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  • Hammond L-100

    Hey y'all. Here's a "conversion" of a Hammond L-100 amplifier that I bought on eBay. I haven't done anything really major to it except for adding controls and a reverb tank. It sounds good. It's loud, but probably not as loud as it should be.

    Click image for larger version

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    I'd appreciate any suggestions or advice on stuff that could be adjusted. In particular, I'm not quite sure what's up with the stage just after the reverb tank.

  • #2
    What ohm speaker are you using and is the NFB resistor of the correct size for the speaker you are using? The stage after the reverb section should be your phase inverter.

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    • #3
      Hammond amp are a conservitive design and are not designed to add any colour of their own. The tubes are just coasting along and not driven hard. see B+ voltage. Power Supply sag, power tube saturation, and preamp tube distortion are harder to get. They make a very good Jazz tube amp but they are no Mini-Marshall, Vox or Fender. Beware of claims of sellers on ebay.

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      • #4
        Originally posted by Cool_Ray View Post
        Hammond amp are a conservitive design and are not designed to add any colour of their own. The tubes are just coasting along and not driven hard. see B+ voltage. Power Supply sag, power tube saturation, and preamp tube distortion are harder to get. They make a very good Jazz tube amp but they are no Mini-Marshall, Vox or Fender. Beware of claims of sellers on ebay.
        Thanks for pointing me to the power supply. I'm reworking it to give 300V to the preamp. (BTW, I didn't buy this because of any eBay seller's claims.)

        I'm puzzled by the choke and 50µf cap on the plate of the 12BH7 reverb driver. Can anyone shed any light on this design? What would happen if were changed to a simple resistor and coupling cap?

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        • #5
          I'm pretty much declaring victory on this project, electronics-wise anyway. Retooling the power supply rail and a few other tweaks won me a bunch more gain. It's a monster now! Kinda Vox AC15ish, and the reverb is pretty sweet. (BTW, it has all the original Hammond-branded tubes.)



          I'll post pictures when I get the cabinet made.

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          • #6
            Pretty cool. I have a working Hammond L-103, and it sounds great, especially the reverb. Like you're playing in a big church or something.

            - Scott

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            • #7
              this is the coupling cap to the reverb coilsspring system , it's cheaper to use a big cap then a transformer and it also cuts out the very low frequencies not going to the reverb set .. take care it has some very high DC tension of around + 300 Volts , that's why it sits in a black carton protection !

              any more problems with the l100 amps ?? mine delivers around 40 watts when modified with 2 x 6L6 all the parts in Hammond amps are very conservatively used you can get easaly the double power out of the suply tranies !

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              • #8
                if you would swop the coil with a resistor you would destroy the whole setup , the coil is there to avoid very low bass frequencies to get to the reverb , they would spoil your sweet reverb sound by rumbling noisess ! remember for high frequencies a coil is a very larg impedance , for lows is it's a shortcircuit.

                all the best from belgium's Classic Hammond Tonewheel Restoration Service (regstd)

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                • #9
                  Likewise, the choke in the reverb driver plate circuit gives more output than a resistor would.

                  You can think of it as a reverb drive transformer with no secondary.
                  "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                  • #10
                    head box pix

                    Thought I'd top off this thread with a couple of pictures of the amp with cabinet.

                    If I had it to do over, I'd make the box an inch shorter and an inch less deep. (it's 10 1/2 by 10 1/2 by 18 1/2.) I was probably overly concerned with keeping the reverb tank away from the power transformer. Oh well, I think it's okay though. Now I just need to fashion some kind of back panel for it.
                    Attached Files
                    Last edited by otto pärt; 01-25-2010, 12:22 AM.

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                    • #11
                      Originally posted by otto pärt View Post
                      <snip>I'm not quite sure what's up with the stage just after the reverb tank.
                      Presuming you are talking about that 68k feeding the cathode, it looks to me like it is a way of having a lower unbypassed cathode resistance while still being able to bias the tube with cathode bias. The result is that the stage gain is higher than it would be with an unbypassed 1.5k (say). It raises the Thevenin source voltage for the cathode resistor from 0V to something higher so that a lower Rk can be used to achieve a given cathode current.

                      That's my take on it anyway.

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                      • #12
                        Originally posted by RudeBoy View Post
                        Presuming you are talking about that 68k feeding the cathode, it looks to me like it is a way of having a lower unbypassed cathode resistance while still being able to bias the tube with cathode bias. The result is that the stage gain is higher than it would be with an unbypassed 1.5k (say). It raises the Thevenin source voltage for the cathode resistor from 0V to something higher so that a lower Rk can be used to achieve a given cathode current.

                        That's my take on it anyway.
                        Thanks. That makes sense, I think. But why must the cathode resistor be small and unbypassed in that circuit?

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                        • #13
                          just got one of these amps, looks likes its going to be fun!

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                          • #14
                            L100 with reverb conversion

                            I have an L100 I just purchased on e-bay. I'm looking at your 2nd and final schematic, but is the 3.9 ohm 1/4 resistor really in series with the 15W 250 ohm one? This doesn't make any sense--the 1/4 resistor will act as a small fuse for the 15W power resistor.

                            Best regards,

                            Dave Adams

                            (a.k.a. swaptronics)

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                            • #15
                              I just did the math. Surprise, the 3.9 ohm reaches 1/4 Watt at 0.253 Amps and at that current, the 250 ohm is 16 Watts. Be sure to use a flame-proof resistor.
                              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                              REMEMBER: Everybody knows that smokin' ain't allowed in school !

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