a little valve info to try to correct some myths..

It has occured to me lately about the myth of the triode.

In the output stage of a valve amplifier, the single ended direct heated triode is the cinderella, the princess, and the push pull pentode as the frog, or worse.

The ethos of having a pentode is really power,

push pull will allow more power, and cancel 2nd harmonic distortion,
paralleling them will reduce output impedance to allow better damping of speakers,
class AB will allow more power, and grid current even more so, so if we wish to design a valve amp for maximum power, we will make it
1/ push pull pentode, with paralled valves,running in
2. class AB, with the suffix 2, to indicate grid current.

ok? simple eh?

how why is the triode, particularly 300b, lesser 2a3, 845 and 211 so venerated?

well it is all to do with the graphs of the voltage and current relations in the input and ouput for these valves.

The 300b has these lines on its graph which are REALLY equal spaced, now when designing an amp the spacing of the lines indicates INHERENT distortion, 2nd and 3rd harmonic, this is inherent distortion in the valve, and there isn't much you can do apart from set the design cleverly and use feedback.

With the 300b, these lines are really even, meaning little inherent distortion, hence a 300b can get away with little or no feedback, which will cancel that distortion, in other words, its not needed.

So this is the crux of it, when a designer, or sales blurb says triodes offer ultimate sound quality, that in engineers speak means they have minimal distortion, as explained above. Other words are fidelity, or linearity, they all mean the same thing.
Its INHERENT minimal distortion. Not to say they don't produce some distortion, but its considered fairly objectionless.

You can correct distortion in other valves with feedback.

But there are different types of distortion.

1/ inherent distortion, namely 2nd and 3rd harmonic. not really nasty. In all valves, some more so than others

2/ clipping distortion, when the output goes past the voltage or current limits of the amplifier, our nice waveform gets flattened or clipped, and generates nasty harmonics.

3/ crossover distortion, this is in class AB or class B, not in class A...this means that when you have one valve...single ended, running in class A, that valve amplifies the whole wave, and it
doesn't 'pass the buck' to another valve to amplify another bit of the waveform. so there is nothing crossing over.
when is passes over to another, class AB is anything from very nearly class A, to very nearly class B..its huge definition. A massive spectrum. If its not class A enough, some crossover distortion will occur.

You can have a push pull in class A, so this doubles power whilst keeping distortion down.
These distortions ,though can be corrected with feedback.

So that's it, when I think of sound quality, I hear other stuff, bass, presence, tangible vocals, this has little to do with inherent distortion in a valve.

Many of you run class B transistor amps, with loads of feedback, and they give superb sound quality. distortion is almost non existant.

you can have a triode in class B, and lots of crossover distortion, or a pentode in class A single ended.

So you have to understand what the engineers mean when they say that, and why that may not tally with what you expect or would like to hear. Its all semantics, definitions, and words.

The inherent distortion is not really consequential as the nasty ones and all types get reduced with feedback anyway, so the distortion of a feedback amplifier is much less than one without,

so is there some magic at work in one of these revered valves? I don't think so, the complex explanation and reasoning is above.
It is just that they have little inherent distortion, which is corrected and reduced much further in other amps with feedback.

when the 300b valve came out, feedback hadn't really been invented, so valves were prized for their inherent linearity.
Engineers think that zero distortion will give perfect sound. So the 300b became a legend. Engineers are still obsessed with zero distortion, but the 300bs godlike status has remained, even tho it produces lots of distortion compared to a feedback pentode or transistor amplifier.

I hope this is reasonably intelligible, it got more complex than I had wished.

 

Surely you meant class AB? I don't think there are any pure class B audio amplifiers.

On the subject of feedback, how are some solid state amps (SimAudio, Gryphon to name but two) able to get away with zero feedback?

Michael.

 

including my cyrus apa7's
see here for an interesting article on zero feedback.

 

Naim amps are Class B.

Class AB is pointless, worse than Class B as soon as you're out of the Class A zone.

(Class A means that the output devices never turn off, Class AB means that they turn off sometimes depending on load and level, Class B means minimal bias for minimum distortion, or only one side of the ouput can be off at any one time, Class C means both sides of the output can be off together. Many so-called 'Class A' amps are actually Class AB, early Krells, the MF A1 series, etc etc. Class C has applications in RF amps, and Class D output stages, where if both sides of the output are on at all simultaneously there's a big bang.)

Paul

 

I think first to pre-empt any flack from triode lovers, my intention was not to discredit the 300b, merely to say that there is no mysterious magic at work with it, and its high regard is purely because of its inherent linearity....linear from the word line (LINEar) where a straight line means no distortion.
If you operate it wrongly, it could be subject to all other nasty kinds of distortion, as with any other amplifying device.

transistors usually produce more distortion than valves, but the absence of excessive phase shifts and the output transformer means you can apply lots of feedback to reduce this to really low levels.

I don't know how all tranny amps do it, I am not that clever, some by really clever design, some by posh transistors, but some do indeed run their output stages in class A, so you don't get the crossing over effect, which induces nasty distortion. They sometimes have feedback in the earlier stages.
Generally, if it says zero feedback on the tin, it means feedback from the output stage, to the input. This is global or loop feedback.
There may be local feedback where a stage is feeding back into itself. This is again definitions, and meanings and interpretations.

Musical fidelities a1 is supposed to be 8 watts of class A, 12 watts of AB, so when you go above 8 watts, its in AB.

Naims bias is about 10mv, across the emitter resistor, which still places it ever so slightly on, meaning its very toward the B end of AB.

Having listened to a few amps, I have heard class Bs that sound better than class As.

The one thing I can be fairly sure of, is that a simpler designs generally do sound less technical. Transistor or valve.
And I believe, that if there is any truth in the S.E. triode claim, it is because the circuit is so minimal and simple. I think complexity kills music. some tranny amps are more complex than others, partly to get the measured performance superb. So this could be a truth of sorts, the better the amp technically, it MAY not be as musical. I remain with my ears and head open to be convinced otherwise.

 

Which Krells are AB? I find it hard to believe that something that generates as much surplus heat as say the KSA 50 or 100 (the early models) and needs so many output transistors can be anything other than class A.

I remember they got in some trouble for that – they sold the A1 as a 20 watt class A amp, i.e. they lied.

Densen are zero feedback too, though I still don't quite understand what it means!

Tony.

 

Imagine a great long line.

At one end is class A, at the other, class b.
in between is AB. This can be anywhere right next to class A, to right down the other end to class B, where Jap amps, and naims reside.
So if these big krells are AB, and make a shed load of heat, chances are they are reaching very close to the class A point.

Think of it like the solar system, the sun is your class A, pluto class B, Ab could be right by the sun, or right by pluto!

To extend that further, why does class a make less power, well if its going in a straight line, (not orbit) through the sun, the centre spot, and its right by it, it doesnt' travel far, wheras, if its jupiter distance, its got a long way to go back and forth, this is not strictly correct in practise, but is a rough analogy.

 

If you can get someone to tell you what the standing current of a Class A amp is then it is easy to work out what power it can do in Class A into a given load. Double the current, square it and multiply by the load.

So if a Class A amp idles at 1.8A then it could do 103W/8Ohm, 51.5W/4Ohm, 25.2W/2Ohm etc. 100W/8Ohm requires +-40 volts, so our idle dissipation is 144W and we cannot claim Class A at anything over 51 W into 4 Ohm etc, whereas we'd expect to be doing 200W into 4 Ohm on our spec sheet.

So to be 'true Class A' into loads as low as 2 Ohm we can either be a 6W/8Ohm amplifier or bullshit a bit.

I think Krell now have a much more elegant means of biasing their amps, so 'Class' becomes much harder to determine. In reality 'crossover distortion' isn't a big deal, but it's worse in a Class A/B operating outside its Class A area than in a properly configured Class B.

It's worth pointing out that 'Class B' doesn't mean no standing current, as I pointed out in the post above that implies Class C. In a Class B amp the standing current is set so that the output devices hand over to each other as smoothly as the physics allows and with just the right amount of overlap to make that happen.

Paul

 

Paul, do you have any more calculations and formulae+ info. cos I have forgotten a lot of stuff, and the info. is hard to find, too?

 

I think it all follows from Ohm's Law.

The trouble is that generalising gets you into all sorts of trouble, and I've made an error in the above generalisations regarding power outputs peaks and averages.....

There are a bunch of good articles at http://sound.westhost.com/articles.htm just remember that being cynical about hifi bollocks doesn't change the basic engineering....

Paul

 

Firstly, Data - great post.
Secondy, 'venerated' - great word :MILD:.
Thirdly, go class 'T' :JPS:.

 

Hi,

The Western Electric 300B triode valve is revered for a number of reasons.

The valve came on the market in the 1935. The forerunner, 300A came out in 1933. The main difference was that the bayonet pin were rotated 45 degrees. This was done so that the valve could be used as a replacement to a 205A valve.

They had many uses. They were used in transmitters & various amplifiers at the time. The Miltary used them alot as did amateur radio. They were very popular in the early day of cinema sound systems. Another use was in power supplies as regulators.

Their main strength was the power output. They could produce 6 to 9 watts. This was at a time when most triodes struggled to produce more than 3 to 5 watts. Only a few transmitting valve could produce more at the time but a number of these used circuits that require higher voltage supplies.

Another thing in it's favour was that they had a long life. They can last up to 40,000 hours. They do have low distortion. As data has mentioned they are very linear devices.

All this gave the valve a popular life. The early amps were single ended which made use of the properties of the valve. The valve was used in the 2nd world war so production was stepped up of these items. In the 50s, push pull amps, tetrode, pentodes, beam power tetodes came more to the fore offering more power. Certain distortions were reduced. The output triode went out of fashion to the masses. Production still continued due to the fact they were a large number of piece of equipment still using them. The rise of the semiconductor also added to it's downfall.

Pockets of valve users around the world reported good results using them. They had a purity of sound which a number of solid state ampss couldn't match. This note in Japan where old stocks of the valve were bought from the states. Over a period of time the momentum & interest increased in the valve. By the 80's/ early 90's so a number of amps using 300B in single ended & push pull mode.

Production of the Western Electric version stopped in 80s but started again in the late 90s due to demand. When production stopped a number of companies made their version of the valve.

The original (older) versions of the Western Electric 300B were/are in demand pushing up prices. These are rated to be, by some, the best ones. They were built to a high standard & high cost.

All this has helped to build up the legend of the 300B.

Like with any valve amp others things are very important to the sound of the 300B. Things like output transformers & power supplies are needed to be carefully design to give the best results.


SCIDB