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Thread: Gallien Krueger MB Fusion 500 schematic

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    Member Nyqusit5's Avatar
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    Gallien Krueger MB Fusion 500 schematic

    Hi all,

    I am looking for a Gallien Krueger MB Fusion 500 schematic. This amp has a hybrid preamp and the one on my bench produces a lot of crackling (not the tubes). A schematic would be much appreciated.

    Thanks in advance!

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    The MB Fusion series has the digital power amp modules, manufactured not by GK.
    For this reason GK provides technical information only for preamp.

    First of all, designate what module is out of order - preamp or power amp.

    If the preamp is out of order, send its board and assembly revision.

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    Member Nyqusit5's Avatar
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    Thanks Doctor,
    The fault is located in the preamp. The board's reference is "206-0540 C1".

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    Quote Originally Posted by Nyqusit5 View Post
    Thanks Doctor,
    The fault is located in the preamp. The board's reference is "206-0540 C1".
    Here it is.

    Good luck.
    Attached Files Attached Files

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    Member Nyqusit5's Avatar
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    That's great. Thank you very much!

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    Does anybody have the schematic for the rear panel board?

    Not the digital power amp module as I know that is proprietary - I am talking about the board that has the Direct out XLR, Footswitch jack, tuner jack, send and return and headphone/line out connectors.

    Thanks in advance!

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    GK was kind enough to send me the IO schematic.
    Attached Files Attached Files

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    Quote Originally Posted by Nyqusit5 View Post
    Thanks Doctor,
    The fault is located in the preamp. The board's reference is "206-0540 C1".
    I have the same revision of the preamp with a channel not working properly, can someone re-attach the schematics?

    Thank you very much

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    Member Nyqusit5's Avatar
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    I'd love to but I can seem to find the file attachement function anymore...

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    Supporting Member Jazz P Bass's Avatar
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    Not too sure what the issue is.
    Both attachments open fine.

    I have attached a 'zip' file of both of them.
    GK 500 Fusion Pre.zip

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    Member Nyqusit5's Avatar
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    I can only see yours which opens fine.

    Can you explain how you attached this file? Even inAdvanced Reply, I can see no option for attachments.

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    I have just opened them with no problem.

    By the way, what a ridiculous design, clearly mandated by Marketing Dept. and damaging design quality.

    They should have left the original OPp Amp design which is excellent and added a couple unconnected tubes lighted by blue Leds from below, but ... using a 12AX7 as an Op Amp? ... ridiculous.

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    Juan Manuel Fahey

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by J M Fahey View Post
    I have just opened them with no problem.

    By the way, what a ridiculous design, clearly mandated by Marketing Dept. and damaging design quality.

    They should have left the original OPp Amp design which is excellent and added a couple unconnected tubes lighted by blue Leds from below, but ... using a 12AX7 as an Op Amp? ... ridiculous.
    I can't say I've run into a design like this for the tone circuits. Looks like it would be noisy by the time you get to the Masters.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Nyqusit5 View Post
    I can only see yours which opens fine.

    Can you explain how you attached this file? Even inAdvanced Reply, I can see no option for attachments.
    When you are in the 'advanced', you have to scroll down the page and click on 'manage attachments'. Also, in the top row of the advanced toolbar, there is an icon of a paperclip over an envelope. That will also open the attachments window.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nevetslab View Post
    I can't say I've run into a design like this for the tone circuits. Looks like it would be noisy by the time you get to the Masters.
    Check schematics side by side: they reproduced their excellent 40 year old design, fully active tone controls, add-on "shape/contour" equalizers, etc. , all based on Op amps and perfectly competent designs, by ghastly performing (in that job) single triode stages .

    As a comparison:

    an ideal Op Amp has:

    *infinite gain (real ones 50000 to 200000 X so no bad at all)
    - a 12AX7 stage has 50X gain and you must tweak it a lot to get, 60 or 70X This is a MAJOR problem.

    * infinite input impedance., in practice from around 1 M in bipolars to hundreds of MB in Fet input ones.
    - 12AX7 needs a 1M grid reference, and has enough capacitive reactance that gain suffers *just* above the Audio band, even so itīs the only area where it matches Op Amp performance

    * zero output impedance, in practice 10 to 100 ohm if we consider current capability and close to zero with enough NFB
    - 12AX7: get lucky if you get less than 40k output impedance . This is a MAJOR problem.

    * an OpAmp is designed for NFB
    - a single triode stage is designed for open loop . This is a MAJOR problem.

    * an Op Amp has + and - inputs, lots of flexibility, can be used as inverting or non inverting stage.
    - 12AX7 .... are you joking? This is a MAJOR problem.

    * typical Op Amp can routinely source/sink 5mA ... enough for a ton of real world applications.
    - 12AX7 can sink a couple mA, and source whatever the load resistor allows, typically from 0.5 to 1 mA ... tops.

    My point is you can NOT have a properly working Op Amp circuit (besides a very basic gain stage) and just replace each Op Amp section by a single 12AX7 triode ... what were they thinking?

    Of course, a MI amplifier populated by half a dozen glowing glass bottles will sell like hot cakes ... or so they expect

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