Bad Output Tranny?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by anais, Dec 22, 2009.


  1. anais

    anais Member

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    Ok, I put a new pair of matched tubes in a Roland Bolt amp I picked up, and they lit fine.. let her warm up and nothing. they were reacting to the volume/playing.. but I got no output at all to the speaker.

    I then broke out the testers.. the Ik at 19.6 but Vp was reading like negative 15.. is this an output tranny problem? I tried to read up online about testing them but it seems like chinese to me so I figured I would ask.
     
  2. SatelliteAmps

    SatelliteAmps Member

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    Pull the tubes. Take a multimeter and set it to resistance. Check between each outside lead of the OT and the center tap. Should read somewhere between about 50Ω and 500Ω on each side (should be close, but probably not exact). If one side is way off from the other, or way higher or lower than these values, then you might have a problem. My guess is that you have a bad tube. Can you try putting the old ones back in to see if the amp still works, or was it dead when you got it? If it was dead when you got it, it could be any number of things, but the first thing would be to check voltages.

    If you are unsure of working on amps that are live, dangerous, and potentially lethal, then take this to a proper tech or get someone's help who knows what they are doing.
     
  3. anais

    anais Member

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    I checked it with a few different sets of tubes.. still nothing.

    The test on the OT reads 84.4 ohms on one side and 75.7 ohms on the other so it seems to be matched up pretty well.

    when I was testing with the amp live the speaker was making some little crackles.. if that means anything. would really like to fix it myself as opposed to taking to an amp tech
     
  4. slider313

    slider313 Silver Supporting Member

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    Have you tried another speaker?
     
  5. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    Is it possible in the process here you've somehow blown the ht fuse (assuming this amp has one!).

    What is the voltage on the screens of the output tubes (what kid of tubes are they, btw)?
     
  6. mark norwine

    mark norwine Member

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    Not possible.

    If you have 19.6mA** going up the cathode, you have to have a B+ potential on the plate.

    Since this amp is fixed bias, neg voltage is applied to the grid. Are you measuring the grid and thinking you're on the plate?

    {**I assume mA, but you didn't give a UOM, so I can't be sure}
     
  7. anais

    anais Member

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    I am using amp head tube testers.. they measure plate voltage and current...

    I just want to figure out why the darn thing wont produce a sound.. I tried 2 different cabinets that both work with different amps... may be a little over my head?
     
  8. anais

    anais Member

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    so I went over it again and tried jumping the effects loop.. now it makes a LITTLE BIT of sound.. nothing audible over the strings of the electric guitar strings.. very very low output.. but something is there.

    ALSO, when I tapped the pre amp tube (which I have tried several) it made a decent amout of cracking as well.. still nothing anywhere near how loud it should be at 60 watts.

    anyone else have any ideas?
     
  9. mark norwine

    mark norwine Member

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  10. Prairie Dawg

    Prairie Dawg Member

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    Or, at least someone to walk you through it hands on. It's not the simplest guitar amp around.
     
  11. anais

    anais Member

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    I know right? Fender stuff.. even newer fenders I am fine with!

    This seems to be the only roland tube line made.. and its kinda pissing me off haha
     
  12. SatelliteAmps

    SatelliteAmps Member

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    Was it dead when you got it?
     
  13. anais

    anais Member

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    I got it as a gift, and the first set of tubes I put in it blew the fuse right away.. I put a new fuse in and the Tubes started sparking (probably should have said this first right?) so I shut her off and tried another set of tubes.. nothing, another set- nothing still..
     
  14. Nolatone Ampworks

    Nolatone Ampworks Silver Supporting Member

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    You've cooked something in the circuit I suspect, or the HT fuse. What version of the bolt is this, and does it have a HT fuse outside the chassis?

    Even if it's the HT fuse, gotta wonder what caused IT to blow. I suspect if you open the amp up it may become more obvious what's going on (burned components, etc).
     
  15. SatelliteAmps

    SatelliteAmps Member

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    So it came with no history, and you never turned it on as it came stock? If so, take it to a tech. trying to chase something like this down without knowing any history or anything is going to be very difficult online.
     

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