Bogner Uberschall twin jet Troubleshooting

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Thread: Bogner Uberschall twin jet Troubleshooting

  1. #1

    Join Date: Oct 2009

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    Bogner Uberschall twin jet Troubleshooting


    I own a Bogner Uberschall Twin Jet with EL34's (jorg installed them straight from the factory)b and had some issues with it lately that I'd like to consult on with you.

    We are recording our first demo now and there's alot of pressure in the studio and I guess that what made me turn the amp on stnadby mode without a speaker load - I've owned a tube amp before and it never happened to me, I feel really bad about it but let's forward with the story.

    The amp stayed on standby for around 5 miutes without a speaker load till we realized it and turned it immidaitely off.

    We turned the amp on again and there was a sound, but after awhile I started hearing strange noises from the amp itself(not from the cab), 15 minutes later the channel light's flickered and the amp didnt make more sound, it did turn on though, it smelled like something was burned inside.

    I took it to a tech, the tech inspected it and said that the fuse was blown and that the power tubes may have got a strong hit and that he's not sure they'll hold on but that I can try using them.

    I took the amp home, played through it for 2-3 hours, everything was o.k, but on the studio the day after when we had another recordng session, the amp stopped making sound after around 15-30 minutes
    of playing, and this time we verified taht everything was in place. This time there was no flickering, the amp just stopped making sound without anything happening, just like that.

    The tech said that if it was on standby without a speaker load than the amp itself couldnt be damaged and it's the power tubes.

    I have a set of TAD EL34 BSTR I can install, I just wanted to consult with you guys and ask:

    1. Is it really most likely the power tubes that are causing the problem?
    2. What is the plate voltage of an EL34 twin jet from the factory?
    3. Where is the trim pot placed?
    4. Around where should I bias the tubes?


  2. #2

    Join Date: Sep 2008
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    The burning smell is bad, bad juju. Either you lost/started losing caps, or the OT, neither of which is good news. Id start with the tubes and see how it works.

    If you pop the chassis out, are there any melted or otherwise defunct looking caps or other bits? Anything ooze out of them or the OT.
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  3. #3

    Join Date: Oct 2009

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    Well the tech that inspected it didnt recognize any of these issues, I also played through it later and it worked just fine, I suspect that the burnning smell was from the blown fuse?
    I read alot of posts claiming there shouldnt be a problem runniong the amp on standby without a speaker load, this is very weird..

  4. #4

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  5. #5

    Join Date: May 2011
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    Did the tech check the screen grid resistors?

    These could have gone.

    I have no idea where the trimpot is placed but it should be pretty easy to spot once you open it up.

    be very careful though.

    To find out the plate voltage connect the negative lead of your multimeter to the chassis (bare metal) and measure at pin 3 on any of the power valves with the meter set on the highest DC setting usually around 600v.

    Also do this with one hand in your pocket.

    then once you have this stick it in the top bit on here and select EL34 and click calculate.
    Weber Bias Calculator

    This will tell you where you should be biasing.

    Does the amp have external bias points?

    Sorry i cant be much more help with this i dont know much about that particular amp.

    If you could post some internal photos i may be able to spot something.

  6. #6

    Join Date: Dec 2008
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    Wirelessly posted (BlackBerry9700/ Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/245)

    I'm a total noobb but when my friend did a similar thing he fucked up the output tranny.

  7. #7

    Join Date: Sep 2008
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    Yeah the OT is the thing id be worried about. I've blown my share of fuses and never once have they made a burning smell. Thats textbook Caps/OT problems

    Anyways, swap your fuse, change your tubes (dont worry about an initial bias, they'll be ok enough to check if the amp works), and give it a whirl.

    As for biasing, do you have a bias probe?

  8. #8

    Join Date: Jun 2006
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    I had the same thing happen with my Uber (original, not TJ). I replaced the fuse and tubes, and it's been fine since. Your mileage may vary though...

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