Metropoulos Forum

When building an amp, how do you know which OT primary to connect to which half of the power tube string? I know if you get them reversed it can lead to instant red-platting and oscillation/squeal. So, how do you know if you've wired them reversed or not before you turn the amp on? Let's assume the OT is not a standardized or historic recreation where you can just use the lead colors.

In some cases the OT used will have colors that match the original schematic/layout. This still does not guarantee correct orientation though. If you are building a circuit with nfb, it is always wise to leave some slack in the primary wires in case you need to reverse them. If your circuit does not use nfb, it does not matter which wire goes to which side.

Interesting. So really... there's no way to know which one goes where? You just have to try it and hope you got it right on the first try?

This has happened to my multiple times. for EL34 tubes, no red-plating, but that siren sound is crazy loud and fortunately tells you right away that they are reversed.

This also happened to me with KT66 tubes....the tubes didn't red plate, but they did spark internally after a few attempts damaged the tubes. this sucks, since KT66 are definitely more expensive.

I dot know of a way to measure / test before firing up the amp If someone knows, this would be good info to share.