Another transformer question

[Randall]

I know not to run a signal into a tube amp with no load, and why there is a shorted switching speaker jack on Fender style amps, etc. But what about OTs with multiple secondaries? Say I wire up a 4/8/16 ohm OT to just the 4 and 8 ohm jacks, and put grounding switches on both jacks with the idea that I will only be using one side at a time. What do I do with the 16 ohm tap I don't plan on using? I'm plugged into the 4 ohm tap, the 8 ohm tap is grounded, and the 16 ohm is unused.

And what about PT's with unused windings, like say a bias tap when the amp is cathode bias? Or multiple HV secondary taps? Do we simply cut and tape them off?

[R.G.]

I know not to run a signal into a tube amp with no load, and why there is a shorted switching speaker jack on Fender style amps, etc. But what about OTs with multiple secondaries? Say I wire up a 4/8/16 ohm OT to just the 4 and 8 ohm jacks, and put grounding switches on both jacks with the idea that I will only be using one side at a time. What do I do with the 16 ohm tap I don't plan on using? I'm plugged into the 4 ohm tap, the 8 ohm tap is grounded, and the 16 ohm is unused.

There are two issues lurking in there.
1. In a transformer with a functioning magnetic field and tight coupling (OTs qualify in almost all conditions), if you load one winding, you load them all. Each winding can be conceived of as a window into a single bucket of magnetic energy in the core. So for multiple secondaries, if you load one of them, none of them can go off causing sudden death scenarios; again, this depends on them being "tightly coupled", and OTs almost always are.
2. OTs are the victims in unloaded death setups, mostly. The actual method of death is high voltage spikes induced into the OT by oscillation or funny behaviour in the power amp, usually a power amp with feedback. The spikes and high frequency oscillation cause high voltages on all windings because of the behaviour of the leakage inductances, which is more abstract than you want to get to, I think.

So if you load one of them, all the secondaries are about equally OK.

And what about PT's with unused windings, like say a bias tap when the amp is cathode bias? Or multiple HV secondary taps? Do we simply cut and tape them off?

Yes. This is a different setup. Opening the output on a power transformer cannot cause funny oscillations and spikes back in the power generator station, so every turn stays well behaved. All turns still share the same voltage, but it never gets out of bounds. For PTs, you just have to make the open windings not cause problems with short circuits or accidental overloads.

What's different is that an OT can sometimes make the power amp destroy it, and a PT can't affect the generator and can't set off the death cascade.

Generally.

[loudthud]

You can't short both the 4 ohm and 8 ohm taps. When a speaker is connected, you can't have a short on any of the other taps. You need a switch to select the tap, then ground that if the speaker is not connected. If there is a 16 ohm tap, just leave it unconnected to anything.

[Randall]

Once again, helpful smart people on MEF to the rescue! I'm certainly glad I asked that question. I felt something in my understanding wasn't right. here's what I was thinking, and have already had my chassis fabricated and plated, so I'll have to go with it. Bass amp, want to have 4 and 8 ohm option, no switch. One jack for 8 ohm tap, two parallel jacks for 4 ohm tap. That way, if I run an 8 ohm cab, there's the jack. If I run a 4 ohm cab, use one of the 4 ohm jacks. If I run two 8 ohm cabs, both into the 4 ohm side. I like it. I was trying to protect it by using the switching jacks, but now it looks like I will have to leave them as open jacks and just be careful not to forget to plug in a load.

Does this sound right?

[Bruce / Mission Amps]

Buy a 5 watt 270 ohm to 470 ohm resistor and put it across the 8 ohm or 16 ohm tap to ground... it will always be there, draw virtually no real current and any speaker load would swamp it out completely anyhow... but it will have enough "load" on the OT to probably save the OT, the power tubes or sockets from a disaster if you run the amp with no speaker load at all.

[Randall]

Thank You Bruce! Brilliant. I admit I don't exactly understand the math of why it works, but I trust your advice for sure.