Problem with my Bogner. Correct diagnosis (tubes)?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Rampage, Jan 8, 2008.

  1. Rampage

    Rampage Member

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    Location:
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    I picked up a Bogner Shiva and I went through the following:

    - I fired the amp up and all was well for about 5-10 minutes of playing then a very noticeable hum/hiss began. This isn't my first tube amp by any means, so I know there has to be at least something wrong. The sound was very shrill and steady. It was also on both channels, therefore I believed it to be a microphonic tube in either V1 or the PI.

    - I tapped on every tube with a pencil eraser while the amp was on, however I couldn't really hear a noticeable tapping sound coming from the speakers.

    - I let the amp cool overnight and took the tubes out today. One of the 12AX7s (an ARS Chinese) has a very noticeable jingling when it is gently moved around. I believe that this tube is my huckleberry.

    - I moved around all the tubes and put the believed culprit in V4 to see if there was a difference. I started up the amp and played through it really quick and the sound is completely muffled; the channel and master volumes needed to be very high to even get a level lower than what could be considered bedroom volume. This to me confirms the idea of a shot preamp tube or tubes.

    - Summary
    • All tubes light up
    • There was a very shrill and steady hiss/hum
    • I checked the tubes and one 12AX7 had a noticeable rattling
    • I moved the tubes around and the amp is now incredibly muffled (makes me believe this is a preamp problem)
    I am going to order some new tubes from Doug's Tubes, but prior to doing so, I wanted to see if anyone had any other ideas about what could be going wrong with the amp.
     
  2. devbro

    devbro Member

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    tubes...........be sure to put Chinese preamp tubes in it. Don't waste money on NOS for your Shiva - been there done that.
     
  3. twangbanger

    twangbanger Supporting Member

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    Yep, get some 9th gen. chinese tubes from Dougs and I put a tung sol reissue in V1 and like it alot.
     
  4. Swarty

    Swarty Member

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    It sounds more like a power tube issue.
     
  5. jspax7

    jspax7 Member

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    Always keep a good spare 12AX7 to troubleshoot your amp.

    Put the new tube in V1. If the problem persists, V1 is not the problem.

    Put the old tube back, and try the new tube in V2.

    Continue the process until you find the culprit. (V3, V4, etc...)

    One preamp tube is cheaper than a new set, and a new set may be unnecessary.

    Having said that, when you replace the power tubes, get a matched quad. (If your amp uses 2 power tubes) Have the bias set, and you'll have a backup set of power tubes in case one fails.

    Always carry at least 1 spare preamp tube, and one set of power tubes. (Don't forget to pick up an extra fuse...)
     
  6. Rampage

    Rampage Member

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    Location:
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    Alright, well I replaced all the tubes, but I am still experiencing the main issue, but I think I may know part of the reason for it.

    I got the new tubes yesterday (Doug's Tubes is fast!) and rotated the preamp tubes throughout the amp and found that I had two bad preamp tubes in there. I also replaced the powertubes and everything was going well.

    I played for about 40 minutes with no problems. This was with the back panel off the amp.

    Today I reinstalled the back panel and after playing for about 20-30 minutes, that ultra annoying high pitched squeal came back.

    This is leading me to believe the amp is getting too hot and this squeal is a result of that. I have a Trafficgreen2+ Shiva (new version with the variable boost and bright switches) which lacks the fan that prior Shivas had. I am wondering if the high plate voltages of the EL-34s in the Shiva could be overheating the amp and causing this annoying squeal. I have owned quite a few amps and it is easy to tell that this isn't just natural tube hiss or white noise. It definitely doesn't belong.

    I have swapped all my cables and taken everything out of the chain, but the noise is still there.

    Here are some things about the squeal:
    - Only starts after about 20-30 minutes
    - Goes from no squeal one minute, to a faint squeal which then increases in volume rapidly
    - The squeal cuts through playing
    - The squeal exists when the guitar volume is off
    - Happens both when there is something in the loop and when there is nothing in the loop
    - When the amp is switched to standby, you cannot hear it through the speakers of course, but you can hear a squeal coming from somewhere

    Taking this to a tech is my last resort at the moment. I really cannot afford the fees right now and I don't want the amp to be sitting in a shop for a month or longer.
     

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