PLEASE HELP ASAP!!! Anyone ever heard of this problem?!?

[jehu]

i have a 97 mesa dual rec 2 channel. tonite it started "whistling", only stopping when the stand by is on. when i changed the rectifier setting from diode to tubes, it reduces the sound significantly but it is still slightly there. it's almost like a kettle whistle but not quite as high pitched. at first i thought it might be a tiny tear in one of my speakers. either way baby both the head & the cab & haven't changed the tubes in about 6 months. for the record the tubes are jj 636's. has anyone ever heard of this?!?

thanks for helping.

 

[mikey383]

It's a microphonic tube. Tubes will pick up outside interference when they become microphonic, and that creates the high pitched whistling you are describing. Take a pencil or other similar non-metallic object and tap on the power tubes. Three should sound similar, and one will probably sound different. That one is your microphonic one. Just replace it with a new one.

 

[Platypus]

I would check your preamp tubes out

 

[jehu]

It's a microphonic tube. Tubes will pick up outside interference when they become microphonic, and that creates the high pitched whistling you are describing. Take a pencil or other similar non-metallic object and tap on the power tubes. Three should sound similar, and one will probably sound different. That one is your microphonic one. Just replace it with a new one.

i definitely thought it could be the tubes, but i have just never actually heard an amp make that sound. what would happen if i replace a single tube, won't it be unmatched?!?

thanks so much for your help!

 

[jehu]

I would check your preamp tubes out

i assume with the same pencil trick as the power tubes?!?

thanks

 

[srf399]

Your owners manual has an entire section that tells you how to deal with this problem. Best think to do is pull the preamp tubes ONE AT A TIME.... and trying the amp with the one preamp tube replaced with a new one. This will isolate the problem tube. If one is gone you probably should think about re-tubing a t least the preamp tubes. They're pretty cheap.... and will only run you about $60

 

[recto-robbie]

Its a micrphonic preamp tube. With the amp on,, lightly tap each preamp tube with a pencil and the faulty one will either sound different than the rest or may amplify your tapping above the rest of them. Replace.
Or replace each and every one of them with a new tube one at a time until you find the culprit. I find that the pencil trick usually works and if it does, most likely it will save you much time compared to replacing each tube individually.

And no it wont be mismatched,,, preamp tubes dont have to be matched,, they are swap and go and they dont have to be Mesa tubes either. The power tubes need to be matched.

 

[jehu]

Its a micrphonic preamp tube. With the amp on,, lightly tap each preamp tube with a pencil and the faulty one will either sound different than the rest or may amplify your tapping above the rest of them. Replace.
Or replace each and every one of them with a new tube one at a time until you find the culprit. I find that the pencil trick usually works and if it does, most likely it will save you much time compared to replacing each tube individually.

And no it wont be mismatched,,, preamp tubes dont have to be matched,, they are swap and go and they dont have to be Mesa tubes either. The power tubes need to be matched.

1st off thanks. i knew the pre amp tubes didn't have to be matched. just so i have it right tho, tap the pre amp tubes with the wood side of the pencil w/ full power on & stand by off, correct?!?

thanks you so much guys!!!

 

[recto-robbie]

Yes tap the with the wood part of the pencil. People say use a pencil because I assume someone will try tapping a hot tube with a screw driver or something and wind up with some major issues.
And yes you want your amp on,,, it doesnt have to be cranked but it does have to be loud enough to hear. The microphonic tube will be heard through your speakers. The last time I had one that went microphonic on me, it whistled like yours and got worse with more gain.
When I tapped the good tubes I noticed nothing different but when I tapped the bad tube I noticed that the whistling would either go away briefly or come on louder. I replaced that tube and troubles went away.
Good luck and tap nicely,, dont smack it.

 

[mikey383]

Yeah, preamp tubes...that's what I meant to say. Sorry for any confusion.