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Thread: Carlsbro Fatboy

  1. #1
    Supporting Member
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    Dec 2009

    Carlsbro Fatboy

    Anyone got a schematic for the 30w Carlsbro Fatboy amp?

    I mailed the factory and the original drawings were lost during change of ownership.

  2. #2
    Senior Member mozwell's Avatar
    Join Date
    Sep 2009
    Sorry for the late reply. I dont have the sch for this amp, but we shoudl be able to sort you out.
    The guys/girls here are a happy (mostly) & helpful bunch.
    1. what is the problem ?
    2. does the amp have send / return jacks, if so, check this out The Dreaded Switching Jack Problem
    3. pictures always help, companies often use the same circuit in many amplifiers, so there may be something "close", include pictures of inside where the electronics are.
    4. Is it a valve amp or transistor amp
    5. check any fuses, there may also be some inside.
    6. try with a different speaker

    some carlsbo schematics carlsbro 50top, 60tc, 100top, bassline 50, bassline 80, colt120 sch

  3. #3
    Supporting Member
    Join Date
    Dec 2009
    Thanks for the reply. I was struggling with the amp for a while trying to figure out the signal paths. The amp was intermittently cutting out, but whilst it appears to be a conventional layout (3xECC83 + 4xEL84) there's a lot of solid state in there to confuse matters, along with meandering traces on a very large PCB (which also carries the mains side) that make this more difficult to 'read'. For instance, the PI is a mixture of the two triode halves plus a transistor pair in the linked cathode circuit. There is also a quad multiplexer IC - presumably to carry out switching along with a few J175 in there . Quite complex for a single-channel amp and it would be interesting to see the whole thing as a schematic to properly relate to the entire design.

    Anyhow, the problem ended up being an intermittent short in the toroidal OPT. It measured fine on the bench, but when in place the cable routing was causing the primary to short. I unwrapped the covering and where the flying leads were attached to the windings they had pulled away slightly allowing the solder joints to touch. The fix was just to re-locate them, secure the flying leads and re-tape the toroid. The leads run in PVC sheathing so for extra security the whole bundle is cable tied to prevent movement within the sheathing.

    A couple of other jobs needed doing - the gain boost also gave massive volume boost (faulty J175) and the reverb tray had an OC send coil (straightforward repair).

    The amp now sounds superb - the clean sound is superbly detailed and despite a short two-spring reverb has a lot of shimmer. The tone shaping is where this amp excels and despite the internal complexity it's very easy to get great sounds from a simple front panel layout.

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