What happen if you use the wrong impedance output tap of the OT?

[Alan0354]

Ok... So the general consensus is that a greater than 100% mismatch is best avoided. And valid reasons have been given. But Alan0354 keeps questioning the replies or reinventing the question in an attempt to find the favorable answer he wants. So, as my final statement on the matter, Alan0354, do as you like. It's your amp. It will probably be fine working with impedances from 4 ohm to 16 ohm load or tap provided your amp doesn't already stress the power tube or OT maximum tolerances. Many players do this every day without issue. The ones that do have a problem have places to go where amps get fixed. It all works out.

I am just trying to get a theoretical reason, I thought it is nice to know what is the limit and the reason of the limit.

[Alan0354]

If you run too high a load impedance (say 16 ohm speaker on a 4 ohm tap), you'll exceed the screen grid dissipation rating of your power tubes. The first big power chord you hit, the screen grids will melt and short the tubes out. Or if you're lucky, the screen resistors will fail open circuit.

As far as I know, the voltage spikes that cause burnt OTs and tube sockets are the result of a really serious mismatch, an open circuit even.

I did some experiments years ago with deliberate mismatches, to try the various protection circuits that amp makers used. The 200 ohm resistor across the speaker jack was good at stopping the spikes, but it couldn't save the screens.

That make sense, a mismatch of 4 to 16 likely to cause the spike in voltage that hurt the tube rather than the OT. So what you are saying is when the plate/screen voltage shoot up due to the high primary impedance from mismatch will cause much high voltage swing at the plate and screen and burn the tube. That is a good explanation, thanks.