 6L6's and 6V6's both used about 6600 primary for a pair. So, if the above "example" tranny were determined to be good for a pair of 6L6's at 8 ohms for a 6600 primary, there is no reason the same tranny couldn't be used for 4 6V6's into a 4 ohm load with a 3300 primary. Get it??? It's all about the turns ratio and the capacity of the transformer.

HTH

Chuck

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3. And while we are at it, do you happen to know WHAT old PA amp it came out of? If we knew that, or even just what tubes it was associated with in the amp, then we'd have a pretty good idea what the transformer was all about.

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4. Thanks for all the help and so fast! Actually the tranny is still in the amp, but its missing the name plate. The leads are cloth insulation, so there are no colors to help. At one time it had an octal socket for outputs, but that has been butchered and a phone jack added to 1 lead It appears there are 2 ground leads and 3 taps. As opposed to applying voltage to the secondary, couldn't I just apply voltage to the primary with a wall wart and measure each tap?? Thanks again
D

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5. Originally Posted by DaBreeze
As opposed to applying voltage to the secondary, couldn't I just apply voltage to the primary with a wall wart and measure each tap?? Thanks again
D
If you do that you probably won't get any/accurate enough VAC measurement on the secondary.

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6. Consider that the primary normally converts like 100-200v of signal down to 10-20, so your wall wart will step down to a very low figure that way.

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7. Originally Posted by tubeswell
If you do that you probably won't get any/accurate enough VAC measurement on the secondary.
Are you serious? I've done this for years with no ill effects (I thought) and I've been able to verify my results with a signal gen at various frequencies- seems accurate to me. I just do that math to calculate the reflected impedances.

One forumite mentioned keeping a wal wart with alligator clips soldered on for just such a purpose!

I have pages and pages of data from all of my recycled/spare transformers. If all that data is bunk I think I'd have to give up on building amps altogether! Wanna buy some inaccurately cataloged parts?

Sorry to hijack!

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8. In my defence I was considering a typical 9V to 15V supply, which at say 29:1 would make the secondary read .31V to .51V and I wasn't thinking about the millivolt setting when I replied before. My bad

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9. Originally Posted by tubeswell
In my defence I was considering a typical 9V to 15V supply, which at say 29:1 would make the secondary read .31V to .51V and I wasn't thinking about the millivolt setting when I replied before. My bad
I didn't mean to offend- I just felt the ground shaking underneath me!

onward with the thread!

jamie

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10. Having said that there still might be an issue relying on a millivolt reading on a secondary winding for accuracy when leakage inductance (which causes the voltage to change with loading) is taken into account. I've never tried the walwart on the primary myself. I have always used it on the secondary, so I've never verified its accuracy. I must pull out a few Ots this w/e and see.

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11. OOps, too late to edit too...

I said:

Originally Posted by Chuck H
No, this is from one end of the secondary to the other, omit the center tap (it should be disconnected) for this measurement.
That wouldn't be the secondary, I meant primary. Just thoought I'd point it out before someone else did. Or DaBreeze followed this info with no usable results.

Chuck

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