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What's going on with all the extra diodes here?

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  • What's going on with all the extra diodes here?

    LEDs 401 and 402 look to be the usual clipping diodes. But what of the rest? I see one lights up on the panel when overloaded.
    This circuit is bass heavy and mushy in that regard. There could be more crunch too.
    Any tweak ideas?

    ALSO, I was wondering if this reverb tank will work for an upgrade. The stock one is too short. The Gibbs below is approx 185Ω. The Rivera schematic lists 150Ω in and 2250Ω out.
    https://www.ebay.ca/itm/Vintage-Gibb...8AAOSwld9dSkDI

    Thank you.
    Attached Files
    Last edited by Guitarist; 08-08-2019, 05:18 AM.

  • #2
    What is the amp model?
    That reverb tank is not a good match. It is giving resistance values but the numbers on the schematic are impedance, not resistance.
    From the numbers you quoted, it probably needs a 4BB type tank to match impedance. 4BB3C1B are fairly common. You may have to play around with the grounding scheme of the jacks.
    What is the number on the stock tank?
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #3
      Rivera M100
      I returned the amp but it's just 10 mins away. But I'll ask him to find out! He agreed to upgrade the tank.
      I just loved the Hammond reverb sound.

      Thanks.

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      • #4
        Originally posted by Guitarist View Post
        LEDs 401 and 402 look to be the usual clipping diodes. But what of the rest? I see one lights up on the panel when overloaded.
        This circuit is bass heavy and mushy in that regard. There could be more crunch too.
        Any tweak ideas?
        In my view the real/main clippers are Leds Q401/402

        I see the bridge Q403/4/5/6 as a crude noise gate, in a way similar to what Peavey adds in its highest gain models.
        eavey uises 2 antiparallel diodes in series with signal, so it axes anything below 700mV peak .... way-too-much and itīs very noticeable.

        While this bridge, which "should" stop signal passing from left to right, since wherever you go you find some reverse diode against you, in practice lets "a little" through, since the bridge is biased by a very low current through symmetrical R40? (unreadable) 180k on top and bottom.

        I have seen similar circuits where threshold is adjustable by manipulating such bias current, I bet here they found an acceptable value and let it there.

        Not sure what are D409?/410 for but they are also forward biased by a small current through 330k resistors .

        In general we think signal *voltage* but sometimes signal current is just as important.

        EDIT: now isnīt it funny that people spend more money, carry more weight, all to have tube sound ... and actual distortion comes from diodes and Op Amps?
        And muddy distortion to boot

        Wonīt even mention JCM900
        Juan Manuel Fahey

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        • #5
          Thanks for the reply, the was generally what I was thinking with the bridge - a gate of some sort. Well it all works, thank God!
          I like how the schematic says, "Adjust vale for high end tone." Well, many of us do love diode distortion, oddly enough A 9V battery is somewhat more convenient than a full-on 400V power supply!

          Also, Ch 1 is the more gainier/crunch side NOT Ch 2!

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          • #6
            Originally posted by J M Fahey View Post
            EDIT: now isnīt it funny that people spend more money, carry more weight, all to have tube sound ... and actual distortion comes from diodes and Op Amps?
            And muddy distortion to boot

            Wonīt even mention JCM900
            That part of the scheme corresponds to the Slavemaster section. If I remember correctly itīs a rather strange circuit incorporated into the loop and assignable by footswitch capable of generating an overdistortion after the two channels.

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            • #7
              It's a (push-pull) tube power amp emulation. The steering diodes clip signal peaks and also introduce crossover distortion above certain threshold. Outcome is similar to push-pull tube power amp clipping.
              Allows to add 'post power amp' style effect processing without actually dummy loading a real tube power amp. Consult manual for better explanation.
              BTW, this is basically an adoption of old Quilter circuit from a Pignose amp (30/60). Modern Quilter amps employ similar circuitry but implement dynamically varying clipping threshold to simulate power supply sag and associated effects. ;-)

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