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Need some help with some EVH LBX II mods

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  • #16
    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?
    Yes, R50 would change the green channel's level by only 5%, while it would reduce the blue channel's level by a factor 3:
    (390k +180k)/180k.


    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.
    A cathode bypass cap would even increase the gain of V2-A. Similar to bypassing an emitter or Fet source resistor.
    Last edited by Helmholtz; 05-02-2020, 10:39 PM.
    - Own Opinions Only -

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    • #17
      They didn't want the increased gain and low end response a cap would add.

      Do the math, without R50 there is a 10k resistor to ground from that grid. If you add R50, it puts 180k in parallel with that 10k. I leave it to you to calculate the result. SO it won't much affect the green, but in blue, it makes the voltage divider with R42 cutting the signal to that grid by roughly half.






      I get 9.5k, not enough difference to matter
      Education is what you're left with after you have forgotten what you have learned.

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      • #18


        How does that look? If good, and there isn't a better/easier way to tame that blue monster, then any other advice on how to best implement it would be great (switch rating, shielding concerns, etc).
        Last edited by calan; 05-02-2020, 11:50 PM.

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        • #19
          The more I'm thinking about it, I may just use wiring and a switch to add R50 first...and if that isn't enough, then the gain pot mods.

          Thoughts?

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          • #20
            Originally posted by calan View Post
            The more I'm thinking about it, I may just use wiring and a switch to add R50 first...and if that isn't enough, then the gain pot mods.

            Thoughts?
            Try it and tell us how you like it. You could even use a lower value resistor like 100k for some more (1/4.9) attenuation.
            - Own Opinions Only -

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            • #21
              Originally posted by Helmholtz View Post
              Try it and tell us how you like it. You could even use a lower value resistor like 100k for some more (1/4.9) attenuation.
              Not sure when I'll open it up (enjoying playing it right now ), but probably in a day or two. I will definitely post my results.

              Anything to watch out for as far as shielding, current rating, etc if I remote mount a switch going to one leg of that added R50?

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              • #22
                Took the bottom off, and R50 is indeed empty, and easily accessible. Can it be this easy?

                I have already moved the standby switch to the front (where the power indicator used to be), so I could use the hole in the back for a bypass switch for R50.

                Anybody see a reason this wouldn't work?

                Any problems with running the wires back there that I should be aware of? (RFI, noise, etc)


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                Last edited by calan; 05-03-2020, 05:00 AM.

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                • #23
                  Before planning all the wires, may I suggest you just tack R50 in there and see if you like the results, and if it does what you are hoping. If it is good, you can worry about wiring then. If you don't like it, no time was wasted. And you may find switchable might never be used. I have converted some small amps from fixed bias to cathode bias and made it switchable, customer's request. They try it both ways, like one of the ways, and never switch it again.


                  I don't know from wires, your resistor is wired right to a sensitive grid, and your new wires will be crossing the chassis. Potential for instability.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #24
                    Originally posted by Enzo View Post
                    Before planning all the wires, may I suggest you just tack R50 in there and see if you like the results, and if it does what you are hoping. If it is good, you can worry about wiring then. If you don't like it, no time was wasted.
                    Sounds like a perfectly logical suggestion. Appreciate it.

                    I'll poke around with a VM, but would there be any residual voltage there after the amp has been off a while? I don't want to pull that mess to get to the filter caps, but I'm not in a hurry to die over less gain either. LOL

                    It acts like it has a voltage bleed circuit on power off, but I don't like to take chances with high voltage.

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                    • #25
                      If you are EVER in doubt about residual voltage or a charged circuit, FIND OUT. Get out your meter. If you can't get right on a cap, get on something connected to that cap. Your main B+ voltage goes to the center tap of the output transformer. Can you reach that connector pin? And from ther it goes to the plates of the power tubes. I know you can access that pin.

                      But there should never be much voltage on R50.
                      Education is what you're left with after you have forgotten what you have learned.

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                      • #26
                        Originally posted by Enzo View Post
                        If you are EVER in doubt about residual voltage or a charged circuit, FIND OUT. Get out your meter. If you can't get right on a cap, get on something connected to that cap. Your main B+ voltage goes to the center tap of the output transformer. Can you reach that connector pin? And from ther it goes to the plates of the power tubes. I know you can access that pin.

                        But there should never be much voltage on R50.
                        Yeah...I probed everything to make sure it was dead. No problems.

                        SO...

                        I only had a 100k and a 68k on the bench, so I series'd them up and tacked them onto the R50 pads, and (drum roll please...)

                        Huge improvement!

                        The gain knob now has much more resolution, and the amp just sounds cleaner. Still plenty of gain on tap, but not like a can of pissed off bees. I can actually max out the blue gain and have a tone that is usable, for that really saturated singing lead stuff. Back it off to 12 o'clock, and it's pure 80's Marshall. Taking it down to 9 gives a nice heavy breakup/crunch tone. (I couldn't get it past 10 or so before, and nothing close to a cleanish crunch). The actual tone doesn't seem to have changed at all and there is no added noise.

                        Now I'm curious to try some other resistors, and A/B the values. I may see if I can find a 150k or 200k trim pot to play with and safely wire it outside the chassis, probably through that big ole' switch hole in the back.
                        Last edited by calan; 05-03-2020, 06:47 AM.

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