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Plexi with Cathode Bias?

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  • #1

    Plexi with Cathode Bias?

    Im working on a Plexi 1987 Clone and cant get the fixed bias Circuit to work. is it possible to change to cathode bias and what do i have to change, thanks for any suggestions, here is the schemo im refering to
    http://www.drtube.com/schematics/marshall/1987pljp.gif
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  • #2
    It is possible, but unlikely. The thing to do when you fixed bias doesn't work... is to fix it.

    What do you mean it doesn't work? You get no bias? The bias is too high or too low? It won't adjust? Do you get voltage at the bias supply but not at the tubes? Just what is wrong?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      enzo, I've build the Power supply and the bias supply apart from the rest of the circuit and im just testing this while waiting for the PT to be shipped to me. I have a laboratory PT where i can adjust Voltage and with this i checked my "Power Circuit".
      - First I build the Power Supply with a full Bridge Rectifier ( not as in the Schemo)connecting the negative side to ground which makes a perfect B+ line, but connecting the bias supply couldn get me more than - 20 millivolt
      bias voltage.
      - The i thought that maybe the connection of the full bridge rectifier negative to ground makes the problem and changed to a just two diodes, now i had a bias Supply with about -22 Volt ( now thats something that can be tweaked to the right value) But now my B+ starts with 50 volt and goes up about 5 volts by the minute, stopping at about 100 Volts and i did not wait for it going up further. so at this point i thought why not change to cathode bias, wich i had working in all of the amps i build so far.
      Im using 1N4007 Diodes, they are only good for 1 Ampere maybe this is the Prob?
      - Of course im a noob and shurely missed a point, but i did not want to bother with this and had good experience with cathode bias

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      • #4
        Rex, are you using a separate winding? Or taping from one ~ ac leg? If the later, don't forget to use a cap between your bridge recto and bias diodes. Check ampeg schemos!

        http://www.schematicheaven.com/ampegamps/v4bpowramp.pdf

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        • #5
          In the original drawing, the B+ is derived from a center tapped winding with CT grounded. The two diodes form a full wave rectifier providing the B+ to the first filter.

          You could change that to a full bridge, but with the same PT your voltage would be doubled. Or use a lower voltage PT.

          But the bias supply on the original is derived off one side of the HV secondary on the PT. That relies on the CT being there. That one half of the B+ winding serves the bias rectifier through a dropping resistor. When you went to a full bridge, you eliminated the CT ground connection. Now there is no longer a complete circuit for the bias feed. That is why there is no bias voltage for you.

          Nothing in the B+ or the bias supply circuits draws anywhere near a whole amp. 1 amp rectifiers are more than enough.

          When you went to two diodes and dropped the ground connection, you no longer had the full B+ supply, so that slow charge up is the result.

          If you are getting the proper PT, then just wire the amp like the schematic. Ground the CT, use the two rectifiers. It will make fine B+. And then the bias tap off one side will work right. When you changed to a full bridge, without understanding how it worked, you wound up leaving out things the supply needed. Like complete circuits to ground.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            thank You Satamax, Thank You Enzo, this is very helpful, I will wait for the PT to be shipped to me and check everything with the CT connected. ( You were right Enzo i had never before build an amp with a PT with CT) Now as this PT has also a Secondary with 50 Volt i could also realize the bias according to this schematic: http://marstran.com/1966.gif
            (of course tweaking values etc.)
            But just for the fun of it, is it possible to change to cathode bias? maybe even switchable? How could it be done?

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            • #7
              Got it working without CT

              Satamax, your reply opened the door! I found an other thread were a guy had a similar Prob and now i did it according to the ampeg schemo (see my attached schem of the working bias supply) now it works even without a center tap, well now i am much more relaxed, and the PT came today as well, hope i get the amp up running before easter

              thank You again, this Forum is so great!
              Attached Files

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