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Hammond A-100 Overdrive

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    Hammond A-100 Overdrive

    Hello, Hope all is well with everyone.
    Two questions that I appreciate your input on.
    1. Any suggestions to get a good mean growl on my A-100 and Leslie 31H ?
    Without getting too Loud. yea right !!
    Im very happy with the Jazz sound I get, but I really want a hard rock Growl.

    2. A blaspheamous question - Can I wire a footswitch in parallel with the existing Leslie HalfMoon Switch ? So I can have both ?

    Any help appreciated.

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    #2
    1. A power attenuator or Kon's zener diode circuit.
    2. I'm sure it can be done but someone else might give you the details.
    A100, X77, M3, M100, E100
    Leslie 147, 145, homemade road Leslie
    My youtube channel

    Comment


      #3
      Originally posted by kingsboard View Post
      1. Any suggestions to get a good mean growl on my A-100 and Leslie 31H ?
      Without getting too Loud. yea right !! Im very happy with the Jazz sound I get, but I really want a hard rock Growl
      Hi kingsboard.
      You can drastically increase the internal gain levels of the AO28 preamp by disconnecting the C15 51 pf (picofarad ) feedback capacitor which is wired across input and the output of the V4A 12AX7 tube, and you can disconnect the C22 24 pf to 90 pf variable gain trimmer capacitor which is the variable feedback capacitor wired across the input and the output of the V4B 12AX7 tube. The C22 gain trimmer capacitor is physically located inside the top section of the aluminium covered expression swell control "dog house" box, and the C22 has a screw on it which allows you to change it's capacitance setting from 90 pf ( maximum clockwise setting of the screw for the softest levels) to 24 pf ( around ten 360 degree anti-clockwise turns of the screw for the loudest levels), but disconnecting the C22 results in noticeably even more gain levels than what you get by setting it to the loudest 24 pf setting.

      Disconnecting the C15 and the C22 drastically increases the gain levels of the V4 12AX7 tube to be at full strength so therefore this allows you to get an overdrive growl within the AO28 preamp so that you can then get the growl within the AO28 when playing the organ at full volume but you can still keep the listening volume level setting of the Leslie 31H at a comfortable or unobtrusive level.

      Of course with the AO28 gain boost created by disconnecting the C15 and the C2, you can then also drive the Leslie 31H at full blast for an even more heavy overdrive up to Jon Lord / Marshall levels.

      If your A100 organ is set up so that the Leslie 31H receives the speaker output signal coming from the the A100 AO39 power amplifier, then you can create more gain levels from the AO39 power amplifier by shorting out the R1 and the R2 47K input resistors, and by disconnecting the C1 and the C10 0.1 uf ( 100 nf ) input grounding capacitors .

      You can wire up a 4PDT switch on to the AO39 power amplifier chassis to switch achieve this.

      However an important thing to be aware of is that the AO39 input passive filter which includes the R1, R2, and the C1 and the C10 along with the R3, R7, R8 and R9 resistors creates a bass boost effect which is the reason why the A100 series organs produce a warmer or fatter bass response through the Leslie when the A039 power amplifier output signal is sent to the Leslie.
      If the signals from the AO28 preamp GG output terminals are sent to the Leslie thus bypassing the AO29 power amplifier, then the bass response is not as warm or fat as what it is from the AO39 power amplifier.

      WARNING: There are dangerous high voltages in the AO28 preamp and in the AO39 power amplifier so therefore proper safety precautions must be observed whilst working with the AO28 preamp and the AO39 power amplifier.
      The organ's AC mains cable must be disconnected from the AC mains, and all the power supply electrolytic capacitors in the AO28 and the AO39 must be safely fully discharged before you begin to do the modifications.

      Originally posted by kingsboard View Post
      2. A blaspheamous question - Can I wire a footswitch in parallel with the existing Leslie HalfMoon Switch ? So I can have both ?
      You can wire up a DPDT switch to switch the C15 and the C22 in or out of circuit, and I recommend that you drill a hole on the AO28 preamp chassis and then mount the DPDT switch there at a suitable location as close as possible to where the C15 and the C22 are located in the AO28.

      The reason for positioning the DPDT switch there is because the V4 12AX7 tube circuit is a very high impedance circuit which is very sensitive to stray cable capacitance which causes treble loss if the wire lengths between the C15 and the C22 and DPDT switch go beyond a certain length. Keeping the two shielded cables wired between the C15 , and the C22 and the DPDT switch as short as possible will avoid creating treble loss issues.

      The treble loss caused by cable capacitance is the reason why I strongly recommend that you DO NOT wire the DPDT switch on to the front rail panel next to the Leslie half moon switch because this would require a two long shielded cables to be wired between the C15 and the C22 inside the AO28 preamp and the DPDT switch mounted on to the front rail panel, and the stray capacitance such long shielded cables will produce a noticeable treble loss.

      Likewise I strongly recommend that the 4PDT switch for the AO39 power amplifier be mounted on to the AO39 amplifier chassis close to the V1 12AX7 tube in order to avoid treble loss issues arising from cable stray capacitance

      All the best.
      Kon.
      Last edited by kziss; 06-12-2019, 08:51 PM.

      Comment


        #4
        Something I’m pretty sure Jon lord of deep purple did was he ran it out to a Marshall cab and ran it through a ring modulator on the way to give it his signature tone. If you found a way to wire in an effect loop, which I’ve seen lots of schematics for, you could buy a ring mod (my suggestion from moog) and get some pretty sick growl tones. Or just crank the organ up all the way. Ring mod organ would also probably sound cool through a leslie, never heard that since Jon lord used a Marshall cab and I don’t know of anyone else that used a ring mod.
        (2x) Hammond M3 Leslie 145
        Hammond L100 Leslie 120

        Fender telecaster, Stratocaster, twin reverb
        Epiphone Les Paul, Carvin

        Comment


          #5
          Wow ! I Love this place.
          1. Power Attenuator
          2. Kon's Zener Diode Circuit
          3. Kon's Amplifier(s) Modifications ( From The Man Himself ! )
          4.An Effects Loop
          Thank you for this Gentlemen !

          I heard that also overdriving the power amp tubes in the Leslie 31H may be an alternative

          Comment


            #6
            Originally posted by kingsboard View Post
            Wow ! I Love this place.
            1. Power Attenuator
            2. Kon's Zener Diode Circuit
            3. Kon's Amplifier(s) Modifications ( From The Man Himself ! )
            4.An Effects Loop
            Thank you for this Gentlemen !

            I heard that also overdriving the power amp tubes in the Leslie 31H may be an alternative
            If you want to preserve your vintage Jensen V21 treble driver and your power amp tubes, then go with options 2, 3 or 4.
            An effects loop will be the most practical option: you can derive all of your overdrive from guitar pedals - this will allow you to experiment with different flavours instead of being stuck with whatever you can get out if your Organ/Leslie, and you’ll be able to keep the volume down which will keep your vintage components and your neighbours happy
            As a bonus, a loop will allow use of other effects such as delay, phaser, wah, etc.
            Current:
            1971 T-202 with Carsten Meyer mods: Remove key click filters, single-trigger percussion, UM 16' drawbar volume correction. Lower Manual bass foldback.
            Korg CX3 (original 1980's analogue model).
            1967 Leslie 122 with custom inbuilt preamp on back panel for 1/4" line-level inputs, bass & treble controls. Horn diffusers intact.
            2009 Marshall 2061x HW Plexi head into Marshall 4x12 cabinet.

            Former:
            1964 C3
            196x M-102
            197x X5
            197x Leslie 825

            Comment


              #7
              I have both a foot and half-moon switch on my Model A and 122. I just hooked it to the two wires going to the half-moon. Works fine. Only caveat...the footswitch only works when the half moon is in the chorale position. Flip the half moon to tremolo and the foot switch has no function. No problem for me because my foot switch is momentary. I play more in chorale and just use it when I want to hit tremolo for a short bit.

              Comment

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