Hi all. I just tried to change the tubes in my Mesa Mark IV from EL34s back to 6l6s. I put in the new tubes, turned the amp on, and when I took it off standbye it makes this terrible humming noise through the speaker. So I switch back to the tubes I used before, but now the amp won't even turn on, not even the fan. One thing I did notice: one of the new tubes didn't have that central bit that all the pins surround, but instead it had what is best described as a nipple of sorts; do you guys think that made any difference? Somebody please tell me it's not dead.
Check the fuse?
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Doesn't sound reassuring.

Post it to me, it may as well be bin fodder...


On a serious note, no idea. Doesn't an amp need biasing for different types of tubes?

And it may well be just a fuse.
^No, Mark IVs have a fixed bias and can take EL34s in the outer two sockets, which is what I was doing before.
Definately change the fuse(s) before anything else. If that fails it could be an OT gone or something so then it would be best to take it to a tech.
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Fixed bias is definitely a wrong choice of words that amp makers have used. Fixed bias actually means the bias can and does need to be adjusted.There are a lot of great articles on biasing out there. Take a look.
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