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  • Pictorial of Biasing a Marshall Jubilee.

    Tonight, after playing my Jubilee, which has had the same JJ E-34L's and Tungsol pre's for a couple years, I decided to do a tuneup on it. I have a bias tool I bought off Ebay for around $15, my volt/amp/ohm meter, and some contact cleaner. After pulling the chassis out, I sprayed a little bit of contact cleaner in the pots and pushed a cable in and out of each jack, so they're good as new. Next, I put the bias tool in between a power tube and the amp, turned on my meter, and noticed that the bias was on the cold side after a couple years of using it lightly.....it read 29mA. EL-34's and E-34L's usually sound best between 35 - 42mA.

    My method is to have the amp chassis plugged up with the cab and guitar, and hit some chords, while adjusting the bias pot. I hold the chassis up on an angle with my meter laying on a transformer. I'm EXTREMELY careful to not electrocute myself by using a plastic handled screwdriver, and I protect the cab by laying a towel on it. I always bias by ear, trying to get the amp to sound as punchy and strong as possible, but make sure the numbers fall between 35-42mA. After a half hour of making adjustments and hitting chords on the guitar, I decided that it sounded best at approximately 40mA.
    These aren't new tubes, so it was just an interim bias/checkup. It sounded best between 38 and 40, so I went a little hot, and loved the sound at 40mA.
    I checked it on both channels and both tubes, just to make sure it sounded healthy as ever. Here's some pics!

    Set to DC Amps

    It reads a little low at 28-29mA. 6L6's are usually 32mA, but EL-34's should be around 38mA

    I turn the chassis up on it's side, while turned on, plugged into a cab, with guitar plugged into the front.


    Find the bias pot, but be careful not to touch ANYTHING. Use a plastic handled screwdriver. I hit the guitar to make it Kerrrang!...then tweak the pot so the amp is sounding bold as can be. Stay in range though.





    Ah, the British tone just kills. 97 Matchless Chieftain, and 87 2550 Jubilee!!!
    Last edited by Gearjoneser; 02-14-2007, 02:00 AM.
    Originally posted by Boogie Bill
    I've got 60 guitars...but 49 trumpets is just...INSANITY! WTF!

  • #2
    Re: Pictorial of Biasing a Marshall Jubilee.

    Vault. Now.

    Comment


    • #3
      Re: Pictorial of Biasing a Marshall Jubilee.

      Nice job Joe! So slightly OT I was wondering if you change out your cabs on your Jubilee. you know slant for 3 months, straight for 3 months. Something like that.

      Luke
      “That which we do for ourselves dies with us … that which we do for others lives forever.”

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      • #4
        Re: Pictorial of Biasing a Marshall Jubilee.

        Those vertical trimpots always make me a bit nervous when I'm working on a Marshall. Kinda nerve-racking working on an amp like that when its on .

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        • #5
          Re: Pictorial of Biasing a Marshall Jubilee.

          Originally posted by krankguitarist View Post
          Those vertical trimpots always make me a bit nervous when I'm working on a Marshall. Kinda nerve-racking working on an amp like that when its on .
          You wanna know why I put a towel on the cab? A few years ago, I was biasing a head like this on an angle. Stupidly, I used a metal handled screwdriver, and accidentally touched something, while my other hand was holding the chassis up. I got a zillion amps running through me, and was stuck to the chassis for a few seconds. Naturally, I let go of the chassis, and it slammed down on the cab, and I fell backwards!

          I was afraid the tubes might have been damaged, but everything was fine. However, there was a few marks on the top of the cab, where the head slammed down on it. It scared the crap outta me, and coulda killed me.
          I threw that metal handled screwdriver in the trash, and went and bought a set of plastic handled ones at the 99 cent store.

          So, let this be a lesson to you guys wanting to bias your own amps. You've got to be VERY careful with a live amp, and poking a screwdriver in there.
          You could literally die if you make a stupid mistake. Tube amps throw out some serious amperage!
          Originally posted by Boogie Bill
          I've got 60 guitars...but 49 trumpets is just...INSANITY! WTF!

          Comment


          • #6
            Re: Pictorial of Biasing a Marshall Jubilee.

            Originally posted by Gearjoneser View Post
            You wanna know why I put a towel on the cab? A few years ago, I was biasing a head like this on an angle. ....Tube amps throw out some serious amperage!
            That story is METAL! :
            Originally posted by Scott_F
            On that day, should I ever be so unlucky, I will expect an unholy assault of pure metal mayhem attacking all my senses with a little tiny voice in the background screaming Effing Hails!

            Comment


            • #7
              Re: Pictorial of Biasing a Marshall Jubilee.

              That's so vault worthy..
              Originally posted by Guitarist
              Honestly, I like Scott's words. "There is a rhythm to life. Ride the waves."

              And keep in mind that while nothing lasts forever, nothing is lost.
              http://soundcloud.com/adrian-czarnecki/tracks

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              • #8
                Re: Pictorial of Biasing a Marshall Jubilee.

                very informative, brother.

                yes. this must be in the vault.
                -Greg

                Guitars:
                2016 Ibanez RG6003FM (JB/'59nJ)
                2018 G&L Tribute Legacy (Hot Rails/stock/stock)
                2019 Squier Affinity Telecaster HH (Esquire'd w/ JB)
                Applause/Ovation AE28 Acoustic/Electric

                Comment


                • #9
                  Re: Pictorial of Biasing a Marshall Jubilee.

                  Gnarly.

                  How do you set the volt meter up? Auto-ranging? DC current?

                  And where can I find one of those slick little bias tools? Is that what it's called? search Fleabay for 'bias tool'?

                  Thanks!
                  J. 'Moose' Kahrs
                  mixer|producer|recordist
                  mooseaudio.bandcamp.com

                  Originally posted by the guy who invented fire
                  All you need to make a record is a mic, some tape and maybe some bad reverb...

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                  • #10
                    Re: Pictorial of Biasing a Marshall Jubilee.

                    Originally posted by J Moose View Post
                    Gnarly.

                    How do you set the volt meter up? Auto-ranging? DC current?

                    And where can I find one of those slick little bias tools? Is that what it's called? search Fleabay for 'bias tool'?

                    Thanks!
                    Look on ebay for the bias tools. You can look under "tube bias" "amp bias". They usually can go for $15-50 depending on features.

                    +1 for vault
                    Epiphone Dot (Seth/Seth)
                    Fender Hybrid Telecaster (APTR-1/JD)
                    Peavy Predator (aps2)
                    Squier Standard Jazz Bass
                    Seagull S6 CW Mahogany w/ L.R Biggs Duet II

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                    • #11
                      Re: Pictorial of Biasing a Marshall Jubilee.

                      i want one of those SOOOO bad. awesome thread GJ.
                      Originally posted by ratherdashing
                      Ah yes, a Gibsom Res Paur. Good fortune comes to buyer!

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                      • #12
                        Re: Pictorial of Biasing a Marshall Jubilee.

                        nice thread, vault-worthy.
                        7 FREE TRACKS OF ROCK - driftrocks.bandcamp.com

                        PARTY - pulsepartyband.com

                        In mother Russia, pedal overdrives you.

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                        • #13
                          Re: Pictorial of Biasing a Marshall Jubilee.

                          I came home tonight, and turned the amp on. I biased it last night around 11, so I couldn't turn it up too loud. Now that I've gotten to hear it at a decent volume level, I can tell that it sounds thicker and richer, and the gain has tightened up. I figure 40mA isn't too hard on E-34L's, which are 10% higher output than regular EL-34's, which I probably would have left at 38mA.

                          Bias tools come in the mA or voltage versions. I like the mA version better, so if you shop for one, buy the mA tester. I think I paid about $15 for it.
                          I think my volt/ohm/amp meter was around $50, but I've had that since I was 18, going to electronic tech school. Every guitarist with tube amps should have those two things. There's no reason to pay $40 for a bias, when a person with basic electronics skills can handle that task.

                          I can't say it enough though....BE EXTREMELY CAREFUL when poking around inside a live tube amp. Those amperages are no joke...they're very lethal.
                          The most important thing is to have plastic handled tools, and some people even wear gardening gloves to insulate themselves from being grounded accidentally.
                          Originally posted by Boogie Bill
                          I've got 60 guitars...but 49 trumpets is just...INSANITY! WTF!

                          Comment


                          • #14
                            Re: Pictorial of Biasing a Marshall Jubilee.

                            What do you do if an amp doesn't have a bias pot?

                            Comment


                            • #15
                              Re: Pictorial of Biasing a Marshall Jubilee.

                              Bob over at Eurotubes is making bias probes for around $25.00, which is cool if you already have a meter. If you don't he is selling cheap multimeters for $10.00 as well. Pretty sweet deal as you know the stuff will work when it gets to you (and if it doesn't he'll make it right). I'm not an employee or anything, but I think it's cool that he's offering a (relatively) low cost solution for the average player.

                              Nice post Gearjoneser.
                              2007 Strat ('78 bridge, a2 Pro neck)
                              1976 Strat (Antiquity 1 set)

                              Fender, Mesa, Marshall Amps

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