Tubes Asylum

Push-Pull Tube Amps for Dummies: Questions 1 & 2

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I've been listening to tubed stuff for quite a few years now, but I really don't understand much about them other than changing tubes when they go bad, rolling tubes to make myself even more neurotic than I already am, & adjusting bias.

On the subject of bias, what effect does bias have on what one hears from the speaker? I have noted on a previous amp that I used to listen to, that as the output tubes started sounding old & tired, the bias would start slipping downward & I would correct the problem by screwing the bias pot in until I broke down & replaced the tubes. But assuming that the tubes were fresh & serviceable, what effect would
1) less bias current than called for
or
2) more bias current than called for
have on the sonic effect heard from the speakers?

Question 2 is: what is the relationship between a push-pull amp's power & the number of output tubes it runs with? This question does seem like a no-brainer (I did title this as a "for dummies" question) but for example, I once owned a pair of mono-blocks that ran with a total of 8 6550s (4 per monoblock) that was advertised at 110 wpc in class AB. I now own a stereo amp that runs with 12 EL34s & was advertised as producing 100 wpc in AB. Is this due to the difference between the EL34 & the 6550, or is there something else that is different going on inside the two amps?

thanks/Matt

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