New amp, tube issues

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by rollyfoster, May 27, 2015.

  1. rollyfoster

    rollyfoster Supporting Member

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    I just took delivery of a new to me Redplate RP40 and have a couple of problems with it.

    1. A power tube (6L6) has a glassy rattle. If I press on it the rattle stops. Switched positions between the power tubes and tried re-seating them and it's still there.

    2. If I tap on the amp or knobs I can hear it through the speaker. It's like a metallic echo kind of sound.

    It's a cathode bias amp with (2) 6L6GC

    Please tell me this is just as simple as replacing the tubes.
     
  2. xtian

    xtian Member

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    Yes. You have a microphonic tube. Not at all unusual, unfortunately.
     
  3. rollyfoster

    rollyfoster Supporting Member

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    Ok thanks. I guess the next question is can I just replace them with a matched set as long as they are GC? I ask because I had to specify "boogie spec" when ordering for my Mark IV but maybe that's just a mesa thing.
     
  4. DaveKS

    DaveKS Member

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    The rattle and microphonic tube are probably 2 sep things. Go along each tube and lightly tap with chopstick or pencil, listen for which one makes a tinking sound when you tap the glass. Usually it's preamp tubes that go micro, though it can happen on power tubes to, but their usually not that severe sounding.
     
  5. pdf64

    pdf64 Member

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    To confirm it's all from the power tube, try the same test above but with the phase splitter tube (closest to the power tubes) removed.
    Check with Redplate; it may have separate cathode bias resistors for each tube, in which case you could just replace the bad tube (it will likely need to be a 6L6GC, though maybe 6L6WGB = 5881 could cope).
    http://www.redplateamps.com/uploads/1/6/1/6/16163768/rp40_operations_guide.pdf
     
    Last edited: May 28, 2015
  6. rollyfoster

    rollyfoster Supporting Member

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    Will try that out, thanks. I do have a spare new 12ax7 I can try in there, although it isn't the preferred long plate.

    Edit: Doesn't seem to be the phase tube. Tapping on it still comes through and even rattles the power tube. If I apply pressure to the 6L6 it greatly reduces the noise transferred as well as the rattling.

    I suppose it got damaged in shipping since the tubes weren't removed.
     
    Last edited: May 28, 2015
  7. Blue Strat

    Blue Strat Member

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    Preferred by who? There's no consensus on what's "best". Also, long plates are more likely to give you microphonic issues...especially in a high gain amp.
     
  8. rollyfoster

    rollyfoster Supporting Member

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    Preferred by Redplate per the manual.
     
  9. RussB

    RussB low rent hobbyist Silver Supporting Member

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    What brand tubes are the 6L6GC's?
     
  10. RussB

    RussB low rent hobbyist Silver Supporting Member

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    That's just weird. The preferred 12AX7 tube should always be the tube you prefer
     
  11. rollyfoster

    rollyfoster Supporting Member

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    Svetlana. The red ones. The phase tube is a Mullard and the others are JJ

    I'm not sure if I'm using "microphonic" properly but if I press upwards on the bottom of the tube it stops rattling and when I remove my finger it makes a sound like blowing in the top of a glass bottle so it is doing something that is transferring extraneous sounds to the speaker. And there is tiny amount of static.
     
    Last edited: May 28, 2015
  12. EFK

    EFK Supporting Member

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    That may or may not have anything to do with the issue. Many, many amps ship with tubes loaded and if well-packed are no more prone to tube damage than with tubes removed. Any tube, new or 'vintage' can be microphonic right out of the brand spanking new (or new old) box, or can become microphonic after an hour of use, or 10 hours, or 100 hours etc. There is no way of which I am aware to test for this, short of sticking a tube in a combo amp and hanging in there for the ride. Sometimes those orange high-temp silicone damper rings will help with this, sometimes they will not. Unfortunately you might replace a microphonic but otherwise perfectly functioning tube with another and develop the exact same issue. It's the nature of a combo amp, or even, a head if sitting on a heavily vibrating speaker cab.
     
  13. rollyfoster

    rollyfoster Supporting Member

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    Here's a short video of the problem:


     
  14. mc5nrg

    mc5nrg Supporting Member

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    What happens when you play thru it? I would expect some sound if you tap on a tube. Its a problem if that creates a runaway howl...
     
  15. DaveKS

    DaveKS Member

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    That's def severe microphonics, but sometimes that thump sound can just be vibration transferring through chassis to the real culprit, usually a preamp tube but can be a power on occasion. You need to pull off all those tube covers and go along and lightly tap with chopstick or sharpie. Go down line using same force, thump, thump ,thump, then you'll find one that tinks, you can actually hear yourself touching the glass. That's the worst one, replace it. Then lather, rinse, repeat. When it gets down to all a bunch of thumps, then it gets harder to tell which one because chassis transfer of vibration. At that point it's almost better to have some known good tubes and switch them in one at a time.

    I've had a few preamp tubes that were that severe over the years, but I don't think I've ever heard a power tube that was that bad, but anything is possible. I really think you've got a combo of a slightly micro power tube and a really bad preamp tube or maybe two.
     
  16. rollyfoster

    rollyfoster Supporting Member

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    I pulled P2 and it was rattling like a mofo so I replaced it. The noise was still happening in the power tube so I guess they were working in tandem.

    I ended up retubing the whole thing except for the phase inverter tube. Unexpected having just received the amp but at least it was just tubes.
     

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