My amp broke down, need some advice

Discussion in 'The Squawk Box' started by djuul, Nov 6, 2017.

  1. djuul

    djuul Member

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    Hey guys,

    So yesterday i was kinda ready to rock out and after firing up my Jet City 50 head it didnt make any sound. After some troubleshooting it produced the faintest saddest amount of sound with the gain and master on 10. This makes me believe one of my powertubes wore out.
    Now here is the thing, the amp normally runs on 6l6's but i have none lying around. I do however have some EL34's that i could pop in and see if the powertubes were really the problem. i figured if i keep the master low i couldnt really harm the transformers or anything. is this a bad idea or is it ok as long as i keep the master low?
    Secondly i havent rebiased the amp yet so its still on the factory bias settings. and i happen to know Jet City biases these amps really cold in the factory to prolong tube life. i have some electrical engineering background so could i do this myself or would that be the best way to get to rock out with John Bonham in the afterlife?
     
  2. robbym

    robbym Junior Member

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    I had sort of the same problem with my Marshall JTM-30. Putting a short cable (pedal cable) in effects Out / In (as a loop) fixed the problem.. Not sure if it's any help
     
  3. JohnnyN

    JohnnyN Old School Premium Member

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    Do not replace 6L6 tubes with EL34s unless the amp is designed for it!
    Yes you can adjust the BIAS yourself, but your fingers will be very close to about 475 volt, so do a lot of research, get the right tools and take the necessary precautions!
     
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  4. sonar

    sonar Senior Member

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    I'd check preamp tubes first. Grab one or two new 12ax7's and roll through the positions. Modern 9 pin pre's like 12ax7's have a pretty high failure rate.

    If one of the power tubes is dead you won't get any sound at all. If one tube is failing you usually hear a lot of hum, although low output could happen.

    And stick with 6L6's.
     
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  5. C_Becker

    C_Becker Dat Gibson smell Premium Member

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    Yep, preamp tubes would be my first guess too. Start with replacing V1 and go from there.
     
  6. KenG

    KenG Senior Member

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    When the amp is on and the volume is turned up place your ear very close to the speaker. IF you hear amp hiss I'd start with the preamp tubes.
     
  7. djuul

    djuul Member

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    It doesnt really produce noticable hiss, but it could very wel be one of the preamp tubes. i associated the sound with transformer hum.
    thanks for the help. Guys, ill visit the shop tomorrow to get some extra tubes.
     
  8. Phil W

    Phil W Senior Member

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    It's unusual to see four clichés in one sentence.
     
  9. Big John

    Big John tastes like chicken V.I.P. Member

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    djuul, if the amp is still running on the factory preamp tubes, all of them should be replaced. Jet City used (and still uses to my knowledge) ultra cheap and basically garbage preamp tubes from the start. The two 50w combos I had did the exact same thing that you're going thru. Putting in new 12AX7's took care of it, plus the amps sounded better then they did before the no-volume problem.

    If you still want to convert the amp to EL-34's, just swap out three resistors and you're good. Change the screens from 470Ω/2w to 1kΩ/3w ceramics, and then check the bias to determine the new bias range resistor value; probably a tad lower than the stock value. With your background, this stuff should be a walk in the park.
     
  10. djuul

    djuul Member

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    Thanks for all the help guys!
    I got a new set of tubes in there and i fixed the problem, the problem was in V1 so when i changed that one is came alive again. I also get some 5881's for the poweramp. I only wanted to test the el34s to see if the problem was in the poweramp. I also have a biasprobe on the way the change the ice cold factory bias.
     
  11. sonar

    sonar Senior Member

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    Congrats!

    Nice to read that you're pro-active about maintaining a tube amp. It should serve you well as you continue the guitar journey.
     

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