In routine use of a tube amplifier, it is highly recommended that the power output of the amplifier is always connected to a speaker cabinet or suitable load prior to powering on. The speaker cabinet (rated at either 4-, 8- or 16-Ohms) must always be connected to the corresponding speaker output of the amplifier. Neglecting to do so can lead to partial or complete destruction of the output stage of the tube amplifier.
The majority of tube amplifier designers protect their products with fuses or alternative suitable protection systems; however, some amplifiers are still insufficiently protected. It is therefore impossible to predict the behavior of all amplifiers on the market in the instance load (be it 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” is used to describe any product that provides a load to the amplifier. The main parameter of the loadbox is its impedance, expressed in Ohms. Impedance matching between a loadbox and amplifier is paramount and as such should be actioned wherever possible (i.e. an 8-Ohm loadbox must be plugged to the 8-Ohm speaker output of the related amplifier).
The power sent by the amplifier to the corresponding load is turned into heat; as such, it is vital to refer to the cooling recommendations stipulated in the manual of the respective load; overheating may cause damage, both to the loadbox and to the amplifier.
Always connect the speaker out of your tube amplifier to an appropriate load (be it a speaker cabinet or suitably rated loadbox).
Setting the Output Volume for an Amplifier
The correct use of an amplifier with a loadbox requires caution. Due to the inherent silence while playing through an amplifier with a loadbox, it is easier to accidentally run the amplifier beyond the reasonable limits set by the manufacturer - more so than when using a speaker cabinet in conjunction with the amplifier. This can lead to faster tube wear and, in some cases, damage to the amplifier or loadbox.
Considerations When Using a Loadbox
When 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.
Please note that the “sweet spot” - the optimal level of the amplifier’s power amp volume - is rarely obtained at maximum volume. In addition, the volume control of the amplifier is usually logarithmic, meaning the volume increases quickly within the first half of the accompanying potentiometer’s rotation, reaching its maximum at 12 o’clock, Therefore, maximum volume of the amplifier can be reached even if the volume potentiometer is not set at its maximum value.
When reaching the maximum value of the aforementioned output potentiometer, you will likely hear considerable distortion. In fact, most amplifiers sound comparatively poor at maximum volume. In addition, always keep in mind that your amplifier may not have been conceived to be used at maximum volume for a long period of time. Running an amplifier at high volume will cause premature wear of the tubes and possible malfunction or damage at the output stage.
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 prudent course of action is to maintain the volume setup you would use in a rehearsal or live performance setting, rather than following what the volume potentiometer indicates.
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