OT Question

[Chris333]

I have a question about an OT. It's a BF Bassman replacement. The schematic that came with the tranny shows the primaries as red = CT and brown and black as the ends. The actual OT has red, brown and white primaries. I assume the white on the OT is the same as black on the schematic. When I check resistance of the primaries, I get 40.4 ohms from red to brown, 81.9 from red to white and 41.7 from brown to white. From those readings, seems like the brown might be the CT instead of the red.

With it hooked up with red as CT, I get a soft pop when I take the amp off standy, and a faint sound when I touch my meter probe to the grid pin of the tube with the white secondary wire, otherwise no sound at all. Voltages all seem OK, and tried three sets of tubes. I could certainly have something else screwed up since this is a homebrew thing. Any ideas?

[Enzo]

Sounds like brown is the center tap to me too. Traditionally the CT was red and the plate leads were brown and blue, not black. But in this world, anything is possible.

[R.G.]

The simple way to tell is to feed the output with a low AC voltage - 6.3V from a filament winding can often be used - and then to measure the voltage on the three wires. The CT is the one where the voltages to the other two are about equal.

Transformer makers used to have a strict adherance to a color code, but this is an almost quaint anachronism now. Wires colors may be anything unless you have a spec from the manufacturer telling what they used on that one unit.

I voltage test every transformer I use to find out what surprises the maker put there for me. 8-)

[Bruce / Mission Amps]

Sounds like it could be an English wound output tranny where brown is the center tap and white and red are the two outside legs.

[Chris333]

Wired up a different OT this morning and still no sound-- was very close to a tool-throwing fit. Then I remembered one of Enzo's posts on Ampage about using a probe hooked up to a cap and run into a practice amp as a signal tracer. Isolated the problem in about five minutes, fixed a mistake in the PI, and the amp worked with the other OT. So I hooked up a 6.3V transformer to the output of the original OT and measured 109V red to brown, 109 brown to white, and 217 red to white. Wired the OT up with brown as the CT and it works fine. I saw the transformer test yesterday on the Tube Amp Debugging page-- guess I could have figured this out without pestering anybody, but thanks for the help!

I'm guessing this is an isolated screw-up at Mercury Magnetics (don't like to call 'em out, but you shouldn't have to do all that dinkin' around with a pricey transformer). I can't imagine that they would change colors from the schematic on purpose. If they DID change it, please don't put a red wire in the primary of an OT and have it NOT be the center tap!

[Enzo]

In all fairness, if it is colored like a MArshall tranny, that is a clue to most techs. If I see red blue and brown, I feel pretty confident it follows convention. If it is anything else, a few minutes with a meter sorts it out. Just never assume, always check.

Actually it is a good idea to point out the oversight to them for their QC.