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amc cvc3030 bias tap issue

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  • amc cvc3030 bias tap issue

    This is beginning to do my head in this one.
    This amp first came to me about a year ago with burnt out edge connector on the heater terminals . Repair with transformer to tube base wiring , Tested the tubes ok and rebiased as the manual stated. My notes say that the bias was set and there was very little scope for adjustment
    a year later it comes in for dead. Tubes failed. replaced for marshall el34 and tried to rebias. couldn't get them cold enough,. tired another set of electro harmonixs still the same
    B+ 452v unloaded dropping to 430v with tubes in
    Looking at the bias tap i had 30v AC but only -39v DC where to my maths should of been -41v .
    Replaced all caps and bridge rectifier still no change -39v . I check unloaded voltage still -39v
    Incase of conductive board i disconnected the bias tap from tx and made a bias circuit on tag board, Still -39v

    Now if my memory serves me right when i fitted the new tubes the bias probe meter stated that they were running at 58ma per tube less than a minute

    So it looks like what ever i do there will be never enough bias voltage.

    Before i move on to looking to mod the bias circuit with either
    voltage doubler circuit
    capacitor coupled bias circuit ( No HT center tap)
    cathode bias

    any advice will be truly welcome

  • #2
    Looks like the individual bias can be adjusted to ZERO VOLTS!!!! A very poor arrangement. However. Pull your power tubes and dial the bias down to zero volts. Now what voltage appears at the grid pins of the power tube sockets? Looking for potential small DC leakage of coupling caps.

    30vAC gets me about 42vDC, then reduce by a couple of diode drops for the rectifier and whatever the resistance of that tiny fuse might be, and we are darn close to your -39v.
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      Also check your screen voltages at pin4 of power tube sockets.
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


      • #4
        Thank you.
        With the tubes out and the 4 pots dialed to zero i'm getting -37.8v
        With the tube board bias feed disconnected ( Bias unloaded ) i'm getting -38-39v
        Pin 3 and 4 452v unloaded
        The fuse has been bridged no change
        I did remove the bias tap from the connector and mock up another circuit in case of board issues, no increase bias voltage
        Spoke to AMC he said i should have a max of -43v

        when the new tubes are fitted, i followed the bias procedure in the manual, after about 20 secs i could only get 6.5v on the test points. With bias probe fitted it was at 55-58 ma per tube.

        my customer is keen on the this becoming running colder and to change the tubes himself without having to pay me to re bias so cathode bias might be way forward


        • #5
          Just out of curiosity: What is your heater voltage? And your actual mains voltage?
          - Own Opinions Only -


          • #6
            Heater is at 6.5v and mains voltage here is 227v with the amp set to 220/230v tap. Uk mains line voltage was 240v now we are on 230v


            • #7
              Has the mains voltage supplied to your distribution board been reduced? It’s still up up at 240-245V around here.
              The introduction of the 230V nominal was accompanied by a relaxation of the acceptable tolerance, so as to accommodate the existing EU wide situation.
              My band:-


              • #8
                Hi PDF.. Well here in the workshop i'm 230v, the owner is about 10 miles away..
                I'm not sure about his supply. Its just too close for comfort for me. Not impressed with the bias circuit
                Going to look at modding it,


                • #9
                  Good afternoon. I have now fitted a voltage doubler circuit. i can now bias with plenty of adjustment.
                  many thanks for your help
                  Till next time