Express 5:25 with possible microphonic tube problem?

Hey guys,

I have a 5:25 that has been sounding a bit "thin" lately. Using three different guitars if I play certain notes or chords (open G, D, C etc) I get a high pitched ringing which is mixed in with the played notes. Having never heard what a microphinc tube sounds like could this be the problem? I've changed the power tubes but that didn't help. I also swopped the pre-amp tubes around but although the problem changed sound a little it was still there. I live in Dubai and have been trying to track down any 12AX7 to use as a tester but still no luck. I guess I'll have to order a full set from the US and hope that cure the problem. Any comments and suggestions are appreciated!

Grant777 said:

Hey guys,

I have a 5:25 that has been sounding a bit "thin" lately. Using three different guitars if I play certain notes or chords (open G, D, C etc) I get a high pitched ringing which is mixed in with the played notes. Having never heard what a microphinc tube sounds like could this be the problem? I've changed the power tubes but that didn't help. I also swopped the pre-amp tubes around but although the problem changed sound a little it was still there. I live in Dubai and have been trying to track down any 12AX7 to use as a tester but still no luck. I guess I'll have to order a full set from the US and hope that cure the problem. Any comments and suggestions are appreciated!

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" if I play certain notes or chords (open G, D, C etc) I get a high pitched ringing which is mixed in with the played notes. "

Had a very similar issue myself. I traced my rogue noise down to the aluminium spring loaded retainers/heat shrouds for the 12AX7 valves. While someone plays the note that seems to produce the noise apply a gentle pressure using a thin piece of wood to each of the metal shrouds in turn.
On mine it was the middle one that was vibrating and could be heard through the speaker. A gentle pressure with a piece of wood made the noise dissappear.
I removed the shroud but could not see anything wrong with it, anyway I refitted the thing and the noise wasn't there anymore.

Unfortunately it has since come back. It's the same valve causing it and if I gently push against it the noise stops as before. It's not quite as bad as it was before so I just live with it now.

Actually I've just had a thought, a piece of folded cardboard wedged between the the offending valve and it's neighbour might just do the trick.

Good luck with yours.

Very interesting! Thanks. I'm am about to change all the preamp tubes but might give this a go. I did take all the covers off and see if the problem was still there...It was. A friend says I should get out more, play it at a louder volume and that tinitus is my friend

Cheers. G

A high-pitched ringing is definitely a microphonic tube. It's actually feeding back because the speaker is vibrating the innards of the tube, causing it to ring like a bell. I'd try switching it with another tube in the row, one that is further away from the input jack, probably V5, the last one. The manual with the tubes' positions and what they do is available at Mesa's site. The middle tube is V3. The first tube is V1, the tube closest to the input.
Hope this helps.