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Need some help with some EVH LBX II mods

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  • Need some help with some EVH LBX II mods

    I finally got my hands on the schematic for the LBX II, and could use some advice on a couple of mods I'd like to do. I'm a tube amp novice, but have been an electronics tech for a few decades, so it shouldn't be too big of a deal. I think.

    The full schematic can be downloaded here. (I have permission to share it from EVH customer support, if anyone is wondering).

    Preamp:



    I'd like to tame the blue channel; the amount of gain is just insane. Based on a little study of the schematic, my plan is to swap in a 12ay7 in V1 for starters, and then add a resistor (maybe switchable), to lower the blue channel's signal that feeds V2. If I'm looking at it correctly, I think I can just piggyback another resistor across R88 in parallel (shown in red in the attached image), to drop the level feeding V2-A...but I have no idea what value to start with, or if will affect anything adversely.

    As for the blue and green circles I added...I assume those change the frequency response of each channel, as the signals enter the switch. Correct? If so, what effect would changing the cap values there have?

    Output:



    There is a "1/4 power" switch between the PI and power tubes, that does basically nothing. This amp is LOUD, no matter how you slice it. Is there a simple mod I could do in the area I circled in red on the output stage image, that would cut more volume without severely neutering the tone? It appears to be tied to an internal bias trimmer for the power tubes and voltage regulator, but I'm not sure how that circuit works.

    Any thoughts and advice greatly appreciated!

  • #2
    Where is the rest of that 1/4 power switch? Is there a voltage change in the B+ supply? Ah yes there is. Your power tube B+ is gut in half by that switch. The red circle part just adjusts the bias some to compensate. This is a 15 watt amp, 1/4 of that is about 4 watts. That will not be a whole lot less loud. But what it will do is let you break up the output stage easier than full power. There are volume controls on the amp.

    C52 is a smallish cap to limit the bottom end going into the gain channel. A lot of bottom will create mud.

    C46 is a larger cap to allow more full range into the clean channel. Then C45 adds a bit of brightness.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Originally posted by Enzo View Post
      This is a 15 watt amp, 1/4 of that is about 4 watts. That will not be a whole lot less loud.
      Thanks for the reply.

      Could I change a resistor value (or add one) in that 1/4 power circuit to cut the power down to 1 watt or so? Maybe increase the value of R41, if I'm reading it correctly?

      I'd like to run the amp hot, but at a lower volume for bedroom practice, and that switch makes so little difference that it's practically useless IMHO. If I turn the volume down low enough to not bother the neighbors, it loses all of it's character. I'm currently using the output level of a Keely limiter in the FX loop to tame it a little, but it would be nice to have an attenuator that actually does something .

      Also, any thoughts on what I mentioned about reducing the gain on the blue channel? As it is now, I can barely turn the gain knob past 1 if I want "crunch", and anything past 9:00 is more than I ever need. I'd like to lower that gain so that I can get more range and use out of the gain knob.

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      • #4
        Originally posted by Enzo View Post
        ...But what it will do is let you break up the output stage easier than full power....
        I just realized that my wording made it sound like it isn't working. It DOES lower the output, but it's so negligible that I refer to it as doing nothing.

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        • #5
          Also, any thoughts on what I mentioned about reducing the gain on the blue channel? As it is now, I can barely turn the gain knob past 1 if I want "crunch", and anything past 9:00 is more than I ever need. I'd like to lower that gain so that I can get more range and use out of the gain knob.
          Any circuit change after the channel switching relais will influence blue and green channels likewise.

          To only reduce blue channel gain you might wire a resistor (start with 47k) from the right side of R112 (in other words across the Blue gain pot) to ground. You could also make this resistor variable/adjustable.
          Last edited by Helmholtz; 05-02-2020, 05:53 PM.
          - Own Opinions Only -

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          • #6
            The half power switch cuts B+ in half by bypassing half the supply. There is no adjusting that. The bias part of the switch merely compensates. Don't associate power and loudness to closely. Even 1 watt is plenty loud. You want extreme attenuation of loudness, slap an attenuator on the output.

            You have more gain than you want in the preamp, how does turning down the post/master work?

            If I turn the volume down low enough to not bother the neighbors, it loses all of it's character.
            That is pretty much the nature of turning down. Your ears don't hear the same at lower volume. The speaker reacts differently at lower volume.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Originally posted by Helmholtz View Post
              Any circuit change after the channel switching relais will influence blue and green channels likewise.

              To only reduce blue channel gain you might wire a resistor (start with 47k) from the right side of R112 (in other words across the Blue gain pot) to ground. You could also make this resistor variable/adjustable.
              I admit that I know little about tube circuits, but it looks like R88 and R48 are being used to bias V2-A separately for each channel, since R50 isn't in the circuit (supposedly...I haven't opened it up yet). Why wouldn't lowering the value of R88 with a parallel resistor work, since it's only in the circuit when the blue channel is selected?

              Or, is that only affecting frequency response at that point?

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              • #8
                Originally posted by Enzo View Post
                The half power switch cuts B+ in half by bypassing half the supply. There is no adjusting that. The bias part of the switch merely compensates.
                Yeah...I'm a dork. I hadn't even realized that was a DP switch, and didn't see it in the PS.

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                • #9
                  To reduce the gain of the blue channel I'd add a resistor in parallel with the Blue Gain pot and increase the value of R112 so the frequency response is unchanged. If the parallel resistor is 47k (as Helmholtz suggested) make R112 300k. This will give about 15dB attenuation.

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                  • #10
                    Originally posted by calan View Post
                    I admit that I know little about tube circuits, but it looks like R88 and R48 are being used to bias V2-A separately for each channel. Why wouldn't lowering the value of R88 with a parallel resistor work, since it's only in the circuit when the blue channel is selected?

                    Or, is that only affecting frequency response?
                    Sorry, my bad. You're right that R88 and R48 do in fact affect drive levels to V2-A individually for both channels. So reducing R88 may help somewhat. (The resistors don't influence the bias of V2-A though).
                    - Own Opinions Only -

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                    • #11
                      I don't think it's a good idea to reduce the value of R88 to attenuate the signal as it has no other resistor to work with only the output impedance of the tube (35k?) You'd have to use a very low value for R88 which would load down V1-A and also change the frequency response.

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                      • #12
                        Originally posted by Dave H View Post
                        To reduce the gain of the blue channel I'd add a resistor in parallel with the Blue Gain pot and increase the value of R112 so the frequency response is unchanged. If the parallel resistor is 47k (as Helmholtz suggested) make R112 300k. This will give about 15dB attenuation.
                        What power rating would I need on the parallel 47k resistor? I'm guessing there isn't a lot going on there on the other side of the decoupling cap?

                        Originally posted by Dave H View Post
                        I don't think it's a good idea to reduce the value of R88 to attenuate the signal as it has no other resistor to work with only the output impedance of the tube (35k?) You'd have to use a very low value for R88 which would load down V1-A and also change the frequency response.
                        Great info. It's been 30 years since I was poking around in things with a VM for a living (and never anything with tubes), so I'm dusting off the grey matter and reading everything I can on tube theory.


                        Can you explain what that switched r88/r48 circuit is doing, and why it's noted to not use R50? It sort of looks like an RC network, but I'm confused as to why R42 sits between the two selectable values.

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                        • #13
                          Can you explain what that switched r88/r48 circuit is doing, and why it's noted to not use R50? It sort of looks like an RC network, but I'm confused as to why R42 sits between the two selectable values.
                          R42/R48 form a voltage divider which reduces signal for the green channel level by 1/40. R50 would mainly reduce the blue channel's drive level. So installing R50 may also help to tame the blue channel's gain..
                          Last edited by Helmholtz; 05-02-2020, 09:39 PM.
                          - Own Opinions Only -

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                          • #14
                            A local copy:
                            Attached Files

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                            • #15
                              Originally posted by Helmholtz View Post
                              R42/R48 form a voltage divider which reduces signal for the green channel
                              Ahhh... I see that now.

                              Originally posted by Helmholtz View Post
                              R50 would mainly reduce the blue channel's drive level. So installing R50 may also help to tame the blue channel's gain..
                              ...but still not understanding this.

                              Why would adding R50 not influence both channels? Is it because it's resistance is so much higher than that of R48, and therefore not having as much affect on that voltage divider's output?

                              And while we are in the same neighborhood, why does V2-A not have the cap in parallel with the R87 cathode bias resistor? (I still have a lot to learn on this new tube technology. lol)

                              Really appreciate everyones help here.

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