Help with Transformer Voltage Drop Method

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by twinrider1, Jan 23, 2008.

  1. twinrider1

    twinrider1 Member

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    I'd really appreciate a walk-through on figuring bias by measuring voltage drop on the output transformer. The patient is a '89 Marshall 2210 (pics available). I would put my skill level at being mechanically able, but not yet experienced enough to know the reasons why things happen. (Edit: What I mean is I know Ohm's Laws type things, but I can't voice an amp, decide what to change to change the tone, etc.)

    Thanks.
     
  2. stoo

    stoo Member

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    Power disconnected ..caps discharged!!!
    You'd measure the resistance in each half of the primary winding on the opt. So that'd be from center tap to the plate of the tube.
    Then power up the amp and measure the voltage drop on each half of the transformer. Ohms law says I= E/R. So if you had a 5 volt drop over a 100 ohms...5/100 =.050 amps = 50 ma for the tube associated with that side of the tranny.
    Read all about it here bout 1/3 way down . http://www.aikenamps.com/Biasing.html
    Stew
    Stew
     
  3. twinrider1

    twinrider1 Member

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    Thanks. This matches up with the excerpts from this thread...
    http://www.thegearpage.net/board/showthread.php?t=323260&highlight=transformer



    I have questions about the practical application. Where do I physically probe? I can see the OT and the wires, but not an open point to probe.

    Also, Doyle's HoM has a table indicating White=CT and Brown & Red=Plate. But on the schematic, it looks like Brown is the center tap with White and Red going to the plates. What's going on here?

    I have to say, most of my limited real-world experience is with DC circuits. Comprehending these amp circuits is a challenge. It feels like a balancing act on paths of a big parallel circuit.
     
  4. stoo

    stoo Member

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    If you look at the power supply the B+ should be the first node. ie the first voltage "take off point" after the rectifier. That should be where the CT of the opt is connected. On some ccts this is where the standby switch is located. It's probably where the first filter cap is connected too. The outer ends of the opt are connected to the plates of the o/p tubes (pin3).
    Stew
    ps. Just had a look at the schematic.. Is there a H/T fuse? That'll be in the B+.
     
  5. twinrider1

    twinrider1 Member

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    Thanks, that helps. I've been spending some extra time comparing the actual wiring to the schematic and it's becoming clearer. Embarassed to say it took a minute to figure out my digital meter. I'm used to the Rx1, Rx10, etc. on my old Simpson. I don't see the logic in the 200, 2k, 20k, etc. (it's not autosensing). I pulled a few resistance readings, but with the kids under foot the live measurments will have to wait 'til tonight.
     

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