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1968 Ampeg J12 - Rectifier Diode Replacement

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  • #16
    Hi y'all, I remembered I have not "finished" this amp

    I will leave the rectifier unmolested however will replace the original 2-prong mains connector for a grounded one.

    This belongs to the "basic" questions but I think it's worth asking and leaving some example of how it's done in different amps so anyone consulting the forum in the future might benefit from it.

    This is the current layout:



    What I am intending to do is to:

    - physically remove from the circuit the 2 capacitors marked in RED (C25 .047 1,000 V and C26 .050 600 V)
    - connect the HOT (black) and NEUTRAL (white) wires of the new 3-prong cable to the ON-OFF SWITCH (making sure the HOT wire goes through the FUSE entering from the center lug)
    - i understand that the HOT wire going (after the switch) either to the BLK TRANSFORMER wire or the RED / BLACK makes no difference (it certainly didn't with the 2-prong as it could have been inverted when plugging into the wall)
    - (of course) the new GROUND (green) wire from the 3-prong will go to the amp's chassis

    thank you guys,
    Last edited by TelRay; 05-05-2020, 10:49 PM.

    Comment


    • #17
      The C26 cap is protecting the switch from arcing. Suggest leaving it in circuit.
      C25 is the problematic one which should be removed for safety reasons.
      When you said 'center lug' did you mean the fuse holder? If so, the 'hot' wire should go to the rear lug (furthest from fuse-cap).
      "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


      • #18
        Originally posted by Enzo View Post
        And for that matter I almost never see rectifier failure in those old Fender amps. I think over the years I have replaced more failed bias rectifiers than B+ rectifiers in old Fender amps.
        me too
        If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

        Comment


        • #19
          Originally posted by g1 View Post
          The C26 cap is protecting the switch from arcing. Suggest leaving it in circuit.
          C25 is the problematic one which should be removed for safety reasons.
          When you said 'center lug' did you mean the fuse holder? If so, the 'hot' wire should go to the rear lug (furthest from fuse-cap).
          well, that depends. The most common cap I've seen in Vintage Ampeg mains circuitry is a total hazard, and should definitely be replaced with appropriate X-class capacitors. I've seen the originals fail catastrophically.
          .
          Click image for larger version

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          you don't want to end up on the business end of cap that shorts the mains onto a chassis bolt
          If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

          Comment


          • #20
            Originally posted by g1 View Post
            The C26 cap is protecting the switch from arcing. Suggest leaving it in circuit.
            Ahhhh... this is why I hear a “CLICK” when I turn the amp OFF now
            great catch!

            Originally posted by g1 View Post
            C25 is the problematic one which should be removed for safety reasons.
            thx!

            Originally posted by g1 View Post
            When you said 'center lug' did you mean the fuse holder? If so, the 'hot' wire should go to the rear lug (furthest from fuse-cap).
            yes, it was originally wired with the wire coming from the mains cable going into the center tap of the fuse holder (the rear one one further from the cap) and i soldered the new (hot) wire in the same way. i remember the point was raised when I was working on the TWIN (it was highlighted that the factory wiring was wrong as it had the hot wire going to the cap)

            Comment


            • #21
              Also, make sure that the earth (green wire) goes to its own dedicated fastener, and is secured with a lock nut (or scrape any oxidization or paint away from the chassis, and solder it directly). Avoid the temptation to screw the terminal to a transformer bolt. Transformers vibrate, and can cause the nut to loosen and unscrew.
              If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

              Comment


              • #22
                thank you guys for the replies above, as always, very useful

                I have measured the output tubes POWER DISSIPATION as follows:



                POWER TUBE V1: 19.2 W
                From POINT A to PIN 3
                R= 88.2 Ohm
                V Drop = 4.5 V
                Plate Voltage = 377.1


                POWER TUBE V2: 16.8 W
                From POINT A to PIN 3
                R= 98.6 Ohm
                V Drop = 4.4 V
                Plate Voltage = 376.2


                The tubes show 14 % unbalance which could be improved but the amp is not noisy.
                What I am concerned about is the POWER DISSIPATION value on each tube being too high.
                According to the 7591 Tube Spec this tube should have a maximum power dissipation of 19 W

                I've checked a couple of things around that I think that can be related to this and so far have found nothing wrong or unusual:

                Voltage A = +379 V (nominal 365 V)
                Voltage B = +370 V (nominal 360 V)
                Voltage C = +325 V ( nominal 320 V)
                Voltage W = +15.2 V (nominal 15 V)
                Capacitor C7 = 23.3 uF (nominal 25 uF) new
                Resistor R15 = 147 Ohm (nominal 140 Ohm)

                Do you guys agree on the POWER DISSIPATION being too high? Any ideas?

                Thx!

                Comment


                • #23
                  They average 18 watts, so each is really off 7%. Actually, you forgot to subtract voltage W from voltage plate. Dissipation is figured with voltage across the tube, not just plate to ground. Is ther any DC on your grids, pins 6?
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment


                  • #24
                    Originally posted by TelRay View Post
                    Do you guys agree on the POWER DISSIPATION being too high? Any ideas?
                    A couple things. First, AC power - the voltage at wall sockets is typically higher than it was 50+ years ago when this amp was made. Back then, 117V give or take a couple. It's common to find 125V these days - so all your supply voltages will be riding higher including filament, which means more electrons are "boiling" off the cathode.

                    Second, self biasing designs were set up to run output tubes at or near 100% plate power. You could inch up your cathode resistor a bit, say to 180 maybe 200 ohms. That will knock down your plate dissipation a bit. Go too far and you'll have a noticeable notch in your output waveform, crossover distortion. If your playing style is distortion all the time, never need clean tones, then it's not so much a problem. Also note that gain suffers a bit when you dial down the plate current this way.

                    FWIW I've used JJ 7591 and found them to be tough, good sounding tubes. EH, maybe OK at lower voltage like in your Jet, not so much when strained by high voltage in other amps. And old ones from Sylvania, GE, and other legacy brands - surprisingly long lasting - including "pulls" from old guitar amps, hi fi & PA. Before JJ made their repros, NOS 7591's sold for gold dust money in the 1990's and early 2000's. I don't know how much they fetch now that competent replacements are available new.
                    Enjoy. Every. Sandwich.

                    Comment


                    • #25
                      Originally posted by SoulFetish View Post
                      well, that depends. The most common cap I've seen in Vintage Ampeg mains circuitry is a total hazard, and should definitely be replaced with appropriate X-class capacitors. I've seen the originals fail catastrophically.
                      .
                      [ATTACH=CONFIG]58408[/ATTACH]

                      you don't want to end up on the business end of cap that shorts the mains onto a chassis bolt
                      I specifically stated C25 & C26 in my comments for a reason. Please tell me how C26 could possibly short to chassis?
                      "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


                      • #26
                        Do you guys agree on the POWER DISSIPATION being too high? Any ideas?
                        1) To calculate plate dissipation only the difference between plate and cathode voltage matters. Tubes only "see" and work with the voltage differences between their electrodes/pins. A tube can't know where ground is.

                        2) It is no problem to operate tubes at 100% plate diisipation when they are cathode biased.

                        Edit: I missed some of the posts above.
                        - Own Opinions Only -

                        Comment


                        • #27
                          Originally posted by Enzo View Post
                          They average 18 watts, so each is really off 7%. Actually, you forgot to subtract voltage W from voltage plate. Dissipation is figured with voltage across the tube, not just plate to ground. Is ther any DC on your grids, pins 6?
                          ahaaaaa... so the calculation is more:

                          POWER TUBE V1 = 4.5 V / 88.2 Ohm * (377.1 - 15.2) V = 18.4 W
                          POWER TUBE V2 = 4.4 V / 98.6 Ohm * (376.2 - 15.2) V = 16.1 W

                          Average= 17.3 W (7% unbalanced)

                          is that the right way to calculate and express the results?

                          I will check PIN 6 when I open the amp up again (will be soon)

                          Originally posted by Leo_Gnardo View Post
                          A couple things. First, AC power - the voltage at wall sockets is typically higher than it was 50+ years ago when this amp was made. Back then, 117V give or take a couple. It's common to find 125V these days - so all your supply voltages will be riding higher including filament, which means more electrons are "boiling" off the cathode.

                          Second, self biasing designs were set up to run output tubes at or near 100% plate power. You could inch up your cathode resistor a bit, say to 180 maybe 200 ohms. That will knock down your plate dissipation a bit. Go too far and you'll have a noticeable notch in your output waveform, crossover distortion. If your playing style is distortion all the time, never need clean tones, then it's not so much a problem. Also note that gain suffers a bit when you dial down the plate current this way.

                          FWIW I've used JJ 7591 and found them to be tough, good sounding tubes. EH, maybe OK at lower voltage like in your Jet, not so much when strained by high voltage in other amps. And old ones from Sylvania, GE, and other legacy brands - surprisingly long lasting - including "pulls" from old guitar amps, hi fi & PA. Before JJ made their repros, NOS 7591's sold for gold dust money in the 1990's and early 2000's. I don't know how much they fetch now that competent replacements are available new.
                          Yes! I usually set the voltage at 117 V with a Variable transformer to measure the "historical" values but I gave it to a friend working on his amp.
                          I have EHs now in this amp. I think the Ampeg branded I have somewhere we manufactured by SYLVANIA
                          Good tip on the cathode bias resistor changes

                          Thx,

                          Comment


                          • #28
                            Originally posted by g1 View Post
                            I specifically stated C25 & C26 in my comments for a reason. Please tell me how C26 could possibly short to chassis?
                            Late 60's - early 70's Ampegs often used a type of cap that was housed inside a ceramic cylinder. In recent years (1990's onward) I've seen them fail short, often blowing a coil of metallic capacitor innards across the chassis. As in the photo - that's what we're seeing. One fine day, working on a V4 or V2, it happened right before my eyes. I had just put a grounded AC cable on the amp, and the polarity cap couldn't stand the strain. It shot a sparking streamer of electrified fun a good 2 feet long. The kind of surprise we can all do without. If a similar failure was confined within a chassis, unseen, it could connect anything to anything else, a shock hazard we'd like to avoid. So... now whenever I see those white ceramic cylinders, they get nipped out if they're functioning as death caps - aptly named in this case. Or replaced with proper new caps in other functions. Cheap insurance.
                            Enjoy. Every. Sandwich.

                            Comment


                            • #29
                              Originally posted by g1 View Post
                              I specifically stated C25 & C26 in my comments for a reason. Please tell me how C26 could possibly short to chassis?
                              I dunno yet. Lemme think of something.
                              If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

                              Comment


                              • #30
                                Well, Leo almost had you off the hook there, but he was talking about the polarity cap as well.
                                And I'm not ready to start encapsulating everything in contact with the AC line that could possibly break loose and come in contact with the chassis.
                                "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

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