When the output stage is overdriven, even on dummy load, you can clearly see it on the scope, and this is (a large) part of the shaping that gets missed when we play quietly. .. We will miss out on other parts of the overall sound shaping, reactivity of speakers, cab, output transformers, and overdriven speakers - all this for being able to play quietly (1-2 Watts max).
Akis,
As you note, the “valve” overdrive can be clearly seen, as can the overdrive of a guitar speaker.
1-2 watts may still be too loud for a small space, while not driving the speaker into distortion that is arguably contributes as much of the sound of electric guitar as the amplifier.
A typical 2x12” guitar cabinet plays at 103 to 108 dB at one meter with just one watt, 111 dB at two watts is hardly “quiet”, but still won’t come close to the distortion we associate with that type of cabinet driven with the usual 50-100 watt tube (valve) amp.
To illustrate the effect of overdriving a speaker, a sine wave tone at 82 Hz, the low E note on a guitar, was sent at various levels from 1 to 8 watts (a 9 dB range) to an Epiphone 8” speaker, standard issue in the Valve Jr. combo “5 watt” amp.
Measured at one meter, 1 watt, the second harmonic is about 22 dB down from the 89 dB fundamental, around 8% distortion. At 2 watts, up to around 11% distortion, still relatively “clean” for electric guitar. At 4 watts, the third harmonic now has risen above the second, more “edge”, though THD is still under 14%. At 8 watts, the distortion almost triples to over 28%, with upper third, fourth and fifth harmonics rising. About twice as “loud” sounding (+9dB) but tripling the distortion increases the perceived loudness considerably over what the the SPL (Sound pressure level) alone would indicate.
The 8” speaker used in the Fender VibroChamp XD, also a tube combo “5 watt” amp, has nearly 100% THD driven at 8 watts, the third harmonic only 2 dB down from the 82 Hz fundamental. Of interest, the resulting distortion from that drive level creates a full “E major” chord from the single E (82 Hz sine wave) tone- E, B and G# notes appear in the harmonic sequence.
8 watts into a B&C 8” designed for high power PA midrange use shows little distortion, the third harmonic around 29 dB below the fundamental, about 3.55% distortion.
Guitar speakers designed for higher power amps won’t have enough distortion when played at a only a few watts, I’d suggest trying small guitar speakers for volume reduction in addition to amplifier attenuation if you’d like to satisfy the search for electric guitar “tone” at “quiet” levels.
Art