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Modding a '60s Laney PA100

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  • Modding a '60s Laney PA100

    Hello all,

    First post here. I used to post on the now-defunct Plexiboard so hello to anyone that was there.



    For some years I've been playing guitar through a late '60s six channel 100W Laney PA. 6 preamp channels, effects loop with take-up gain stage (yes, really - advanced for the time), 4 x EL34s. It's the "Supergroup" PA and the power section is identical to the Laney Supergroup guitar amp. The pre is a simple one valve two gain stages per channel with a version of a James stack in between. Schematic available here.

    I've crudely cascaded two of the channels together for gain and made some other small tweaks, but now it's time to mod it properly into a guitar amp.

    I've prepared a little schematic to build an overdrive preamp in channels one and two drawn from the Laney Supergroup but with a slightly tweaked James stack. The original James stack on this had a mid-bump - I've tweaked it to have a mid scoop. The original Supergroup had a Fender-style stack.

    I'm pretty new to this, so I'd be grateful if someone could look over my schematic and tell me if you think it looks ok before I order in all the parts.

    My schematic is here. NB this is just the pre. It goes through to the power section as shown in the original schematic above.

    Many thanks for any input.

  • #2
    Should the second stage have 470k from grid to ground?
    The third stage has 100k plate and cathode resistors. Is it supposed to be a cathode follower?

    Comment


    • #3
      that 470k resistor/cap combo i lifted from the Supergroup preamp section. See here

      Re. the 100k cathode resistor on stage 3, i think that's a mistake. Should be 1.5k.

      Comment


      • #4
        Originally posted by parker_knoll View Post
        that 470k resistor/cap combo i lifted from the Supergroup preamp section. See here
        I was thinking R15//C12 and R16 give treble boost. It won't be as effective if R16 is missing

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        • #5
          that 470k resistor/cap combo i lifted from the Supergroup preamp section. See here
          The two 470k resistors in the original circuit are primarily for mixing/separating the signals from the input triodes. You are already using one of them (including treble boost cap) before the second stage. But as Dave H suggests, it might be a good idea to wire the other one to ground to get enough treble boost. The RC highpass needs to see some load.
          This will actually reduce the level of lower frequencies by 50%.

          The 470k connected to the plate of your triode No.4 will just reduce its available output voltage by around 50%.
          Last edited by Helmholtz; 03-04-2019, 04:53 PM.
          - Own Opinions Only -

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          • #6
            Originally posted by Helmholtz View Post
            The two 470k resistors in the original circuit are primarily for mixing/separating the signals from the input triodes. You are already using one of them (including treble boost cap) before the second stage. But as Dave H suggests, it might be a good idea to wire the other one to ground to get enough treble boost. The RC highpass needs to see some load.
            This will actually reduce the level of lower frequencies by 50%.

            The 470k connected to the plate of your triode No.4 will just reduce its available output voltage by around 50%.
            A belated thanks for these contributions. I will do without the treble booster. I can try it without the 470k on the plate of Triode 4. I guess it was there in the PA version of the amp to reduce to risk of valve distortion.

            Here's the amended version. If it looks viable. I will get the parts ordered. Right now when I turn my amp on it sounds like a school of dolphins and I don't want it to blow up.

            Comment


            • #7
              Originally posted by parker_knoll View Post
              A belated thanks for these contributions. I will do without the treble booster. I can try it without the 470k on the plate of Triode 4. I guess it was there in the PA version of the amp to reduce to risk of valve distortion.

              Here's the amended version. If it looks viable. I will get the parts ordered. Right now when I turn my amp on it sounds like a school of dolphins and I don't want it to blow up.
              Invalid link for amended version.

              nosaj
              Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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              • #8
                Originally posted by nosaj View Post
                Invalid link for amended version.

                nosaj
                Sorry, Google Drive being funny. Try this: https://www.dropbox.com/s/ykmn72s7nr...up-V2.png?dl=0

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                • #9
                  Don't change anything til you figure out how to get that 'school of dolphins' sound into a pedal. Take my money!
                  "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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                  • #10
                    Originally posted by g1 View Post
                    Don't change anything til you figure out how to get that 'school of dolphins' sound into a pedal. Take my money!
                    To do school of dolphins properly you really need the correct vintage gear. A pedal is a poor imitation

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