Power Rating

Figure 3: Transistor: Actual vs. Perceived power, crest factor 12dB
Tube Amps
The nature of tube amps is very different from that of solid state amps. Rather than suddenly
hitting the distortion point, tubes gradually go into saturation and tend to ‘round off’ signals in a
very soft way when getting to its maximum performance. The overdrive created by tube amps is
quite pleasant to our ears and is known by many as a central part of the highly complex ‘tube
sound’ that have been pursued in many bass amp designs.
Again, looking at bass signals, the gradual saturation of tubes essentially ‘compresses’ or limits
the high peaks of the bass with only little resulting overdrive thereby altering the original crest
factor to become quite a bit lower than the original signal with a consequently higher perceived
loudness at a lower power rating – this is a quite known effect by many users, usually referred to
as the difference between ‘transistor watts’ and ‘tube watts’ (fig. 4)
In the example below the 500W tube amp has reduced the peaks, and hereby the crest factor,
also achieving a perceived level and power of 56W RMS.
Effectively this means that the 500w tube amp is perceived to have more power than the 900w
transistor amp by the bass player even though they are vastly different in the actual power
measurable.