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Anyone familiar with a Fender bassbreaker 18/30 ?

Discussion in 'Amp Tech Center' started by Burning Fingers, Oct 26, 2020.

  1. Burning Fingers

    Burning Fingers TDPRI Member

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    This monstrosity of a Fender Bassbreaker 18/30 amp has been brought to me for repair..usually I wouldn't touch this type of amp with a 40 foot pole but it belongs to a friend.

    Channel 2 ( the 18 watt channel ) is insanely loud with the volume on 2 but channel 1 has no sound at all unless I hit the guitar strings really hard..then it gives a weak farting sound.

    The channel switching ,either by the front panel switch or the footswitch, appears to be working fine.

    Fender have been spectacularly useless in not even responding to my request for a schematic.

    I have removed the preamp board and resoldered every joint and reseated all connectors and tried new tubes but the fault remains.

    I had to replace the volume pot and bass pot as in getting the really tight knobs off the pot shafts came out of the pots...talk about cheap and nasty ! :(

    So has anyone dealt with this problem on this model before ? or has anyone managed to get hold of a schematic for it ?.

    Stay safe. ;)
     
  2. Jon Snell

    Jon Snell Tele-Holic

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    If it is similar to the Bassbreaker 15 then check the anode load resistors, all 100k, on the pre amp valves after swapping the valves with known good ones.
     
  3. Burning Fingers

    Burning Fingers TDPRI Member

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    Thanks Jon...I will check those resistors when my patience with this amp returns.

    I have never dealt with a Bassbreaker 15 and can't find a schematic for one so I don't know if there any similarities between the 15 and the 18/35.

    There are only 2 X 12ax7 tubes and 4 X El84 tubes...in 18 watt mode only 2 of the El84's are in play with the 30 watt mode using all 4 power tubes.

    The first 12ax7 is used by both channels and the other has to be the PI..there are 2 large ICs sitting in there as well just to complicate matters.

    Oh, give me a good old fashioned point to point tube amp any day over these modern, accountant designed pieces of rubbish.
     
  4. Jon Snell

    Jon Snell Tele-Holic

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    I have never seen the model you describe but a valve amplifier is a valve amplifier.
    Check the cathode resistors and if, like the 15, it has 3.3M grid resistors, they can drift high value and make the valve cut off.
    Good luck. Screenshot 2020-10-26 at 12.57.29.png This is taken from the 15 service manual.
     
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  5. mkdaws32

    mkdaws32 Tele-Afflicted Gold Supporter

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    It’s hard to say. With only one preamp tube driving the two channels, the ICs could be op amps acting as buffers and/or clean gain stages. A bad solid state buffer or gain stage on one input could exhibit exactly what you are describing.

    My guess on the topology is one 12ax7 using the two stages in the front of the tone stack with one side of the second 12ax7 acting as a current recovery stage after the tone stack and a single stage PI (not a long-tail-pair PI, which would require another stage). This I purely a guess, though. Tone recovery stage could be one of the ICS, I suppose, but who knows?

    I’m more familiar with solid state amps. But my H&K TubeMeister head has only one 12ax7 driving two channels and it uses an op amp ic to drive the input. This Bassbreaker could be a similar input design.
     
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  6. blowtorch

    blowtorch Telefied Ad Free Member

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    I've got the Fender Bassbreaker 30R. And I love it.
    And I'm no tech.
    not much help, I know
     
  7. Burning Fingers

    Burning Fingers TDPRI Member

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    One of the IC's turns out to be a voltage regulator (7815) but the other has no readable markings on it.

    So what with Fender kindly ignoring my 3 requests for info and the unusual design of this amp, it is going back to it's owner in the same state as I got it with the message that Fender don't give a sh*t.

    Do NOT buy an amp ( especially a hybrid amp ) unless you know a schematic for it is available or you will regret it if it needs a repair beyond the most basic of faults.
     
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