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Ampeg V4 wierd channel 1 & 2 interacting

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  • Ampeg V4 wierd channel 1 & 2 interacting

    I have on my workbench '74 Ampeg V4 non-master volume model which acts weird... This amp has two completely separate channels, each one is driven by its own 12AX7.

    It's something I can't understand. When I plug into CH#2 everything is fine, but when I plug into CH#1 sound is quiet and distorted, gets louder and better when... channel 2 volume is turned up. I have resoldered connections on the pcb of both channels but nothing changed.

    In basic words

    1. When I plug into CH#1 - both CH#1 and CH#2 volume pots are working and interacting with each other
    2. When I plug into CH#2 only CH#2 volume works (so as it should).

    Any ideas?
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  • #2
    Check cathode voltages (pin 3) of V1 and V2. Do these change with vol settings?
    - Own Opinions Only -

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    • #3
      Originally posted by Helmholtz View Post
      Check cathode voltages (pin 3) of V1 and V2. Do these change with vol settings?
      No, I have constant 1,9V on V1 and 2,1V on V2

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      • #4
        Is the amp in original condition (no mods)?

        What is the common plate voltage at V1/V2 pins 1? (This is the point where both channels are mixed and could possibly interact.)

        What happens/changes if you swap V1 and V2?
        Last edited by Helmholtz; 12-07-2019, 08:15 PM.
        - Own Opinions Only -

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        • #5
          I'd pull V2 and see if channel 2 volume still interacts when plugged into channel 1.

          Also, check the grounding on the input sockets. I can't see it with this amp, but i had a similar situation with an amp where there was a grounding problem at the input where there was a lot of corrosion. The shorting contact on channel 2 was high resistance and channel 1 grounded through the input circuit of channel 2 and neither socket made proper contact with the chassis.

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          • #6
            Channels are independent, so you can remove V1 (CH1) or V2 (CH2) tube and run the amp only on one that you like

            In this case,
            If I pull out V1 tube then Ch#1 does not work at all CH#2 works good (so, up to specs)
            If I pull out V2 both channels does not work.
            Of course I have checked/changed those tubes (including later V3 12DW7 and V4 12AU7)

            After jack-input inspections:
            The 5,6Mohm resistor on CH1 input jack gives me strange readings, but I'll unsolder it from the circuit and check again. The CH2 jack does not properly "close ground" when cable is removed but I think this only leads to hum when volume put is up without an instrument plugged.

            When those channels meet in the circuit? on the half of the 12DW7? Am I reading the schematic well?

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            • #7
              What's ever more weird, I removed Ch2 wiring (input->PCB) from the PCB to get only I channel connected and its still the same. Both controls of 1/2 channels affect volume. I don't see any modifications on the PCB and components, the rest look stock

              V1 plate voltage is 230VDC, V2 is a bit higher (280)

              Update: I don't have any wire connected to the R15 (I see on other amp guts that its there) But I also dont have any loose wires inside...
              Last edited by boroman; 12-08-2019, 10:07 AM.

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              • #8
                Channels are independent
                No, channels are not completely independent. In fact the plates (pins 1) of the respective second triodes of V1 and V2 are connected and share a common plate resistor (R13). Consequently these plate voltages must be the same. If you pull either V1 or V2, the voltage across R13 must drop as total current is halved.

                Please post all V1 and V2 plate and cathode voltages.
                - Own Opinions Only -

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                • #9
                  Yes, you're completely right.
                  I meant, they are independent to some point, they do join onto one EQ.

                  V1 12ax7 228v
                  V2 12ax7 228v
                  V3 12dw7 319v
                  V4 12au7 209v

                  when V1 tube removed: V1 & V2 are 266v
                  when V2 tube removed: the same (266v)
                  when both V1 and V2 tubes are removed: 325v on both V1 and V2...

                  V3 & V4 voltages stay about the same value with or without pulling those tubes.

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                  • #10
                    At the moment I am only interested in V1/V2 voltages.

                    I assume these:
                    V1 12ax7 228v
                    V2 12ax7 228v
                    are pin 1 voltages? Please always specify pin numbers as each tube has 2 plates and 2 cathodes. Are cathode voltages (pins 3) always close to 2V?
                    What are the voltages at pin 6 and pin 8 of V1/V2?

                    To verify cathode resistor values, pull V1 and V2 and measure resistances between pins 3 and ground.

                    Do you have a scope and signal generator?


                    V1 plate voltage is 230VDC, V2 is a bit higher (280)
                    If these are pin 6 values, they are off. Check pin 8 cathode voltages and measure R5,6,11,12.
                    Last edited by Helmholtz; 12-08-2019, 05:27 PM.
                    - Own Opinions Only -

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                    • #11
                      Originally posted by Helmholtz View Post
                      At the moment I am only interested in V1/V2 voltages.

                      I assume these:
                      V1 12ax7 228v
                      V2 12ax7 228v
                      are pin 1 voltages? Please always specify pin numbers as each tube has 2 plates and 2 cathodes. Are cathode voltages (pins 3) always close to 2V?
                      I'm amazed by your knowledge, Helmholtz!
                      Anyway, yes, those in the last post were from PIN1 to ground..

                      V1 PIN3 = 2.0v
                      V2 PIN3 = 1.8v
                      V3 PIN3 = 60.5v
                      V4 PIN3 = 8.5v

                      What are the voltages at pin 6 and pin 8 of V1/V2?
                      V1 PIN6 = 151v, PIN8 = 2.7v
                      V2 PIN6 = 193v, PIN8 = 1.9v


                      To verify cathode resistor values, pull V1 and V2 and measure resistances between pins 3 and ground.
                      Resistance between PIN3 to ground with both tubes pulled is 3.3kOhm


                      Do you have a scope and signal generator?
                      If these are pin 6 values, they are off. Check pin 8 cathode voltages and measure R5,6,11,12.
                      Infortunately I don't have a scope, as well as signal gen. too, but I may feed some signals from computer. The amp sounds very good on CH2 (on CH1 too when CH2 is turned up as well). No hum, noise, loud output. I have changed the bias resistor and some caps too.

                      R5 = 6.8k
                      R6 = 391k
                      R11 = 6.7k
                      R12 = 390k

                      What confuses me, I don't have anything put in the C22 and some other (like R7) slots - the solder joints on the back look like factory ones (?) I think they might be from V4B ("B" version) or something that shares the same PCB...
                      pic:
                      Click image for larger version

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                      Last edited by boroman; 12-09-2019, 09:01 AM.

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                      • #12
                        Originally posted by boroman View Post
                        What confuses me, I don't have anything put in the C22 and some other (like R7) slots - the solder joints on the back look like factory ones (?) I think they might be from V4B ("B" version) or something that shares the same PCB...
                        pic:
                        [ATTACH=CONFIG]56285[/ATTACH]
                        Notes in schematics says: "R1, R7, C1, C4 will not appear in some units"

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                        • #13
                          So finally we have all relevant V1/V2 voltages. They look good except for the first triode of V1, which has 30% increased plate current. Is R2 (5.6M) ok and correctly connected?

                          Update: I don't have any wire connected to the R15 (I see on other amp guts that its there) But I also dont have any loose wires inside...
                          What does this mean? R15 (3.3k) is the cathode resistor of the second triode of V2 and needs to be connected between V2, PIN3 and ground.


                          From your symptoms I am convinced that the problem is in the V1/V2 circuitry. I suspect something wrong (something shorting or bad connection) on the vol pots board ("64-60-071") or its outer wiring.
                          Last edited by Helmholtz; 12-09-2019, 05:30 PM.
                          - Own Opinions Only -

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                          • #14
                            Originally posted by Helmholtz View Post
                            So finally we have all relevant V1/V2 voltages. They look good except for the first triode of V1, which has 30% increased plate current. Is R2 (5.6M) ok and correctly connected?



                            What does this mean? R15 (3.3k) is the cathode resistor of the second triode of V2 and needs to be connected between V2, PIN3 and ground.


                            From your symptoms I am convinced that the problem is in the V1/V2 circuitry. I suspect something wrong (something shorting or bad connection) on the vol pots board ("64-60-071") or its outer wiring.
                            There's a hole near R15 that is not connected to anything. Will try to show tomorrow. I have pics of guts fro manother V4 and there's a wire that does down somehwere from that point. I don't know it may be other revision of the circuit or something. The bad voltage on the 1st tube may be due to that I unsoldered one of the jacks from the PCB (it was unsoldered when I did the measurements).

                            I will try to resolder it back again, check once again + check the potentiometer board

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                            • #15
                              One very important thing you have not told us is what setting you have the 'sensitivity' switch on (above the volume pot).
                              I just had to replace C2 in my spare, I have it open if we need to verify any measurements.
                              Last edited by g1; 12-09-2019, 06:59 PM.
                              "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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