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mariosr wrote:Sorry torrent61, but you mixed up tube amps and solid state amps, correct is the opposite way
Tube amps cannot mismatch impedance (or just in a certain grade)
and solid state amps can !
If speaker impedance is higher than amp impedance, the amp can’t give away all its power. Part of it gets reflected back into the amps output transformer, heating it up. If speaker impedance is much higher than amp impedance (infinite for nothing connected), all power gets reflected back into the amp causing serious damage to the output transformer.
If speaker impedance is lower than amp impedance, the amp needs to work harder to get the same output level as with an impedance match. Since a tube-amp is limited to how much power it can give, it can never damage itself. The tubes get worn out faster (since they have to work harder) and the amp won’t seem to be as loud as with an impedance match.
As a precaution, most tube-amps automatically short circuit (impedance of load is 0) there output when no jack is connected. This is to make sure that if the amp were to accidently turned on with nothing connected, it wouldn’t cause damage to the output transformer. Just make sure you don’t have a cable without a speaker connected to the output of the amp, as this would bypass the safety precaution!
You can get away with a ½ mismatch for tube-amps. For example, you can connect an 8 ohm output of a tube-amp with a speaker cabinet of 4 or 16 ohm without causing too much harm. But you will lose power and bandwidth causing the guitar to sound thinner and less defined. Never connect a 4 ohm output to a 16 ohm speaker cabinet.
If you need to make a mismatch it’s better to have the speaker impedance lower than the amps impedance (as described above).
If speaker impedance is higher than amp impedance, the amp won’t produce more power than it can give away. It’s therefore safe to turn on a solid-state amp with noting connected to it (infinite impedance). PA technicians sometimes confuse tube-amps with solid-state amps and tell guitar players that it’s okay to turn on their guitar amp with no speaker connected to it, but this can be very dangerous as described above.
If speaker impedance is lower than amp impedance, the amp will increase its output power to drive the bigger load**. Since there is no theoretical limit to how much power a solid-state amp can give, it will simply blow-up when you short circuit (impedance is 0) the output of the amp. Luckily most amps have fuses and short-circuit protection build in.