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  • low gain / high gain inputs

    Hi. After googling many hours, of course I end up asking here. I'm sure this question was made before but I couldn't get the answer by myself so here I am...
    I'm very newbie to the tubes, electronics and amps world (and little english too) so be patient please

    I'm kind of confused about how to wire a low gain and high gain inputs in an amp.
    What I did, which I presume it's wrong, is to keep the high gain input as it was and to wire the new low gain input (I sacrificed the low sensitivity input because I don't use it) after the coupling cap of the first triode but before the tone stack and volume pot that connects to the second triode. So now the high gain input goes through to both triodes of the 12ax7 preamp and the low gain just goes through only one of the triodes of that tube. It works but I had to disconnect the earth from the low gain jack's tip because the first plate shorts to ground! [OF COURSE you dumbass ]
    I supose this is not the best way to do it so could you help me here please!?

    This is what I did:
    Click image for larger version

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    Second, looking in this Marshall schematics:
    http://www.drtube.com/schematics/marshall/jcm800pr.gif

    In the 1987 circuit, the 470k after the volume pot is mixing both inputs like I want to do in my amp or am I wrong?
    In the 2204, the 0.022 uF between the first triode plate and the low input input jack is coupling both triodes and connecting the inputs like my schematic or am I wrong?

  • #2
    In the 2204, both input jacks have 2 shorting switches each, however only one of these switch contacts is wired in the lo input jack. When a plug is inserted into the lo-input, this switch contact is disconnected, and the guitar cable is connected to the grid of V1a vias the voltage divider network comprised of the 470k and the 1MA pot (bypassd by those caps).

    In the hi-input jack both switch contacts are utilised. When a plug is inserted the guitar cable is connected to the grid of V1b via the 68k input resistor, and the plate of V1b is AC-coupled to the voltage divider in front of V1a via the 22n coupling cap and shorting switch in the lo input jack (which is closed when there is no plug in that jack.

    Your schematic is not wired like that at all. Is that any help?
    Building a better world (one tube amp at a time)

    "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

    Comment


    • #3
      Hi tubeswell. Thanks to reply! Yes, you're right, the 22n coupling cap isn't connected like my schematic, but because I want to plug both inputs at the same time, and change where the guitar signal goes with a footswitch connected to both inputs. Lets say you want to have in the 2204 both inputs connected at the same time and you want to switch between them with a footswitch, how would you mod it? I think that's my real question.

      Comment


      • #4
        Originally posted by elushi View Post
        Lets say you want to have in the 2204 both inputs connected at the same time and you want to switch between them with a footswitch, how would you mod it?
        With a multi-pole double throw relay
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

        Comment


        • #5
          This schematic
          is referred in this post and I think this is what you are saying. But when I tried something like that, the first plate was shorting to ground. Why it doesn't happen in this schematic? I mean, the first plate V1a connects to V1b grid via the 470k resistor, but also connects to ground (through -> 68k grid stopper -> low gain jack tip -> high gain jack ring -> GND). What I'm missing?

          Comment


          • #6
            Originally posted by elushi View Post
            when I tried something like that, the first plate was shorting to ground. Why it doesn't happen in this schematic?
            Firstly, if the plate of V1a in your schematic was shorting to ground if nothing is plugged into the Low gain input, then you obviously must've had the low gain input jack wired wrong. However I would not use a non-switching jack there because of all the RF noise you would pick up on that wire if it was just floating.

            Originally posted by elushi View Post
            I mean, the first plate V1a connects to V1b grid via the 470k resistor, but also connects to ground (through -> 68k grid stopper -> low gain jack tip -> high gain jack ring -> GND). What I'm missing?
            In the Marshal schematic the vol pot after V1A goes through a 470k 'semi-mixing'/gridstopper to get to the grid of V1b, and the 68k is just floating because the switch contact in the V1a input jack (which would otherwise be connected to ground) is disconnected with the plug in that jack. So you have a cascaded V1a and V1b.

            With the plug in the High gain jack, the ground switch on that jack is disconnected, so the signal from V1a isn't dumped to ground.

            With the plug in the Low gain input, the V1a grid is grounded through the V1a input jack's dual sleeve switches, and the coupling cap after V1a is grounded through the input jacks ground switch. (there are 3 switches on that input jack)
            Building a better world (one tube amp at a time)

            "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

            Comment


            • #7
              Originally posted by tubeswell View Post
              However I would not use a non-switching jack there because of all the RF noise you would pick up on that wire if it was just floating
              I agree and that's the reason I posted this thread actually, because mi wiring do work, but with a non-shorting switch.

              Originally posted by tubeswell View Post
              the 68k is just floating because the switch contact in the V1a input jack (which would otherwise be connected to ground) is disconnected with the plug in that jack. So you have a cascaded V1a and V1b.
              With the plug in the High gain jack, the ground switch on that jack is disconnected, so the signal from V1a isn't dumped to ground.
              Sorry for mi insistence on this. But if you look on this modified Marshall schematic, when a plug is on the high gain jack, this one obviously is disconnected from the dual shorting switch but it isn't disconnected from ground either, because the ring is connected to ground. Look inside the footswitch as it was drawn in the picture: in that DPDT position, the output jack's tip is driving V1a and the ring is connecting the grounded sleeve via the DPDT. But inside the amp, the ring is also connecting to the low input jacks tip before the 68k res (AS LONG AS AS THE LOW INPUT JACK IS PLUGGED)...
              so two cases : either a)I'm wrong or b)the high gain channel works as long as the low input jack is plugged too, avoiding ground dumping. (But if this is the case, then V1b is getting RF noise too)

              Comment


              • #8
                Originally posted by elushi View Post
                so two cases : either a)I'm wrong or b)the high gain channel works as long as the low input jack is plugged too, avoiding ground dumping. (But if this is the case, then V1b is getting RF noise too)
                c) V1a doesn't dump to ground for other reason no matter if the low gain jack tip it do connects to ground

                Comment


                • #9
                  Enzo already explained how to use an AB switch with a TRS stereo input jack on the high gain channel in that other post. The only other way I can see of making the Park schematic work is with a 3 pole switch on the high-gain input jack.

                  If you want to configure to switch between cascaded or parallel input stages, or just either as a single stage (and maybe that is what you are asking? ) then here is a concept schematic. There probably would be a bit of switch pop as you flick the DPDT (which a large snubber resistor across each of the two sets of poles might suppress, or alternatively smallish high voltage plate bypass caps might suppress), and you might have to lay this out astutely to minimise unwanted coupling. Just a suggestion, not saying it will work perfectly first time.
                  Attached Files
                  Building a better world (one tube amp at a time)

                  "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

                  Comment


                  • #10
                    Originally posted by tubeswell View Post
                    If you want to configure to switch between cascaded or parallel input stages, or just either as a single stage (and maybe that is what you are asking? ) then here is a concept schematic
                    Thanks for the schematic. That's not what I want to do, but following a similar idea maybe I could work it out.


                    Originally posted by tubeswell View Post
                    Enzo already explained how to use an AB switch with a TRS stereo input jack
                    Yeah I know...
                    Originally posted by Enzo View Post
                    Also the other half of the switch grounds the ring contact, which grounds the low gain jack. The 68k resistor there works as a voltage divider to tame the signal from the first stage somewhat.
                    I just don't understand why the V1a doesn't dump to ground in Park schematic but mine does...
                    this was my first attempt where V1a was dumping signal to earth
                    Click image for larger version

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                    ANYWAY tubeswell THANKS FOR ALL YOUR REPLIES AND YOUR HELP!

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