Your amp is probably fine. You would likely experience signs if you had caused damage. With tube amps, they often start sputtering, almost like a gated fuzz, before the damage happens. If you shut them off fast when you notice, you can avoid a repair.
The danger depends on the type of amp. With solid state amps, going from a higher impedance amp output into a lower impedance speaker cab risks damage to the amp. However, mismatch the other direction, low impedance (ex: 8 ohm) output to high impedance speaker cab (16 ohm) is safe. In fact, if you have a SS amp with a 4 ohm output, it will be safe with 4 ohm, 8 ohm or 16 ohm speaker cab.
With tube amps there are two different kinds of risk…
“If your amp’s output transformer is designed for an 8 Ω load:
- 8 Ω tap → 4 Ω speaker = low impedance mismatch (2:1 mismatch)
- 8 Ω tap → 16 Ω speaker = high impedance mismatch (2:1 mismatch)
The mismatch ratio is the same, but the stress on the amp is different.
8 Ω output into 4 Ω speaker (too low a load)
The tubes must deliver more current than intended. This can:
- Heat the output tubes more.
- Stress the output transformer.
- Increase power supply strain.
8 Ω output into 16 Ω speaker (too high a load)
The tubes deliver less current, but the output transformer can develop higher flyback voltages when the speaker’s impedance causes energy to reflect back into the transformer. This can:
- Stress the output transformer insulation.
- Cause arcing at tube sockets or tube elements.
- Damage transformers in severe cases.
Historically, transformer damage from excessive voltage spikes is why many amp techs consider a high-side mismatch (too many ohms) potentially more dangerous than a low-side mismatch.”
Generally, with a tube amps, one step either direction (2:1) is not likely to cause damage. Examples: 16 ohm output to 8 ohm cab. 8 ohm output to 16 ohm cab. What is very dangerous with tube amps are things like 4 ohm output to 16 ohm cab. Or, 16 ohm output to 4 ohm cab.
