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Well, I got 4 new 12AX7 Electro-Harmonix preamp tubes today from The Tube Store, they're balanced and matched with both triodes in each tube rated at 110. I put them in my Mesa F-30 combo, and I'm still having the same kind of feedback problems at low volumes that I was before. I guess it's the 2 EL84s that have gone microphonic on me. I'll have to get new ones and hopefully all will be fine again.
I found this bit of info on EuroTubes' site.
"Tube Noise
All 12AX7 type tubes are microphonic. Some are just more so than others. You can check your pre-amp tubes by turning the amp on, gaining it up and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite). You will be able to hear the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after it’s tapped. If it does ring then it’s what is called microphonic and should be replaced.
In the case of very high gain amps like the Peavey 5150 and Ultra or the Mesa’s and Marshalls you will find that by simply swapping the pre-amp tubes around you can find a combination which will yield the least amount of noise.
Power tubes on the other hand are rarely microphonic. EL84’s seem to be the most susceptible to being microphonic and having filament rattle. The worst combination of course is a combo amp using EL84’s especially some of the high powered one’s. The speaker vibration can rattle those little tubes to death in short order. All power tubes can be checked for being microphonic just like pre-amp tubes."
I found this bit of info on EuroTubes' site.
"Tube Noise
All 12AX7 type tubes are microphonic. Some are just more so than others. You can check your pre-amp tubes by turning the amp on, gaining it up and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite). You will be able to hear the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after it’s tapped. If it does ring then it’s what is called microphonic and should be replaced.
In the case of very high gain amps like the Peavey 5150 and Ultra or the Mesa’s and Marshalls you will find that by simply swapping the pre-amp tubes around you can find a combination which will yield the least amount of noise.
Power tubes on the other hand are rarely microphonic. EL84’s seem to be the most susceptible to being microphonic and having filament rattle. The worst combination of course is a combo amp using EL84’s especially some of the high powered one’s. The speaker vibration can rattle those little tubes to death in short order. All power tubes can be checked for being microphonic just like pre-amp tubes."
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