In one of the page comment sections, Dave K asked this particular question:

You have both SET and UL pentode amps, from the listener’s standpoint, what are the audible differences in sound between the two types?

From a tech standpoint, I know that triode = even harmonic distortion, pentode = odd harmonic distortion
UL pentode = ??

And this was the simple answer I provided:

The UL output stages tend to be very “neutral”. I think if a person is looking for the sound of a piece which is closest to the actual recording then the UL pentode power stage is the best bet. I only say this after listening to a fair amount of classical music live and the exact same performances recorded.

SET stages are anything BUT neutral. They are warm, rich, and full and they provide this unique color to everything they reproduce. This is one of the reasons they are so popular in spite of their typically low power output. They just sound fabulous with so many types of music.

Pentode stages are the most “transistor” like. (I know that’s not much of a description but it’s accurate.) They tend to sprinkle in the complete set of harmonics and inter-mods, so that while they can sound quite good, I find them to be the most cold sound reproduction.

But I was thinking about it afterwards and I’ve decided a more detailed discussion is in order.

When Engineers first started to design simple vacuum tube audio amplifiers, triodes were the name of the game because that’s all there was. So there was a simple choice, SETs which were limited to low power by the predominant 2nd harmonic distortion, or push-pull class AB amplifiers which produced somewhat more power by cancelling the majority of the 2nd harmonic distortion in the balanced output transformer. And all the triodes suffered from a critical flaw: very low power sensitivity. This means that it requires a large swing in grid voltage to produce much power. And this has its own ramifications for overall amplifier distortion.

The first approach to fixing the problems of non-linearity and low power sensitivity was tetrodes. But the tetrodes had additional problems that limited their usefulness as audio power amplifiers. The good news is that those problems we mostly corrected through the addition of an additional grid to control secondary emission from the plate. The result was the pentode (and later the Beam Power Tube) which was immediately very successful with design Engineers. Suddenly there was an audio power tube with both high power sensitivity and good linearity. With the addition of NFB theory (H.S. Black, 1934) Engineers were soon producing high power, low distortion audio amplifiers with relative ease. There was just one problem, listeners did not share the enthusiasm of the design Engineers. The listening public seemed to prefer those horrible low power triode amplifiers.

The issue was soon identified; it was low level but higher order distortion products produced by the pentodes. They were affecting to overall tone. So designers went all in and drove distortion to very low levels <1% using NFB and started shaping the tone with triodes in the preamplifiers. This approached produced amplifiers that while very low distortion, were best classified as “cold” or “sterile” in their sound. The approach was successful to a degree, but there were still significant portions of the population which preferred the “triode” sound.

The answer, while not immediately obvious, was to find an intermediate solution between the “warm” triode conditions, with their accompanying low power sensitivity, and the “cold” pentode conditions, with their accompanying high power sensitivity. This nut was cracked by Alan Dower Blumlein in the late 1930s. The simple solution was to connect the screen grid to the primary of the output transformer at an intermediate location between the B+ end (Pentode operation) and the plate end (Triode operation). He was issued patents on his invention on Dec 5th, 1938 in the United Kingdom and on October 22nd, 1940 in the United States. It was quickly dubbed “Ultra Linear” operation.

This approach to audio power amplifier design was an immediate success. Soon it was a simple matter to design using pentodes with high power sensitivity AND produce tone, that while not as “warm” as the triodes, was comfortably “neutral” and which allowed the music to be faithfully reproduced.

So this is a little longer explanation of how we  got to the Triode/UL/Pentode option set we have today when building vacuum tube power stages. There is a lot more to the story than what I’ve related, including lawsuits, patent wars, and shameful marketing campaigns, but I hope my explaination gives readers a little better appreciation for the differences between these three types of power amplifier stages.