There seems to be a great deal of confusion among players, techs, equipment manufactures and the media, about load boxes, dummy loads, speaker emulators, simulators and the like. The uncertainty, lack of, and in some cases, mistaken information about these devices is truly scary.

We have all heard horror stories about favorite irreplaceable amplifiers that have been severely trashed when connected to a strange load device.

As a repair shop, we have seen more than a few guitar amplifiers that have suffered in this way. Don't get me wrong, I am not suggesting that non- speaker load devices or related products are basically flawed and should not be used.

After all, we were the first people in the industry to design and market a reactive load device, "the silent speaker". On the up side, there is no denying that including a dummy load in your rig can open up a wide range of new possibilities in the quest for the ultimate tone.

Before I start talking about the ins and outs of non speaker load devices, we had better define terms and make sure that we are all playing the same tune. Red Box's, cabinet emulators, simulators and the like are not load devices. They are equalizers that attempt to shape a line level signal or an amplifier direct out, so that the resulting tone imitates that which comes out of a speaker cabinet. The output of this type of filter can not drive a speaker: it is meant to input a PA/ recording board or an effects rack.

WARNING!!! When using this kind of device, your amplifier must be connected to a speaker or a dummy load. Failure to do so may result in serious amplifier damage.

Another type of speaker related product on the market is the attenuator: it is inserted between the amplifier and speaker to provide control of the signal fed to the speaker. In a future article we will talk about attenuators, emulators and simul ators.

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.

In the recent issue of a well known guitar magazine, a well known personality in the tube world answered the question "What causes an amp to lose it's bias?" with "Nothing, What happens is the tubes wear out." And he goes on to say "buy a new set of tubes and be done with it." 

In our opinion that is a rather incomplete as well as inaccur- ate answer followed by a suggested cure that is risky at best and potentially an amp killer at worst.

The "bias" in a tube amp is a control voltage that sets the idle current for the output stage tubes. If the bias is not there or set too low, the output tubes draw too much current which will damage the tubes and may destroy the power and/or the output transformer. If the bias is too high, no damage will result but the amp won't sound very good.

While a shorted tube can bring down the bias supply, a worn or old tube will not. Just putting in a new tube without checking the output stage and the bias supply is bad idea. The tube, when it shorted may have damaged the bias supply or other components in the amp. This is especially important in amplifiers that rely on cathode bias. The cathode bias resistor may have been severely burned and changed value. In which case the amp will be incorrectly biased.

A variety of other problems can cause insufficient bias voltage. The bias supply is built with electronic components such as capacitors and resistors all of which age and can fail. Don't forget, guitar and bass amps get moved around a lot and suffer physical abuse. Broken connections and wires are common. When an amp blows a fuse or you change the tubes, the prudent thing to do is, CHECK THE BIAS!!

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.