[MF_Kitten]

ok, so when playing the Fireball i use for band practise, i noticed a nasty sound coming from it, kind of "on top of" the sound. it was like a really bright as hell tinny screechy noise, kinda like super-high frequency feedback... so i show it to our singer, who owns it, and since the distortion sounded just fine, and the noise was "over" the sound itself, i figured we should change the power tubes, and see if it helped.

so we changed the power tubes, and tadaa, didn´t help

in fact, the problem was worse now. if i had a high gain setting on, and rolled the volume of my guitar all the way down, and then turned up the volume, the amp went screeching like hell!

so we tried leaving it on, and carefully bumping different things on the amp with a rubber covered tool, and we found out that it wasn´t the tubes themselves that were doing it, but something somewhere was picking up something in the amp. if i bumped the top of the amp head itself, it made bright squeals, and if i went inside it and bumped the black metal cover that the tubes were protruding from, the same noise resulted. it was impossible to determine what part was microphonic!

i tried gently tapping the pre-amp tube covers too, and it made the noise no matter where i tapped... so yeah, wtf do i do? :P

[Shaman]

Well, in cases like those, 99% of the time it is a preamp tube gone bad. I have never heard of a power tube go microphonic..

My Invader had a preamp tube go bad after two days of owning it, and yes, I got it brand new.

On my Invader, it was the first preamp tube, which was covered with a metal cover. You should try to remove the cover, if you have one, and try tapping on the tube like you did. The tap might not even be audible, so you should try changing the preamp tubes one by one just to see if one of them is the "broken" one.

Good luck!

[MF_Kitten]

ah, then it´s settled! Dave Weiner also said the same thing, and the previous owner agreed, and said he´d come by with tubes, locate the fucked one, and replace it for us

weeee! :P