design

Identifying Output Transformers

A few years ago I stumbled upon a vacuum tube collector in London Ontario. For whatever reason, he was trying to offload some of his parts, so I quickly called him up and hauled off a box of parts. In this box was the following:

  • 2 Matched JJ EL34 power tubes
  • 10 various 12AX7 pre-amp tubes
  • 13 7-pin and octal tube sockets
  • Power Transformer from colour TV
  • Output Transformer from (probably the same) colour TV

Naturally, these parts are a great beginning to a guitar amp. The first step to a junk amp build is identifying these transformers. The power transformer is simple enough to identify – I bravely assumed the two longest black leads were the 120 VAC input, and then measured the other 7 leads in colour-coded pairs. This gave me a 570V secondary with a center tap, a 5V rectifier output and a 6.3V filament output. Easy peasy.

Now, this output transformer was a bit trickier. This beast weighs a good 10 pounds, and has 9 leads coming out of it.

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Being an Ultralinear  OPT, it has additional taps on the primary for your output tube screens. Sweet. But it also has 4 leads on the secondary, and I have no idea what impedance each one corresponds to. Some transformers have 2,4, and 8 ohm outputs, while others have 4,8, and 16 ohm outputs. I don’t want to just blindly plug in a speaker, since if I got the impedance wrong, I could have a smoldering pile of components.

OPT

The only way to unravel the mystery of this is with the magic of math.

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The first step is to calculate the winding ratio for each of the secondary pairs. I have an old BWD signal generator that I connected to the primary Blue and Brown leads. My signal generator has a maximum output of 2.5V (2.39V measured) but this will do. With this signal going into the primary, I can now read the output with the VAC setting on my DMM. Then I can use the following equations!

Winding Ratio (wr) = V1 ÷ V2

Impedance Ratio (IR1) = wr² x IR2

Output transformers have an effective impedance dependent on which tubes you are using and in which configuration. Since I want to use the EL34s in my junk bin, My primary winding will have an effective impedance of 6500 Ohms in a push pull configuration. I got this number from a datasheet. Knowing the primary impedance and the winding ratio, I can now calculate the impedance of the secondary windings.

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And there you have it! I want to use an old 8 ohm speaker I’ve got kicking around, so I think I’ll use the black and green leads to get 6.06 Ohms on the secondary. I want to have the option to connect other speakers, so maybe I’ll make three separate outputs for all three windings.

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