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Old 01-17-2013, 10:12 PM   #1
Zan595
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6505+ Tube Problems

Aright, I ordered a set of new 12ax7's from Dougstube's and replaced my old ones. Everything went smoothly for about a week. Now, there's an incredibly loud humming noise whenever the amp is on. I opened the back up where the 12ax7's are, replaced all of the new ones with the old ones, and tried again. Same problem. Also, no matter how I arrange the tubes in the back, it seems like there's always at least one or two tubes that don't light up as the others do. What does this problem sound like to you guys?
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Old 01-17-2013, 10:34 PM   #2
R45VT
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Combo or head?


Why did you replace the tubes the first time?
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Old 01-17-2013, 10:36 PM   #3
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Head with an Uber Cab. I replaced the tubes because I bought the head used over a year ago, and the head was starting to produce noticeably more white noise as time went by. Nothing like the noise it's making now, though.

Last edited by Zan595 : 01-17-2013 at 11:04 PM.
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Old 01-17-2013, 10:50 PM   #4
R45VT
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So lets be clear- the noise is present even when no guitar is plugged into it? It makes the noise on either channel?

You replaced all the preamp tubes-? (and since switched in the old ones)

did you replace power tubes?
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Old 01-17-2013, 11:03 PM   #5
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The noise happens with no guitar plugged in,
I replaced every old pre amp tube, I didn't change the 6l6's.

Last edited by Zan595 : 01-17-2013 at 11:04 PM.
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Old 01-17-2013, 11:05 PM   #6
R45VT
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Then you changed the preamp tubes.

The 6L6 are the power tubes. Pull the outer 2 and test for a quick second(only remove them when the amp is switched off). Then put them back in and pull the middle two.

Just have it on long enough to see if there is any noise. Don't burn yourself either. You may have to let the tubes cool down. Don't stick your fingers in the tube sockets either.

If the noise is present in both scenarios than it is time to take it to a shop.
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Last edited by R45VT : 01-17-2013 at 11:07 PM.
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Old 01-18-2013, 06:27 AM   #7
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Sounds like a shop job tbh...

For pulling the power tubes if you plug your cab into half the impedance (16ohm cab into 8ohm, 8ohm into 4) then you can safely leave it on without it having to be a quick check.

If pulling the power tubes doesn't help, try pulling the preamp tubes one by one, starting nearest the input jack. My hunch is failing filter caps on the preamp tube rectifier circuit (you don't need to know what that means, but if the noise goes away when V1-4 are out its a distinct possibility and you can at least ask your tech to check).
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