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kayla
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Joined: 11 Mar 2011
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Location: Central PA

 Post Posted: Wednesday Oct 23, 2013 
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I have a Fender "Evil" Twin. Its a two channel amp with a 25w and 100w setting. I usually keep it on the 25w channel (who needs 100w?). ha but anyways.

i'm tryin to trouble shoot my issue. I've been getting a very high pitched, similar to microphone feedback squeals as soon as i turn on the over drive. I've change my pickups in the guitar, changed guitars, removed pedals and plugged direct and i've still got a squeal every once in a while. It doesn't happen all the time.

The last time i blew a tube it didn't sound like this. The amp just didn't work. So the squealing is confusing. I checked to make sure the tubes were secure. The tubes were changed a few months ago according to the previous owner.

i also check the power supply, thinking we overloaded the circuit (or whatever it is), and didn't fix the problem.

any ideas? i'm thinkin i just need to find a tech to look at it.

thanks!

- kayla.
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StumbleFingers
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 Post Posted: Wednesday Oct 23, 2013 
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It sounds like you may have a microphonic tube - it hasn't completely failed, it's just noisy. You can try a tap test. Tap lightly on each tube with something solid like a pen or a fingernail. If one makes a glassy or metallic sound, swap it out. (I've always done this test with the amp on, which may or may not be foolish.) Or just swap them by trial and error to see which one is the culprit. If you know which preamp tubes are used for the overdrive channel, you can start there.

You might also try lower output preamp tubes if the amp is just noisy by nature. Or it may just need a trip to the shop.
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tornandfrayed
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 Post Posted: Friday Oct 25, 2013 
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Kayla, I have the old set of tubes that was replaced. I only replaced them because I was not sure how old they were and the guy I bought it from had no idea. If you want them you can stop by and get htem. That way you could have something to swap out. Did the squealing just start? Did you try a different guitar?

Dave
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kayla
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Joined: 11 Mar 2011
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Location: Central PA

 Post Posted: Friday Oct 25, 2013 
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tornandfrayed wrote:
If you want them you can stop by and get htem. That way you could have something to swap out. Did the squealing just start? Did you try a different guitar?


ya it just started. i ran it hard from the time i got it from you til now. i've done some research on this particular amp and it seems blowing tubes was one of the reasons it was only in production for a few years. i tried a different guitar, same result. it's just odd, it doesn't happen all the time. couple times i was hitting my over drive to go into a lead and it would start squealing, then one time i went to start a lead and had no volume at all.
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bassist_25
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 Post Posted: Saturday Oct 26, 2013 
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Another vote here for a microphonic tube being the culprit. Oftentimes, microphonic tubes will continue to ring after certain notes but will be dead quiet other times. Sometimes tubes will become microphonic with no warning. You'll just plug in one day and will have a bad tube. I had a nice RCA 12AX7 go microphonic in my SVT last winter. That was a depressing day.
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Ron
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 Post Posted: Thursday Oct 31, 2013 
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Kayla,
If you swap out tubes and the problem remains, it could possibly be that one or more of the tube sockets need to be cleaned, resoldered or retensioned. I used to do tube amp repair many years ago and socket issues were more common than tube failure, especially for amps that spent a lot of time in storage or other less than ideal climates.
I've seen them get so bad that the sockets needed to be replaced.
Here is a link to a socket resoldering/retensioning tutorial.
http://billmaudio.com/wp/?page_id=171
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