hey guys, not all my preamp tubes are glowing, well i can only see 2 with a orangey-blue colour, does it mean that the others are dead? is it possible to run on 2 preamp tubes and all the power tubes?
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Hmm. I think it would just cut the wattage down. Or is that power tubes?

well moral of the story is.. replace all your pre amp tubes.
I'm pretty sure it's the power tubes that you take out to cut down wattage. You should change all you pre-amp tubes, you amp will prabobly sound like **** without all it's pre-amp tubes.
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does your amp sound funny? if not, leave it as is, order some new ones, and if it sounds like they need replacing, replace the 'broken' ones, but preferably all of them.
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I would think the amp would sound not quite right, since preamp tubes shape the sound. I think they should glow. Get some replacements, if the new ones do the same, then at least you have some back ups for when one does fail.
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They're tubes, not lightbulbs. If the amp sounds and works fine, don't worry about it.
If you pull out a pre-amp tube, you'll get more gain, due to the fact that there is more current running through the other pre-amp tubes. You have to know which tubes are which though, as you don't want to pull out V1, the phase inverter, or your reverb/tremolo/effects loop driver.

Anyway, pre-amps tubes don't usually glow a ton, you will probably only get a faint glow. My guess the the ones with the orangey-blue color could be bad if you're getting a funny sound out of your amp. If they're all factory tubes, you might as well replace the whole lot of them, due to the tubes could have been rattled in shipping and whatnot. But, KEEP YOUR OLD TUBES!!!! Sometimes they sound good if you swap different ones out and tubes that sound bad in one amp may sound amazing in another.

BTW...Tubes and lightbulbs are the same principle...current, filament, the whole 9 yards.

EDIT: Just noticed it's a 5150...which tubes are glowing and which aren't?