One of the quirkier and exotic types of power amplifiers are a tube based product known as a Single Ended Triode or SET for short.
A SET is a class A power amp using one output tube, typically a triode, to drive your loudspeaker. They are generally low power devices with poor efficiency – as is any class A power amplifier.
This class of power amplifier has a small, but very passionate, group of supporters who swear nothing more musical exists on the planet. There may be an element of truth in their thoughts.
First, however, we should clear away some of the mystique associated with these power amplifiers so we may better understand the SET.
When I suggest there may be some truth to their revered musicality claims there are three main areas where these qualities come from: single ended, 2nd order harmonics and high voltage – the first two shared by most class A power amplifiers, the third shared by most tube amplifiers.
A single ended amplifier means that all the amplification is handled by a single output device, be it solid state or tube. While certainly the most inefficient means of delivering power, single ended designs are free from the crossover problems found by the more common and efficient output pair we generally see in an amplifier. Score one.
When a single ended amp gets near clipping its distortion products are more heavily weighted towards second order harmonics which are very pleasing to the ear, relative to third order harmonics. Score two.
So far nothing here is exclusive to a SET because a solid state version would have the same properties.
What can be said to be unique to a vacuum tube single ended amplifier, relative to a solid state single ended amplifier, is voltage – something all tubes have in common.
Tubes generally run at far higher power supply voltages than do solid state devices – in some cases ten times higher – and here is where some of the mystery about how you can get satisfactory loudness levels from a SET come into play.
Let’s look a little closer tomorrow.