If I have an amp with multiple secondary taps.
Do I need to still use the one that's appropiate for my amp or can I use any of the taps? I've seen it done so i assume it's okay but only as 8 and 16 ohms switch...
So is it that maybe a 100% mismatch either way is also ok then ? Would I be ok to use the 16 ohm when your amp calls for 4 ohm?
NO. Use the 4 ohm tap with a 4 ohm speaker load and the 16 ohm tap with a 16 ohm load. Use a mismatch at your own risk. You can damage the amp. It isn't worth doing.
One step either way is usually ok but can vary from amp to amp.
Older Fenders could handle it. Some newer ones can't because they are designed so close to component spec's that they can't handle any extra current or heat.
I would not go two steps, like 4 ohm to 16 ohm load or visa versa.
Yep. The OT transforms the speaker impedance to the required plate to plate impedance of the output tubes. There's not really any harm in mismatching impedance (with limits) as long as the OT is well designed. Some early Marshalls blew up when mismatched because the transformers weren't rated even to handle the normal load. Your output tubes will adjust their behavior based on the load presented to them; mismatch high and you will reduce power and bandwidth. Mismatch low and you can theoretically increase power depending on voltage and the values in the feedback loop (if there is one). Some amps have been made with impedance selector switches labelled "Tone," because the mismatch alters frequency response. Gerald Weber once did a blind taste test and found that 9 out of 10 players preferred the sound of an amp mismatched one "step" higher.
