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  • Ampeg v4 Low Signal

    Could use a hand from the pros here on an Ampeg v4.

    Schematic:
    http://www.ampegv4.com/images/schema...VT-22_1977.jpg

    The amp has VERY low signal coming through. I ran a preamp (effects pedal) into the power amp input and got good signal so I am sure the issue resides in the preamp section and have started tracing through the schematic.

    The circuit is pretty damn complicated and I am not familiar with the "V201" (6K11) type tube. Should I have signal coming out of pin 10 or is that just a voltage pin? (the voltage does check ok at 320vDC but no signal is present)

    If not pin 10 should the amplified signal coming out of pin 4 then?
    Thanks

  • #2
    Originally posted by Pryde View Post
    Could use a hand from the pros here on an Ampeg v4.

    Schematic:
    http://www.ampegv4.com/images/schema...VT-22_1977.jpg

    The amp has VERY low signal coming through. I ran a preamp (effects pedal) into the power amp input and got good signal so I am sure the issue resides in the preamp section and have started tracing through the schematic.

    The circuit is pretty damn complicated and I am not familiar with the "V201" (6K11) type tube. Should I have signal coming out of pin 10 or is that just a voltage pin? (the voltage does check ok at 320vDC but no signal is present)

    If not pin 10 should the amplified signal coming out of pin 4 then?
    Thanks
    It may be a bad preamp tube. No signal should be on pin 10, that's the power supply.
    Check R204, R 207 is there voltage on both sides leads of the resistor(s)?
    The audio output of V 201 should be on pin 4. It's a cathode follower.

    Comment


    • #3
      Before you get too far in, give all the preamp tubes a wiggle or pull them in and out of their sockets a few times to clean the pins. It is fairly common with these to have signal dropouts/crackling etc. because of dirty tube sockets. Beyond that the tube sockets often need to be resoldered.
      "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


      • #4
        And make sure that the power amp in jack, switching contact is making a good connection. Try plugging a spare cord from the pre out jack to the power in jack.

        Comment


        • #5
          There is nothing cosmic about these tubes. They are just triodes, like the triodes inside a 12AX7. Only thing is there are three triodes in one tube instead of just two. In the later years of tubes, they thought that might be a convenient thing. And that is why the tube has the extra pins.


          But triodes are triodes, and they work the same whether there are three in a tube, ir just two, or even just one.


          There is no signal on pin 10 because it is a cathode follower stage. Pin 10 is a triode p;ate, and the triode plate in a cathode follower is connected directly to B+.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Thanks for the recommendations everyone. Previously I cleaned the power amp input jack and re-tensioned it but still had low signal. This morning I jumped the switching pole and bingo, good signal. I just replaced the switching jack and all is well there.

            The amp is operating now and the 7027 screens are seeing -52vDC. The next area of concern is a very hot resistor at R55. The picture shows clear evidence of overheating underneath the board. The resistor does measure at 474 ohm so its in spec but very hot under there. The schematic calls for a 7w resistor but not sure what is under there as I have not flipped the board. Would you recommend putting a 10w resistor there for better heat dissipation? Maybe mount it on the top-side of the board as well so its isolated and breathing better? Thoughts?
            Attached Files

            Comment


            • #7
              Originally posted by Pryde View Post
              The amp is operating now and the 7027 screens are seeing -52vDC. The next area of concern is a very hot resistor at R55. The picture shows clear evidence of overheating underneath the board. The resistor does measure at 474 ohm so its in spec but very hot under there. The schematic calls for a 7w resistor but not sure what is under there as I have not flipped the board. Would you recommend putting a 10w resistor there for better heat dissipation? Maybe mount it on the top-side of the board as well so its isolated and breathing better? Thoughts?
              Be sure to check to see if the cover plate can be removed to access the component side of the board before you try to flip the board.

              Heat rises, so the board will get hot. Mounting the resistor on the copper side of the board might help, but if that amp is 40 years old and the board has only slightly discolored, you may be worried about nothing.

              Comment


              • #8
                Those resistors do get hot.

                Comment


                • #9
                  Sorry I meant flip the chassis not the board as there is access from the other side.

                  I will get to that here soon. In the meantime I checked bias current and it is very cold at ~17m/a. (measuring V across plate resistor/resistor value. My plate voltage is 514 making 17ma quite cold. Would you folks up this to 40m/a (~70% of plate voltage)?

                  I could install an adjustable 100k pot in place of R49 (75k) yes?

                  Comment


                  • #10
                    The cold bias is the way they were able to run the power tubes at those high plate voltages and still have some reliability. My personal belief is that all the guys that complain you can't get good enough power tubes to be reliable in these amps are also the guys that have increased the bias. The 3 V4's in my care all have stock bias and I don't have any problem with power tubes or sound quality.
                    So, is there a sound problem or do you just want to meet the mythical 70% bias spec. ?
                    Yes you can put the bias pot there, but better to use a 50K pot with a 50K resistor in series.
                    If it sounds better to you, then warm up the bias, but be prepared to sacrifice tube lifespan and maybe have outright failures.

                    As for the hot resistor, the same amount of heat will be dissipated no matter which wattage resistor you use. All you can do is spread it out with a larger higher wattage resistor, or use a resistor meant to be chassis mounted for heat sinking.
                    "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
                      THanks g-one.

                      I will leave it be then. I did pull the bias resistor to check spec and it measures out fine (74k) so I will just pop it back in. Also the hot resistors are a pair of Dale 5w 1k wired parallel (=10w 500 ohm) so that takes care of my first idea. Going to clean pots and switches and call this one good. Thanks for all the help everyone!

                      Comment

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