The little magic elves trapped inside the glass have different moods. Telefunken regularly gave raises and bonuses to their elves for working hard, whereas Sovtek basically imprisoned them against their will, forcing them to eat nothing but borscht.
Seriously, tube design is partly an art. Tubes work by a weird natural phenomenon where when you heat a metal (the cathode) it emits electrons. Normally they don't get very far because they have limited momentum, but in a vacuum they will jump to another plate (the anode). There's a mesh plate in between that, depending on its charge, deflects electrons, preventing them from reaching the anode. There are millions of variables that effect how this process really works: how pure the metals are that make up the plates, what the exact composition of these metals are, how exactly the plates are arranged (even fractions of an inch matter), etc. Presumably each manufacturer has their own special formula. Sort of like why all fried chicken recipes don't taste the same.
It's like anything else. Materials, design, and construction.
The actual components of the tube, inside, are of varying materials between brands. Also, even connecting wires. Different construction of grids. Spacing of components. The amount of gas in the tube. They are vacuums, but use different getters to get out gas and oxygen...this leaves some gases and some residue which can get on internals. How strong the components are makes a differences...minute vibrations. Durability. tubes can become microphonic to varying degrees....sound can change with vibration or vibration can create sounds.
I dont know why I read this post. I know nothing about tubs. But I sure am glad that I came acrouse it. I cant stop laughing. Elves? I am begaining to think that some of the members are having mental disorders like I am haveing. It is refreshing to know that when I go to the mental hospital that I might find a few members there to talk to about cans and stuff. I have heard that the hospital is staffed entirely by elves.