LOAD: All amplifiers are designed and built to drive a load. The music instrument amplifier's load is the loudspeaker. It is important to understand that the final sound produced by the amp/speaker system is the result of the very strong interaction between the amp and speaker. You can't really talk about the sound of an amp or of a speaker separately, but only about the sound of the amp/load (speaker) combination.
Power tube performance in the output stage of an amplifier are critically dependent upon the load that the tubes are required to drive. At any given signal frequency or level the value of the load seen by the output tubes will determine the power output and not only the total distortion produced but also the kind of distortion (even order or odd order or the combination of both). On the load side, looking back at the driving amplifier, the load (speaker system) sees not only the driving signal but also the dynamic (changing) impedance (resistance) of the output stage which has a major impact upon load or speaker dynamics. .... Like I said, you can't talk about amp and speaker sound separately. To understand how a speaker load and a dummy or non-speaker load work we must add two new terms to our technical vocabulary: resistive load and reactive load. A resistive load is exactly what it's name indicates, a resistor that is measured in ohms and is always the same regardless of the signal voltage across it, AC or DC. On the other hand, a reactive load is an AC resistance or impedance, as in speaker impedance, it also is measured in ohms but unlike the resistive load it's impedance (ohms) changes with the frequency across it. The difference between these two types of loads is clearly indicated in the graph below. The graphs plot the measured impedance, in ohms of one speaker system and three non-speaker load boxes against frequency. The purely resistive load, the Power Soak represented by the broken yellow line maintains a constant resistance (4 ohms) at all frequencies. The reference reactive load is a Marshall 4 x 12 cabinet: solid thick yellow line. This is the classic speaker impedance curve: in the upper bass region the resonant frequency of the speaker causes the impedance to increase five to eight times the nominal four ohms impedance and the same range of increase over a much wider range in the treble region due to the voice coil inductance. The nominal impedance of a loudspeaker is measured at 400 HZ.
If you consider the impedance curves presented here in light of the conversation about the effect of a load upon an amplifier stage, it is easy to see that a resistive load device will make much greater power demands and create a very different frequency response and distortion products than the reactive speaker system. The increased power demand of a resistive load will greatly shorten the life of the power tubes and may damage the output transformer in some amplifiers. In the case of some reactive load boxes, the reactive element (an inductor) is not large enough or not damped or resonated. In this kind of load box, the rising impedance in the treble region can become so high as to effectively leave the amp unloaded. A cranked unloaded output stage can create very high voltage inductive peaks (several thousand volts) sufficient to cause catastrophic arcing in the tubes and in some cases, punch through the output transformer winding insulation ... end of transformer. In our opinion, the load box that is the safest and best performing sonically is the one that closely matches the impedance curve of a real speaker system. ..... if you have a load box that you are in doubt about, send it to us and we will measure it for you.