Is there an advantage to more tubes in an amp?

Hey thanks for the kind words folks. I learn more here than anything else and a lot from the folks in this thread.

above by Mark >> "The only thing I don't dig is how 300b or any pre-historic-triode SE amp can play MUSIC? To me it's the same as to build 1w radio that also can play music quite loud especially if placed soewhere in the corner of empty room..."

What do you mean you "don't dig" Mark? I'm not sure I get your point.? Do you think low powered tube amps are nonsense? I don't think they are for everybody but they do a lot of things quite well.

>> said above by Sean .."On top of this, i've found that tubes (even from the same manufacturer ) have a far wider level of operating tolerances than SS devices"

I thought that fets and mosfets were far less consistent than tubes? A 30% tolerance in transistors is fantastically good isn't it?

I'm not much re SS but I'm told the specs show that one transistor will have twice the capacitance of the other - thus half the speed...ie the NPN and PNP "complements" really aren't a true complement. MOSFETs being the worst in this regard. These capacitance nonlinearities get worse when you have more than two transistors in the output stage. You have large, and dissimilar, capacitances that are all over the place, and drifting slowly with temperature. It takes a lot of current to charge them. I’m not sure that SS is an answer to poor tolerances or stray capacitance issues. They are different problems but they are still there.

Anyway I’m really NOT that well informed about the SS stuff as I don’t really mess with it much. Really just asking. your comment re tolerance surprised me. Tubes have some weaknesses I wasn't sure that was one. I'll have to check it out.

There are lots of handy tricks to bring aging a tubes back into line such as simple cathode biasing which makes a stage almost self adjusting (if the current goes up in one it increases the bias which, in turn, decreases the current.)

Anyway
Cheers
I remain,

Clueless: My experience has been that transistors made by the same manufacturer from the same batch are relatively close in operation. In fact, so long as the manufacturer maintains consistent standards, i've found them to be pretty good from batch to batch. As a case in point, i've been able to pull a dead transistor(s) out of a circuit and install a new one of the same brand with nary a problem. Bare in mind that this is taking place at RF frequencies with multiple pairs ( up to 96 output devices ) running in parallel. Should something be funny, i.e. dis-similar conductance characteristics, the results would show up VERY quickly in terms of distortion and circuit instability.

Obviously, others share similar experiences / thoughts as to what you stated about mismatches, especially between npn & pnp's. Some manufacturers have gone WAY out of their way to avoid such situations. I know that Plinius uses outputs of only one type ( can't remember if it's PNP or NPN ) in their output stages.

I can't say the same thing about tubes based on my own past experiences and those of others. One of the EE's that i used to work with at another job used to work for RCA in one of their tube production facilities. He told me that tubes were all over the place in terms of specs and that they varied the "acceptable performance standards" based upon how each batch of tubes turned out. As such, the "best" of one batch might be equal to the "mediocre" tubes from the last batch. As such, not only did the "best" tubes from each batch vary, so did those that were at the bottom of the barrel.

I found much of what i know about tubes the hard way after purchasing a company that manufactured tube based RF amplifiers. The funniest thing that i've noticed is that specific tubes that are highly desireable in RF circuits in terms of high output levels and ruggedness are supposedly less desirable in audio circuits. The green label Sylvania's come to mind here. Another thing was that tubes that we knew to be of very high conductance were rarely used in audio circuits. While some amps did use 8417's, i've never seen an 8908 or 2057 in an audio amp. I have to imagine that there are reasons why, but my thoughts are that a tube that had high output levels up that high and was quite rugged would be coasting down low in the audio band. The interesting thing is that i'm beginning to see more tube amps using tubes designed for transmitting purposes pop up lately, so maybe my thinking was that far out of line.

Marakanetz: There are SS devices that can put out GOBS of power by themselves. The only problem is that they require both very high voltage and very high current and require tons of heat sinking. This means that the manufacturer has to have very sturdy power supplies capable of very high steady state current levels with enough heatsinking to handle such tasks. As such, they are not suitable for high profit margin operations or where one would need mass production / mass distribution. The weight alone would kill them in terms of shipping. As to whether or not such devices are suitable for use in the AF ( Audio Frequency ) range is also another matter.

As a side note, what are commonly called "ceramic" or "steel" broadcast tubes are basically a hybrid between a tube and a transistor. There are devices like this that can output 15 KW on a steady state basis that you can hold with one hand. The amount of heat generated by such a device is absolutely incredible though and i would not want to be in the same room that it was being used in : ) Sean
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Good stuff! Thanks for all of the replies.

The links are great...I'm working my way through them.