NewReligion
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  • At least I found the right power amp schematic; JMP50B.
    NewReligion
    NewReligion
    If you don’t have a bias from the two would fit into and go to the multi meter, there should be a lead that is solid to pin eight connecting to pin one and then two ground of the power tube.

    Replace that bare wire connection with a one watt one ohm resistor. You can then measure across from Penn eight the pin one mV. It should read about 32mV To be 65% dissipation which will be just fine.
    Dean Swindell
    Dean Swindell
    Hi thanks for the quick reply. Yes I've already done that. The problem is the bias is turned all the way down but the MV is about 27.5 and pin 5 is -41.5. Need to reduce the negative volts to Pin 5. R30 is suspect.
    NewReligion
    NewReligion
    Some Marshall’s do not have a resistor on pin five aka the Swamp resistor.

    Does your app have a resistor on pin five?

    More importantly what is the reading across the 1ohm resistor from pin 1 to pin 8?
    Hi David; don't mean to be a pest, but I need a little advice. My mk III bias is too high when turned all the way down, it only goes down to negative 41.5 at pin 5 and 28ish m.v. on the cathode resistors I installed. I'm thinking of upping the value of the resistor going into the pot (R30). What would you do?
    Hi! I was working on my JCM 900 2500 today and noticed it has the JMP51C preamp. I can't beat the schematics out of the bushes to save my life. The B is clearly closer than the A but not really good enough. Do you happen to know where I can get this?
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    NewReligion
    NewReligion
    Unfortunately I do not. You might ask some of the guys at SLOCLONE forum as their specialty is reverse engineering and having all data. They even make boards for Firsrl V4’s available.

    Wish I could help more.
    I once turned a Fender Showman into a sort of Boogie MK I. There are plenty of mods out there for that, but I can't find one for the MK II OD circuit (OD after tone stack). You know where I can find this?
    Hey what would the JCM 900 Mark III sound like if you disconnect the diodes that are connected almost directly to the input jacks?
    NewReligion
    NewReligion
    Try it. Just lift one side and see if it still has a signal. If so that will answer your question. Less clipping I would think.

    If there is no signal just rum a wire (aka staple) in its place to complete the circuit.
    Hey David - how does your EVL-50 mod respond to the guitar volume being rolled back a bit for something like a Zeppelin or AC/DC level of gain? The reason I ask is that I just sold a Bray-modded Super Lead because I absolutely could not get a decent lower-gain tone doing so. I realize there will be some loss of highs with a rollback of guitar volume, just wondering how your mod reacts to that. Thanks bro
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    NewReligion
    NewReligion
    Yes it does. In addition you can plug into the low and use a over drive if you like and get a nice smooth 2204 pushed tone. David...♫
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    Can you share more info about your Pandora mod on Marshalls (I left you a comment on YouTube too)? I can't believe what phenomenal tone you're getting at such low volumes. I have 4 Marshall amps I'd like to put it into so I can start using them every day- and at gigs- everywhere I play they are FAR too loud- but oh that tone!
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