EL34 50/50 power amp bias controls location?

Discussion in 'The Workbench' started by dave999z, Mar 11, 2010.

  1. dave999z

    dave999z Active Member

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    Could someone tell me where the bias controls are located on this power amp?

    So far I have only opened up the top and don't see them. So I presume I'll have to go further into the chassis (they're probably on the circuit board somewhere?).

    Thanks!!!
     
  2. Jesstaa

    Jesstaa New Member

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    I'm not sure there's any Marshalls with bias trimpots on top of the amp.
    They'll be below the decks in the guts.

    Be careful not to touch things, you know.
     
  3. dave999z

    dave999z Active Member

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    Just don't know how to get to it. Wish someone had a picture showing which screws to take out. I have scoured the web on this. Nothing.
     
  4. Wilder Amplification

    Wilder Amplification New Member

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    It's on board number 60-02 and should be labeled on the board as RV1. RV2 on that same board is the "balance" pot that allows you to balance the phase inverter circuit gain/output in order to allow you to set the "symmetry" of the output section.

    Both sides should have their own board or be cordoned off from each other with markings of some sort.

    Here's the board layout schematic for the 100/100 (should be the same for the 50/50 but with two less power valves per side). If the board layout schematic is correct it should be on the board closest to the front of the chassis (board #60-02) -

    http://www.wilderamplification.com/schematics/Marshall/dualmonobloc/boardlayout.pdf
     
    peterichardz likes this.
  5. dave999z

    dave999z Active Member

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    Thanks very much for the info. Much appreciated! I guess I have an even more basic question, which is how to even access the board in the first place. Do I need to remove the 6 screws that secure the plate holding the tubes? (Or maybe I need to go in through the bottom of the amp?)
     
  6. Wilder Amplification

    Wilder Amplification New Member

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    That I would need a pic of the amp opened up to tell ya how to do it if you can make that happen.
     
  7. dave999z

    dave999z Active Member

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    i'm sorry the photo quality is so bad. my camera sucks and i'm running out the door to work. but if this is not sufficient i'll attempt better photos later.

    [​IMG]

    thank you, again!
     
  8. Wilder Amplification

    Wilder Amplification New Member

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    Anytime...looks to me like it's done through the bottom.
     
  9. dave999z

    dave999z Active Member

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    I'll try taking the bottom cover off tonight and see if the controls can be accessed that way. If so, seems like it's going to be difficult to make the adjustment since the amp has to be running but I'll have to simultaneously turn a control on the bottom of the amp. Maybe if the controls are near one of the edges, that edge can just hang off the table.
     
  10. Wilder Amplification

    Wilder Amplification New Member

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    Or you can turn it on its side.

    However, only because you're new and I'd hate to see you hurt your amp let alone yourself, you have biased an amp before correct?
     
  11. dave999z

    dave999z Active Member

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    Actually, no. But I'm only planning on doing it if it looks doable. Haven't even ordered my replacement tubes yet. I want to pop it open and see how accessible the controls are. If they're right there, then I was going to go ahead and order a Weber bias probe, which seems pretty idiot proof as long as I use one hand and don't go touching random internal components.
     
  12. Wilder Amplification

    Wilder Amplification New Member

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    The Weber BiasRite is your best bet and was what I would've recommended to you had you not already known about it.

    Let me know how things go with that.
     
  13. dave999z

    dave999z Active Member

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    I still apologize for my sucky camera (it's a video camera that has the ability to take bad stills).


    Here's a picture from the bottom:

    [​IMG]


    Here's a picture showing the front half (I put pink circles around what I think must be the bias controls):

    [​IMG]


    Here are closer shots of the front left and front right:

    [​IMG]

    [​IMG]


    OK, so a few questions...

    1. Are those indeed the bias controls?

    2. If so, obviously the only way to adjust the bias on this amp is by placing it on its side?

    3. Would I use any tiny plastic screwdriver-type tool to turn them?

    4. What stuff in there will kill me if I touch it?

    Thanks again! This is really helpful.
     
  14. Wilder Amplification

    Wilder Amplification New Member

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    Each amplifier has its own "bias" control along with it's own "balance" control. Since they didn't mark which was which, you'll have to measure the voltage on each leg of the pot to find out. If you look at the 3 solder joints around the perimeter of the pot access holes, those will be your test points. If you have a multimeter, measure the resistance to ground at the pot's terminals (the terminals closest to the access hole for each pot). One of those pots will have continuity to ground (the chassis) on one or two of its legs. That one is your bias pot.

    The "balance" control is something you would have to have a scope to set so just forget it's even there.

    Yep

    Or a small blade metal one with an insulated handle.

    Assume that all of it will. You'll be working on the thing with it live so you don't wanna be sticking your hand into a live amp if you can avoid it.
     
  15. wkcchampion

    wkcchampion New Member

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    So.. which ones are they? The inside or the outside ones?
     
  16. MerlinVSM

    MerlinVSM New Member

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    Hi
    Onyone, which of these 4 bias pots to use when bias the powertubes. Is it one bias pot per left channel two tubes and one for right channel teo tubes ?
    Is it one bias pot per Power tube? Please, anyone clarify.

    Thanks for help.
     
  17. MerlinVSM

    MerlinVSM New Member

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    It is the outer pots that bias the power tubes (two tubes per pot) acc. to other posts on this forum.
     
  18. wkcchampion

    wkcchampion New Member

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