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Pictorial of Biasing a Marshall Jubilee.

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  • brit84
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    do you just use a single bias probe on one tube or do you test on both? Im new to this but am keeping up so I can do it myself.

    Thanks

    Leave a comment:


  • skyydogg01
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    shameless bump.

    this thing NEEDS to be in the vault.

    someone like me, who is just getting into tube amps, this thread is very valuable.

    Thanks Jonesy

    Leave a comment:


  • SonofaMitch
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    +1 to all, Very Informative, Deffinatly Vault Worthy

    ~Mitch~

    Leave a comment:


  • Fresh_Start
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    Hoffman Amps sells the parts and has instructions for making your own bias current checker.

    Here's an interesting and off-the-beaten-path explanation of biasing an amp from the Tone Lizard. Don't have an oscilloscope yet though...

    Chip

    Leave a comment:


  • Lespaul1981
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    great thread!

    Leave a comment:


  • Gearjoneser
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    Originally posted by J Moose View Post
    Joe ~ What kinda volume were you running that thing at while biasing? I always thought tech's did that with no signal running through, or at least having the amp on standby...but that wouldn't apply full current to the plates would it?
    The amp has to be running to get a reading on the amp meter, but the volume level doesn't seem to change it very much. Ideally, it's nice to have it running at stage volume, when biasing by ear, so you can hear the tightness of the low end thump, which is what I aim for. It's hard to have the guitar strapped on while doing this, so I usually just lean it right next to where I'm working, and pluck the A/D or D/G, and listen while I'm tweaking the pot.

    Here's one thing to know. You never want to turn the bias pot drastically, because it can destabilize the circuit, and you'll get erradic readings on the meter. If the numbers start jumping around, you're better off turning the amp off for 15 minutes and letting the tubes cool, then starting again later.
    Another technique is setting it at 35, play and listen, 36, play and listen, 37, play and listen etc. The main point of biasing is to get the amp sounding as healthy as possible, and sometimes it's hard to tell when you're just fiddling with the bias pot and whacking the strings on the guitar.

    The most important thing, besides NOT electrocuting yourself, is NOT overheating the tubes. If you ever go past the desired numbers, you could fry your tubes. You'll know you're doing the wrong thing if the plates start turning orange. This is why it's not a good idea to do this without a meter on it. At least you know if you're in the ballpark then.

    Leave a comment:


  • J Moose
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    Originally posted by Christobevii3 View Post
    What do you do if an amp doesn't have a bias pot?
    Nuttin'.


    Really...if the amp doesn't have a bias pot then it's a 'fixed' or otherwise self-adjusting bias. At that point, changing a tube is no different then changing a lightbulb.



    Joe ~ What kinda volume were you running that thing at while biasing? I always thought tech's did that with no signal running through, or at least having the amp on standby...but that wouldn't apply full current to the plates would it?

    Leave a comment:


  • BigAlTheBird
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    I'd be too scared of ****ing up and dying, but that is a great post Joe! Definitely vault-worthy.

    Leave a comment:


  • Gearjoneser
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    Originally posted by J Moose View Post
    How do you set the volt meter up? Auto-ranging? DC current?
    Set it to DC Amps. The Auto Range means that the meter will change the metric range automatically. On cheaper meters, if the correct range isn't selected and the amperage exceeds that range, the fuse will blow.

    Leave a comment:


  • Farkus
    replied
    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.

    Leave a comment:


  • Christobevii3
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

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

    Leave a comment:


  • Gearjoneser
    replied
    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.

    Leave a comment:


  • joelap
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    nice thread, vault-worthy.

    Leave a comment:


  • dgk2448
    replied
    Re: Pictorial of Biasing a Marshall Jubilee.

    i want one of those SOOOO bad. awesome thread GJ.

    Leave a comment:


  • bbking1021
    replied
    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

    Leave a comment:

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