Dana
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I went to the PNW Audio Show and got on an email list. A different amp manufacture sent me a PDF 7 pages talking about triodes vs pentodes. I can't find the article online so I'll just post a couple of paragraphs. I don't know anything about this but it sure was interesting and makes sense how two watts can fill the room.
Why?
This is the same type of simplistic thinking which says “to pass a vehicle on the road I want to be in the highest gear possible. Clearly I can accelerate harder in the top gear, with the highest top speed, than in a middle gear, with a lower top speed.” Fortunately, we all know more about cars than that!
In the case of amplifiers however it becomes even more important than simple “gearing” of the output transformer. It also involves the “torque” of the tube itself, and triodes have much more “torque” than pentodes/UL. So not only are we using a better gear with our triode amps, we have switched to a higher torque “engine” by using triodes. So what about triodes, technically speaking, gives them more “torque”?
The first advantage of triodes really is “TORQUE”!
Triodes have much lower plate output impedance (higher “torque”) than UL connected pentodes. For example, the venerable 6550, at typical bias current, has a plate output resistance of 3,000 in UL but only 500 ohms in triode. (As an aside, in “TRUE pentode” mode, with a fixed screen voltage, the output impedance is 50,000 ohms! That’s why you never see a high end audio power amp made this way even though it is capable of even more power than UL. Therefore any subsequent mentions of “pentode mode” imply “pentode/UL”.) This means that to produce a respectably decent damping factor one uses MUCH LESS feedback, 1/6 as much, with a triode amp than with a pentode amp. In fact some triode amps use ZERO feedback – you could never get away with that with a pentode amp!
The second advantage of triodes is their complete lack of something called “partition noise”. A quick search (google “Partition noise in electronics” if you would like to read more) comes up with: Partition Noise: When a circuit is to divide between two or more paths then the noise generated is known as partition Noise. The reason for the generation is random fluctuation in the division. Not a bad definition but let me tell you a bit more about this noise and what it means to the audiophile. Obviously the “two or more paths” are present in the Beam Pentode because their electrons will be split between plate and screen. The triode has only one path – the plate – and so it has no partition noise. Of course this explains why pentodes are inherently noisier than triodes but it does not explain the insidious nature of partition noise in audiophile terms. The big trouble is that partition noise goes up and down with the voltage difference between plate and screen. When they are the same, such as in a UL amp with NO SIGNAL coming through, the partition noise is very small. Amplifier noise is measured with no signal present, because this is much easier, and a UL amplifier can measure just as quiet as a triode amp with no signal present. However, when the music starts playing the plate and screen voltages separate. The screen voltage swings at about 40% of the plate in a UL amp, so when the plate is swinging 100 volts AC the screen is only swinging 40 volts AC, resulting in 60 volts AC between the two!. When the plate and the screen voltages separate this partition noise increases dramatically, especially when the screen voltage is higher than the plate because then the screen becomes more attractive to the electron stream. (Obviously triodes, and triode connected beam pentodes, always have zero partition noise.) The fact that the noise changes as the screen and plate voltages separate mean that this partition noise is modulated by the musical signal itself. Any noise which is modulated by the signal is perceived by the human ear/brain as PART of the signal. When a noise appears to be part of the signal we audiophiles call it “grain”. Of course the higher thinking part of the brain knows that this “dry” or “whitish” or “grainy” sound is NOT a natural part of the instrument or human voice, but the ear/brain has already recognized the correlation and it is impossible for us to mentally take the noise out/off of the music. (True random noise, which is not correlated with the music, is not identified by the ear/brain as being part of the music and so it is much less destructive. This has been observed by many top reviewers, including the legendary Harry Pearson.)
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Why Do Triodes Sound Cleaner and More Powerful (Read 1213 times)
