Power transformer primary colour coding question

[Bloomfield]

I have an old power transformer here that I found years ago that I was planning to use for a build. It is about the size of a Bassman transformer and looks promising for what I have in mind. All of the other power transformers I have here have two black primary leads. This one has one black and one black with a red trace woven into it. I assume they must signify start and finish of the winding because I can't think of any other significance they might have.

Normally I just put my mains fuse and switch in series between the hot (black) of the mains and one end of the primary, and the neutral connected directly to the other, but I just use whichever one comes to hand first. If we were to know which is the start and which is the finish, is there any reason to prefer one way of hooking them up over the other?

[Leo_Gnardo]

Normally I just put my mains fuse and switch in series between the hot (black) of the mains and one end of the primary, and the neutral connected directly to the other, but I just use whichever one comes to hand first. If we were to know which is the start and which is the finish, is there any reason to prefer one way of hooking them up over the other?

In a word, no. As long as you're sure black and black/red are the 120V winding, I'd check the other windings with power applied to make sure they are what you guess they are. Be safe - use some clip leads to your meter & don't make wiring changes while power is applied. Occasionally you'll find transformers that don't conform to standard color codes.

To be super safe, fuse the mains lead, and dial up power slowly with a variac. Good luck, and write back to let us know you survived.

[Bloomfield]

Thanks Leo, I checked it all out years ago and everything was as you'd expect to see. I was just curious why they would distinguish those two leads; I've never seen that before.

[J M Fahey]

Not really needed here, where we have a single primary (do we?), but many PTs come with dual 115V primaries to allow series or parallel wiring forn Export/International use.
In that case it is *needed* to identify start/end at each winding for proper phase.

They might be used to it and mark beginning/end always.

[mozz]

Can you list all the wire colors? Get your meter out and measure too.

[Bloomfield]

I just had another look. There is just the one primary as described. The HT secondary is two red and a red/yellow centre tap as you would expect. There are two filament secondaries, one of which may have been green earlier in its life. The other is a slightly different yellowy brown.

The resistance of the primary is approximately 2 ohms. The HT is 27.1 on one side and 29.4 on the other. Both filament windings are very low resistance, about .1 ohm.

At a line voltage of 122.7 I got 301-0-301 for the HT, and the filament windings were 7.07v for the greenish pair and 7.46 for the other. Reversing the primaries increased the readings on the filaments by .03v but that is probably not significant.

I think I'll go ahead and use it; hopefully one of the filament windings will give a suitable voltage under load. It is quite possibly an organ transformer made to run a whole bunch of preamp tubes and a small power amp. I'll see how it works. If it's not up to the job a Bassman transformer will drop right in it's place. Still curious about the primary colour coding, even if it makes no difference.

Andy