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  • 5x3 DC voltage on pin 5

    Just built a home brew 5x3 cathode bias and hammond trannies,and on measuring voltages on first start up i am getting dc voltage on both pin 5's of the jj 6v6,here are the voltages throughout the amp =

    12ay7
    pin 1=150v
    pin 2=0v
    pin3=2.42v
    pin 6=148.6v
    pin 7=22.2 mv
    pin 8=2.42 v

    12ax7
    pin 1=195v
    pin 2=0v
    pin 3=1.51v
    pin 6=232v
    pin 7=26.65v
    pin 8=66.8v

    6v6 v3
    pin 3=443v
    pin 4=410v
    pin 5=40mv
    pin 8=31.65v

    6v6 v4
    pin 3=440
    pin 4=409v
    pin 5=77mv
    pin 8=31.6v

    gz30
    pin 2 and 8=5.09ac
    pin 4=160v
    pin 6=160v
    pin 8=440v

    voltage across 469r resistor is 30.75v which gives about 13w and i think its in the ballpark for jj 6v6.

    havnt plugged guitar in yet but have a dummy load attached and a little scared as only built a couple of kits before and this is my first homebrew,i am thinking i am going to have to bring the voltages down and try and use a 300r resistor i have waiting.but will wait for advice from you guys before i proceed.

  • #2
    Whats the pri Z on the output tranny? With 8ka-a the model I plugged your values into looked pretty good. A solid 100% dissipation at idle, max dissipation at around 16W, but that's what I've seen with my 6V6 tubes. And an output transfer curve straight as an arrow. Hope you're looking for loud and clean!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

    Comment


    • #3
      Thanks for the reply.

      Output tranny is a hammond 1760j at 4000k.
      Loud and clean isnt exactly what i am looking for ,so whats the best way to address the loud and clean?

      Comment


      • #4
        Originally posted by strat100 View Post
        Thanks for the reply.

        Output tranny is a hammond 1760j at 4000k.
        Loud and clean isnt exactly what i am looking for ,so whats the best way to address the loud and clean?
        At 4000 anode-anode you might be in meltdown territory for 6V6s. Just a thought: Why not put in 6L6s, since that's the direction the OT seems to be going?

        There are plenty other options though. don't get discouraged by what I've said. Things I've seen suggested on this forum include:
        Recto tube that drops more voltage (lowers B+ and everything else) or D@MN BIG resistor in front of B+ node
        bigger resistor before screens to help control dissipation (I'll have to look at that to see how it affects the tube operating curve)
        other solutions are more esoteric, like a variac in front of the amp to bring line voltage down to 'vintage' levels
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

        Comment


        • #5
          Thanks eschertron food for thought definately,got an nos 5y3 and some zeners on the way so will see what happens when i give them a try and have a good think on the 6l6's after getting the amp in good order.

          Comment


          • #6
            Originally posted by eschertron View Post
            At 4000 anode-anode you might be in meltdown territory for 6V6s. Just a thought: Why not put in 6L6s, since that's the direction the OT seems to be going?
            How about doubling the load impedance to reflect back double the primary Z (so 8K).
            "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

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            • #7
              Thanks G-one but as a relative newcomer who has probably chosen the wrong transformers and has no idea how to double the load impedance, maybe it would be best to put some 6l6's in as the tranny no doubt is best for them or change the output tranny to a deluxe reverb one.
              Last edited by strat100; 10-30-2014, 07:25 AM.

              Comment


              • #8
                I think he meant double your speaker impedance? Just a quick&dirty way of using something you might already have on hand... unless you already have a 16ohm in it.

                ??? :P

                Justin
                "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
                "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
                "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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                • #9
                  ot has 4,8 and 16 ohm taps and have the 8ohm speaker into the 4 ohm tap

                  Comment


                  • #10
                    Originally posted by strat100 View Post
                    have the 8ohm speaker into the 4 ohm tap
                    That's what I meant. You have doubled the load impedance for the tap. Having a 16ohm speaker on the 8 ohm tap would have the same effect.
                    So you will reflect back double the specified primary impedance. If the transformer is spec'd as 4K, the way you now have it loaded makes it effectively 8K.
                    So you seem to be covered as far as the impedance issue goes, but you still have to contend with the B+ issues. If you are too high for the 6V6's you will need to decrease B+ or consider switching to 6L6.
                    The reason I was trying to avoid the 6L6 is that you said you were not interested in loud and clean, so the 6V6 are probably more of what you are looking for sound wise.
                    "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

                    Comment


                    • #11
                      fired her up tonight with a borrowed nos 5y3 which brought the B+ down to 404 but still with a 470 ohm resistor,so still need to address the B+ but the amp was very loud and very clean all the way to 12,luckily no problems with noise infact extremely quiet so i am very pleased and cant wait to lower the B+ and get some grind .

                      Comment


                      • #12
                        Just an update;added some zeners to the ct and brought the B+ down to 361v and put a 300 ohm on the cathode and that put me at 12.4 watts and the amp has some break up now,not as much as expected but that could be down to the cannabis rex speaker,thinking of getting a jensen jet lightning to see if that helps.

                        Comment


                        • #13
                          Are you using a 12AY7 in the V1 position (indicated in post #1)? Drop in a 12AX7 to see what the amp sounds like with the output tubes slammed
                          If it still won't get loud enough, it's probably broken. - Steve Conner
                          If the thing works, stop fixing it. - Enzo
                          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                          Comment

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