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A (set of) query(s) about Valve Amps

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DISCLAIMER - I am posting this in multiple places, for maximum exposure/to get as many answers as possible, so sorry if you get fed up of reading it multiple times.....!)

Anyway......I was watching a video reviewing and comparing small valve amps (albeit guitar.....15/20W) , with a variety of speakers (10" and 12") and valves (types and quantities...ECC83's and 81's, EL34's, 84's and 6L6's, and anywhere between 2 and 6 tubes)
Now I know different valves give different sounds, and different arrangements of components used also influence characteristics....
So......during the review they described the amp with the most valves (6, 3 pre and 3 power) and least (2, 1/1), both same power, as 'lacking headroom' (which I get to mean that they were starting to distort with little provocation.....) So...what does the number of valves have to do with sound/character
Also two amps had exactly the same valves in the same configuration, wattage and speaker sizes......but did not sound that similar.....(both best sounding imho) Is the difference down to the other parts of the amp being different from each other?
And finally.....it is obvious that number of valves has little to do with wattage (most and least valves were both 15W, and 20W had 3 and 5 valves), so what determines power?

I am sorry if these are just stupid questions, I was just curious.....

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The number of valves has little to do with the sound character, that's mainly determined by other components. The number of output valves has a lot to do with output power, but so does the valve type, as do the power supply and output transformer.

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A valve (tube) is very close to an electro-mechanical component because of the size, parts and manufacturing tolerances. It is no wonder that the parts differ that much from each other within one model. This is actually very much the same with nearly any other electronic component.

Now I simplify a lot, but overall you might say, that the system works through choices of manufacturing and testing:

Plain and simple resistors have nine (yes, 9!) tolerance classes: from 0.05 to 20 %. Batches have certain tolerances and the process has many parameters. Accuracy has its price. There are these manufacturing lines that produce a variety of carbon resistors without specific resistance. Then these parts are classified and they get their markings. Remember: a cheap 1 kohm carbon film resistor with 20 % tolerance may lead from 800 to 1200 ohm unit - the difference between these two is 400 ohms! Don't talk about precision, talk about price. And how many of these cheapest resistors your amp had?

It is not only the single valve that affects the performance. It is the parametric jungle that the signal sees while wandering through the different (sic!) components. If you need to copy some design accurately, beware that it is expensive. Even digital systems suffer from the same issues, because parts from resistors to microprocessors all have the same production issues. Some microprocessors are faster and still they come from the same silicon and the same process.

Please take a look at Claude Paillard's tube "factory". Notice those simple tools and give a thought to tolerances.

https://www.youtube.com/watch?v=EzyXMEpq4qw

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It's hard to generalise between number of valves and headroom or character.  Preamp valves are not only used as simple gain stages - the number can vary depending on the approach the designer has taken, and whether any are used in auxilliary roles like reverb, active EQ stages or FX loops. 

And in power amps, a pair of EL34 (for example) could be used in amps ranging from around 20 watts to 80 watts depending on the voltages and topology, and with or without a valve rectified power supply. 

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