white noise when switched from standby

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bno, Jun 29, 2005.


  1. bno

    bno Member

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    I am resurrecting an early '70s Peavey Mace that sat in the basement/garage for about 10 years. It was rescued from the aftermath of a fire in '95 and it had been sitting disassembled all these years. I have it working again but when I turn it on, wait a minute to warm up and then flip the standby switch, there is a white noise kind of 'whooshing' sound (think "jet airplane") that lasts for about 5 seconds and then slowly fades. Also, all of the pots are really scratchy and jumpy (humidity/water damage, I'm sure).

    However, once its on, had a chance to settle in and the levels are tweaked, the tone is actually very nice. It was modded in the early '80s at Kolbe Sound to replace the 6L6GCs with Mesa EL34s. I'm only using the inner 4 tubes so its basically a nice warm 100watt British tube 2x12 combo with old Carvin v12s. I like it.

    What's the whooshy sound? Should I care? Replace the tubes? Is there something I can use (aerosol?) to clean out the pots and even them up? Its probably just going to be a unique sounding studio amp, unless I can get it to behave.

    Thanks in advance.

    bno
     
  2. bno

    bno Member

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  3. unclej54

    unclej54 Guest

    i haven't actually experienced your jet engine noise but any time there's a problem that goes away after a bit i first think tubes and then caps. try a little process of elimination. remove one of your pre-amp tubes before turning it on. then turn it on like you normally do. if you still get the sound turn it off, replace that tube and go to the next one and so on until you've checked all of the pre-amp and power tubes. if you start it up and the noise is gone with one of the tubes removed replace that tube.

    if, after removing all of the tubes one at a time the noise is still there then look to your capacitors. someone with more experience may correct me but when caps get old they handle the charge that they take in different ways than intended and that often results in unusual noises. you're electrolytics probably need to be replaced anyway after sitting that long.

    as far as the pots go there are many good cleaners on the market. i personally use radio shack's contact/control cleaner and lubricant on pots and it works fine. just give each pot a little squirt in the small hole on the side and then turn the pot up and down several times and that should take care of that.

    good luck with it.
     
  4. bno

    bno Member

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    Based on info I gleaned elsewhere on the forum I just picked up a can of DeoxIT.

    I'll guess I'll be doing the tube shuffle next.

    I'm only willing to go so far with the project. I wonder if I can do the caps myself? It might be good experience - even if I wreck the amp......
     
  5. unclej54

    unclej54 Guest

    i'm going to strongly suggest that unless you know how to bleed the caps that you get someone else to do it. they hold enough charge that it can actually...literally...kill you. i can't stress enough that you can seriously hurt yourself if you don't know what you're doing...

    but do the tube change thingy first and if that fixes your noise problem you're good to go. if that doesn't fix it then you'll have to decide how much money you want to put into it to have a qualified tech fix it.

    as an alternative i'd give you twenty..no wait..thirty two dollars and twenty seven cents for it right now and pay for the shipping. (that's all i've got in my pocket at the moment):D
     
  6. bno

    bno Member

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    Ah well (sigh), now its gotta go to someone's bench. Loaded the full compliment of EL34's, one tube flashed blue and it started to hum. Swapped out the bad tube, and went back to 4 - this time a different tube did the blue thing and the one next to it went bright white and it popped the fuse.

    re: caps - yeah, I think 350volts would stop your heart - literally. Clear!!! - bshzzzt!!. For the time being I'll stick to building computers, household wiring and repairing appliances and electric tools.

    Anyway. Thanks for the help and advice.
     
  7. TheAmpNerd

    TheAmpNerd Member

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    Well that might be kinda tough cause all the ones
    I've seen don't have pre amps tubes.

    The have a combination of nice op amps,
    and cmos chips that you can't find any longer

    They make little hi dollar replacement cards
    though, thinking they run about $ 40 each
    for the cmos chips.

    Isn't silicon fun. : )

    Good luck with your amp though.
    Check the bias supply and resistors really good.

    Along with all the electrolytics in the amp, I'd
    replace all of them and clean clean clean everything
    in the amp really really well. If the amp was smoke
    filled from the fire...no telling what is going on.

    Be safe.
     
  8. unclej54

    unclej54 Guest

    oops..my bad..i've never worked on that particular model so i was giving him general troubleshooting guidelines.
     
  9. TheAmpNerd

    TheAmpNerd Member

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    I figured as much.

    Plus pulling and replacing transisors isn't
    the easiest thing either.

    Op amps are better, if you get the "right"
    replacements.
     
  10. bno

    bno Member

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    Yeah, fixing this one is beyond me. Truth is, back before it got wrecked those six EL34s put out a very warm and fat clean tone and club volume level. I'll send it out to Sommatone when I have the cash to bring it back.

    Thanks for all the advice. Greatly appreciated.
     

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