Will an underrated speaker stress a rectifier tube?

[leydenjar]

i.e. for example, say I have a speaker rated at 30 watts and i'm using it in a 40 watt amp that can definitely put out 40 watts, if it doesn't blow the speaker quickly will this situation stress components (like a tube rectifier) in the amp?

I wouldn't do this but I've seen it.

[Diablo]

The output transformer sends current to the speaker voice coil. The impedance of the voice coil is fixed by the design of the voice coil and the speaker magnet circuit and influenced by the cone resonance. The impedance of the VC effects the load line that the output tubes operate on, so can alter the current drawn by the tubes. A low wattage rating on a speaker means that the VC can't handle high watts over an extended time without burning up. If the speaker impedance matches correctly to the amp, and the speaker VC is operated in a range where it doesn't burn up, then the only impedance change you will see is from heating the VC. So a low wattage speaker may see more impedance change under heavy use because it has thin wires in the VC. If you burn up the VC, the impedance changes a lot, and this can draw lots of current through the tubes.

[Jazz P Bass]

Very good reply Diablo
Another tack is 'are you playing the amp at 40 watts?'

[leydenjar]

Got it.

Think of the speaker like a resistor.

Too many watts = resistor burns up = excess current draw = trouble

[Jazz P Bass]

Typically though, speaker coils will fail open circuit.

[Steve Conner]

Which means boom for a tube amp