What is a loadbox?

What is a loadbox?

In the normal use of a tube amplifier, it is highly recommended that you always connect its power output to a speaker cabinet prior to powering it up. The speaker cabinet (2, 4, 8 or 16 Ohms) must always be connected to the corresponding speaker output of your amplifier. Not doing so can lead to partial or complete destruction of the output stage of the tube amplifier.

Most tube amplifier makers protect their products with fuses or other protection systems, but some amplifiers still remain not or insufficiently protected. It is impossible to predict the behavior of all the amplifiers on the market in case of use without a load (a speaker cabinet or a loadbox).

The electronic term that describes the speaker cabinet with respect to the amplifier is the "load": we say the cabinet "loads" the amplifier. The term "loadbox" fits any product that embeds an electronic load. The main parameter of the loadbox is its impedance, expressed in Ohms. An 8-Ohm loadbox must be plugged to the 8-Ohm speaker output of the amplifier.

The power sent to the load is turned into heat, so please follow the cooling recommendation of the loadbox otherwise overheating may cause damage, both to the loadbox and to the amplifier. The Torpedo Reload, Live, Studio and Captor are loadboxes. This term indicates that these products feature a load which can electrically replace the speaker cabinet while dissipating (transforming into heat) the power coming out of the amplifier.

The embedded load in the Torpedo products is reactive: it embeds a specific circuit to simulate the complex impedance of a real speaker. This kind of system is widely used in the industry to silently test amplifiers.

Is the use of a loadbox totally silent?

We usually talk about "silent recording" when a loadbox is involved. If we compare the loadbox solution to a traditional cabinet miking solution, it is obviously several orders of magnitude quieter, but you will still experience some minor sounds, noises, that have to be taken into account:

  • Your guitar or bass strings can be heard. This is obvious, but it can be disturbing, depending on your environment.
  • You may hear some noise coming out of your Torpedo when playing, like there is a tiny speaker inside the box. This is perfectly normal and there is no reason to worry. The sound is produced when power goes through the coil of the reactive load embedded on the Torpedo. The vibration is related to what power comes out of the amplifier connected to the Torpedo and to the signal’s frequency content (notes played are heard). Your amplifier may also produce similar noise, at the output transformer’s level. Such noise is usually not heard, simply because it is normally overcome by the sound coming from the loudspeaker.
  • The Torpedo embeds a fan, as there is quite a lot of power dissipated into heat inside the box. We selected a so called "silent fan", but as it is running fast, it is never entirely silent. This said, you can consider that, in normal use (hearing your guitar through monitors, or headphones), you can barely hear that fan.

Care should be taken when using a loadbox

The correct use of your amplifier with a loadbox requires some precautions. Because of the fact that you may
be playing "silently," it is much easier to accidentally run your amplifier beyond the reasonable limits set by the
manufacturer than when you are using an actual speaker cabinet. This can lead to faster tube wear and, in
some cases, to more serious inconveniences.

Keep in mind that the "sweet spot" — the perfect running point of the amplifier, the one that will give you the
tone you are looking for—is rarely obtained atmaximum volume. In addition, the volume control of the amplifier
is usually logarithmic, which means that the volume goes up quickly on the first half of the potentiometer’s
rotation, reaches its maximum at 12 o’clock, and will not change much beyond that point. Therefore, you can
reach the maximum volume of your amplifier even if the volume potentiometer is not set at maximum.

By reaching the maximum output power of your amplifier, you will hear a lot of distortion, which may not
sound as good as you could hope. In fact, most amplifiers do sound rather poorly at maximum volume. Always
keep in mind that your amplifier may not have been conceived to be used at maximum volume for a long
time. Running an amplifier at high volume will cause premature wear of the tubes and possible malfunctions or
damages at the output stage.

  • When first testing the amplifier at high volume, monitor the color of the tubes and the general state of the amplifier. Red-glowing tubes or any appearance of smoke are signs of a problem that may result in partial or complete destruction of the amplifier.
  • The fact that the volume control of your amplifier is not set at maximum doesn’t mean your amplifier is not running at maximum volume. A good habit is to keep the usual volume setup you would use in rehearsal or on stage, rather than just following what the volume potentiometer indicates.