Why so many tubes?

One would think after 117 years of evolution (1904, invention of the vacuum tube) and with only 4 other components involved (R, C, L, transformer) engineers would have reached the optimal configuration for a tube amplifier by now.

But there’s another ’science’ involved. It’s called Marketing ... the art of tempting people to spend top dollar.

Ya- you'd think. They've been making automatic transmissions for a while too, and yet they still fail in the middle of nowhere.


There are actually innovations still occurring with vacuum tube amplifier tech (I can call out a couple of patent numbers if you like). Electrolytic caps keep getting better- and the better they get, the more effectively you can bypass a power supply. Plus we have computer modelling now so you can simulate and optimize circuits (including tube circuits) better than you could back in the days of slide rules.

Paralleling tubes or going push pull can increase power but also can muddy the sound.

The word 'can' in the statement above prevents it from being false. Going PP can also **decrease** muddiness by decreasing distortion. The trick is to not combine PP circuits with single-ended. Otherwise you get some emphasis of the 5th harmonic (this is old news; Norman Crowhurst was writing about this 60 years ago...).


A fully differential amp (with no dedicated phase splitter) will have dramatically less distortion than a single-ended amp. The harmonics generated will fall off at a faster rate as the order of the harmonic is increased as well. But it still falls off with an exponential decay, which is what you're looking for if you want the amp to sound right. The difference in overall distortion can be dramatic- as much as 3 orders of magnitude lower for a given power output; about 1 magnitude lower for a relative output (IOW if both amps are driven to clipping). That translates directly to increased detail and smoother presentation.

Yeah but Ralph you are Da Man with stuff like this so I have to ask, what about going the other way? Why not use one great big high power tube? The reason I ask is some guys I respect are going that way with SET and real happy with it. Of course everything else still matters too but might there some inherent advantage to using fewer tubes?

Not really (if we're just talking about power tubes). All tubes are imperfect. So you run with one tube you deal with its imperfections. When you parallel a great number of them the differences tend to iron out and act more like the tube is supposed to in the data books. But you have a lot more current capacity so more power- without a distortion downside as long as the driver circuit can handle the grid capacitance of so many tubes. And that's not really a problem- we've been doing that for decades.

To give you an idea of how different the distortion figure can be, almost any SET when driven to clipping will make about 10% distortion (for this reason a good number of SET manufacturers spec the power at the 1% or 2% power level. rather than clipping). By comparison as an example, our M-60 makes between 0.5% to 1% THD at full power, but that power is 60 watts, compared to an amp that might be making only 8 watts. You can see that if the M-60 is then operated at the same power levels as the SET that its distortion will be much lower- probably 2 or 3 orders of magnitude lower (since both amps are zero feedback and their distortion decreases to unmeasurable as the power is decreased).


Distortion obscures detail, and it adds coloration since the ear interprets all forms of distortion as some sort of tonality. So even if the amp has a good distortion signature (which will cause it to sound smooth and not harsh) it simply will be unable to express detail in the way that an amp with the same but much lower distortion signature can.


So while the guys you know might be quite happy, that isn't the same as hearing everything out of their recordings that they could. One power tube is fun, it has a nice gothic steam punk appeal and SETs have a good distortion signature even though its quite high. But you can do better. 

There's a 50cc class of racing motorbikes. Back in the 50s and 60s it became all about how many cylinders you could get to displace only 50cc. Things got real small and because things really didn't weigh all that much revs were over 20,000! So they put a stop to it and 50cc is now limited to one cylinder.


In the audio world this would be akin to using a lot of really small tubes to make an amp but it would be a really poor analogy.

Seems like a complicated way to reproduce music. And yes a little bit of laziness involved, but electrical engineers make lousy writers as the subject is a difficult exposition.
So some of tubes are the same as the power transformers? And I am open to SS to being more complicated.

From this I get that tubes are complicated and transistors are moreso. That's correct. The alternative is no music. That makes me crazy and then it gets complicated :)

If direct path is preferred in the speaker by most, why the acceptance of a glass army in one's amp/preamp?

Not sure what is meant by 'direct path'... But if you need a lot of power, you might need a lot of tubes to make that power. If so, they'll be in parallel, which means the signal is fed to all of them and they all work in tandem to make that power. In that particular case, the signal path complexity is the same whether one power tube is used or 20.


We have a good number of power tubes in our amps because the power tubes, which are triodes, don't make a lot of power. But those amps only have one gain stage in the entire amp! That's about as direct as you can get.