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EVH 5150 MK3 mini head reducing the gain

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  • EVH 5150 MK3 mini head reducing the gain

    Hi

    I had a customer see me with this Mini head and he want to reduce the noise and gain
    He is a heavy metal player.
    I did say that the head is what it is and with 4 pre amp tubes with one channel + a crunch he is better off using it or selling it, But he want to keep it.

    Ive played around with tubes changing for 5751 and 12au7 etc but this does not please him

    Any guys out there tried a few mods and worked


    BBB

  • #2
    Is the amp model an MKIII or an LBXII?

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    • #3
      its a fender made Mk111 type PR3736

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      • #4
        Never heard of it. (PR3736)

        The MKIII came in a 50 & a 100 watt version.

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        • #5
          It does say mk3 on it but it's a 15 watt version with el84 will put up a picture tomorrow. The pr3736 is on the bottom I've never seen them before

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          • #6
            I think that would be the 5150 III LBX (lunchbox)
            Schematic:
            Attached Files
            "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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            • #7
              Sounds like the LBX to me.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                There is also the LBXII.
                It's missing V3.

                EVH_5150_III_15W_Head_Tube_Functions.pdf
                EVH_5150_III_LBXII_Head_Tube_Functions.pdf

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                • #9
                  Thank guys for that .. as this amp does not say LBX on it a didn't know what it was as ive never seen them before . It is what you lot say it is.

                  Now Modding is not my strong point. The owner uses it with the crunch button in. He want to reduce the gain and get rid some of the hiss.

                  BBB

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                  • #10
                    That would be V3, the 'Red' channel.

                    I don't see any controls on that circuit.
                    It's pretty much 'balls to the wall'.

                    I would scope it to try to see what the guys gripe is.

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                    • #11
                      Thanks jazz bass. I'll have a check with him what he wants. Sometimes these sort of customers are just not worth the hassle on jobs like this.

                      Bbb

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                      • #12
                        IF its the first circuit posted, I can suggest a few attenuators , just a single extra resistor each, you tack solder them one by one until hes happy.
                        Each should take about 10/12dB (back of the envelope calculations)
                        * 220k from junction (Net) R69 - V1a2 (pin 2 - grid) to ground. As near as possible, preferrably to far end of C67 or R96 to avoid ground loops.
                        * 150k or 220k in parallel with R88 (820k)
                        * 100k from Net R72-V2b7 (grid) to ground. Preferrably grounded end of C66 or R95

                        These should be more than enough, wed be knocking over 30 dB gain (and noise) , in fact probably 1 or 2 will be enough.
                        Way better and more predictable than tube swapping and keeps original bias at each stage.

                        You ***miiiiight*** add small toggle switches, similar to pickup phase or coil splitting ones for each attenuator (use twisted cable inside or shielded wire, those grids are sensitive) to let owner mix and match at will ... but that doubles job price, you are *redesigning* his amp, way beyond plain modding. Your choice.

                        I would leave the Red channel, around V3a-b untouched, even with a tamer amp hell still need to get the fury back for solos and such. In any case that distortion will be somewhat tamer because you will not hit it that hard from the preamp.

                        Test them and post results, but always with customer side by side and with his approval, so later he does not get cold feet and accuses you of messing with his amp or something.
                        Last edited by J M Fahey; 09-13-2017, 10:45 AM.
                        Juan Manuel Fahey

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                        • #13
                          Thank you Juan manuel fahay. I'll contact the customer and let him know. Bbb

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                          • #14
                            There's the gain knob, that does actually turn counterclockwise too.

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                            • #15
                              Just to let you know ive have done the mods below and it seems to work well. massive reduction in the hiss.
                              I'm letting the customer try it and let him make that decision

                              " 220k from junction (Net) R69 - V1a2 (pin 2 - grid) "
                              " 100k from Net R72-V2b7 (grid) to ground. Preferably grounded end of C66 or R95 "

                              bbb

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