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