Hi guys, quick question. My amp just recently lost a fair amount of gain on the lead channel, bringing it down from "Brutal" to "Not Brutal". It's been shifting between having that extra gain kick in and not having it. I happen to like the extra gain (it's kind of why I bought the amp) and am thinking it's time for new preamp tubes. But before I buy anything, I'd like to know:
-How I can check to ensure that the problem does in fact lie with the preamp tubes
-How I can tell if I need to replace all 6 of the tubes (expensive) or just one or two (much less expensive)
-Anything I can do to increase tube longevity (I have a combo amp if that matters)

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FrustratedRocka you are a legend

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The man clearly knows his shit.

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one of the best, educated and logical posts I've ever seen on UG in the Pit. Well done good sir.
I test to see if a tube is microphonic by tapping it with the eraser end of a pencil...if a tube is microphonic many things happen....loss of gain, tons of feedback, etc. 311 has a video somewhere where he shows how to do this.

EDIT: if your problem is just on the lead channel...if you tell us what the amp is, we might be able to figure out which preamp tube(s) are for that channel, then you could start by testing that tube. It may very easily the culprit...
Last edited by eyebanez333 at Aug 3, 2010,
You can start by just buying one 12AX7 and trying it in different positions until you find the culprit. If it's anything but the phase inverter, swap 'em all out, they're due. For a test valve like that get a balanced one.
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swap your v1 and v2 and see if the problem goes away.
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