Two Tubes Redplating (See Inside for Free Bonus Symptoms!)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by The Smith, Aug 30, 2008.

  1. The Smith

    The Smith Supporting Member

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    Hi,

    I have an AC-30 style amp. The other day (right before a gig) , the following problems began to occur:

    *Turn on and get volume at about 5% of what it should be (95% drop). What volume there is, sounds distorted. There's also a fairly loud hum once I take the amp off of standby. Oh, and there is some intermittent crackly pops (that last less than a second) and—I believe—signify the transition between working properly and not (though I haven't tested this enough to confirm it yet). After a minute or two, the amp would work properly and played fine for the first set.

    Left everything on during intermission so as to minimize a reoccurrence. Same problem when I pick up my guitar again for 2nd set: about two minutes of heart attack while everybody waits for me, and then it plays fine.

    Yesterday, I checked out the back of the amp and found two el84s to be redplating while the amp is malfunctioning. After a minute, I heard the crackle-pop, my volume comes back and the tubes cool down. There are 4 el84s, it's the two on the right side (as seen from the back) that are getting hot. Again, the problem is intermittent.

    My first thought is to switch the two power tubes on the right with the two on the left and see if the redplating follows the tubes or stays with the tube positions. I'm puzzled that two tubes are redplating, not one.

    The amp does have a 1/2 power switch, and I briefly tried diagnosing it in half-power, but the problem was too intermittent to draw conclusive results.

    Your thoughts and insights are greatly appreciated!
     
  2. goneracin

    goneracin Member

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    put new tubes in it. one tube may have a loose component in it, causing the problem, or it has some other issue. I tell all my customers with a problem that 95% of the time, any amp issue is a tube, doesnt matter how new they are. Its the easiest and cheapest first troubleshooting thing to try.

    Also, always bring a complete set of tubes to a gig. at least 1 preamp tube, a set of power tubes, and a recto if so equipped. Esp in an amp thats cathode biased.
     
  3. rockon1

    rockon1 Member

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    Also make sure the sockets are tight and clean. Something as simple as a loose socket or dirty socket can cause similar problems. A bit of electrical contact cleaner and a mechanics pick can go a long way-provided you make sure the caps are drained first! Bob
     
  4. WesKuhnley

    WesKuhnley Member

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    Your Dealer and the manufacturer should be taking care of you on this one. Try giving Valvetech a call, contact info is on their website.
     
  5. The Smith

    The Smith Supporting Member

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    Thanks all, I put in two other tubes I had lying around and problem solved.
     

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