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1968 Ampeg J12 - Rectifier Diode Replacement

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  • #1

    1968 Ampeg J12 - Rectifier Diode Replacement

    hi guys (I'm starting to thing I have more amps than I need, yessss... by I like the different "colors" in the "palette")

    Bought this 1968 Ampeg J12 some 15 years ago and just decided to recap it now.

    I am wondering what type of diodes should I use for the rectifier:



    Have also looked into some older posts in the forum referred to this amp and have seen many "complaints" about the output power. Yes, I too wished it was a little bit louder so I do not need to carry the Twin around.
    I've seen recommendations to check R7 and C7... I might take a look just in case.
    I have also seen discussions about the tubes, in my case it carries the 7591 Power tubes as described in the schematic.
    Tags: None
  • #2
    1N4007

    I'll go look at the "more power" post and see if I have anything to add. The best answer will probably already be there. More efficient speakers.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

    Comment

    • #3
      Why change diodes if they are ok? Newer or "better" diodes won't increase voltage or amp power and semiconductors don't age like ecaps or tubes.
      - Own Opinions Only -

      Comment

      • #4
        Originally posted by Helmholtz View Post
        Why change diodes if they are ok? Newer or "better" diodes won't increase voltage or amp power and semiconductors don't age like ecaps or tubes.
        Ah, yes. This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

        I ignored for the moment that there may not be a reason to replace the existing diodes and just answered the question. So if you NEED to put in new diodes then 1N4007 is a perfectly good choice. If the diodes you have are working then you don't NEED new diodes.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment

        • #5
          My automatic reaction is 1N4007 also, but looking at the schematic is specifies 800v 6A diodes. 1N4007 are rated 1A (30A surge). Is 1A enough for a small amp like this? Probably. But the specified parts are like 42 cents at Mouser, so...

          Mouser search: 800v 6A rectifier. and always check the "in stock".

          Oh, and I certainly agree with both above: why are we replacing good diodes?
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            Originally posted by Enzo View Post
            ... certainly agree with both above: why are we replacing good diodes?
            thank you guys!
            quick answer to your question:
            - i don’t know if they are good (i know i could test with my multimeter but also that it should be checked with a diode diagnose tool i do not have)
            - i have changed all caps (electrolytics, film, ...) and still have some hum (much louder than the super quiet twin i worked on before)

            - this could be bias (the amp has no hum or bias balance)
            - this could be that the amp has a two prong mains connector (that i intend to change)
            - there could be grounding issues somewhere (haven’t found them yet)
            - or... it could be the diodes

            Comment

            • #7
              If your diodes were bad, it would likely be blowing fuses.

              Post or link the entire schematic. Your heaters have a center tap, and it goes to W. OK, I found one, but others may not, so link. W is where I expected, the power tube cathodes. That elevates the heaters for hum abatement.no hum balance needed.. But that ONLY cures ONE type of hum. Each type hum has its own cures. The amp is cathode biased, so there is no bias supply.

              Do ANY of the controls have ANY effect on the hum?
              Education is what you're left with after you have forgotten what you have learned.

              Comment

              • #8
                There is a schematic linked in the opening post.
                The heater winding CT connects to the power tubes' cathodes (looks like + 15V).
                - Own Opinions Only -

                Comment

                • #9
                  You are correct, sorry. The blue text of the link doesn't contrast the black text around it enough for my old eyes, I guess.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment

                  • #10
                    Hey TelRay,

                    The Jets referred to in all those other threads complaining of low power are a different model from yours, the J12-D with 6BK11s, usually fitted with 6C10s. Just thought you should be aware of that before applying advice given on those. Be glad you can at least get tubes for yours!

                    I'm with everyone above - don't worry about the diodes. Make sure the tubes are correct for the amp & that someone didn't put 12AU/AY/AT/DW7s in it. Try a more efficient speaker. How are the power tubes?

                    As far as gain/volume, I'm not sure about the numbers here, but yours at least has 2 gain stages and then the phase inverter; the D model only has 1. Good luck with it! If you ever get a chance, look up a GU-12. Nice little combo, a step up from the Reverbojet, does Stones material quite nicely. At least, mine did before my friend blew it up.

                    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 -

                    Comment

                    • #11
                      thank you guys, your comments always add value and contribute to my education
                      I will refrain from touching the diodes for the time being :LOL:

                      About the volume issue... it’s a small amp yessss. I just wished I could use it for band practice, but it’s not loud enough.

                      I will try to assimilate what’s been said about the hum before opening the amp again.

                      Comment

                      • #12
                        Well the schematic provided for the thread below seems to be the same as the one posted here. Judging by the symptoms and what little discover has been made so far I REALLY suspect the trem circuit. Modeling this amp on Spice says it's just as loud as any other amp with two small bottles at the indicated voltages. Speaker efficiency not withstanding. I covered it on this other thread in a little more detail.

                        https://music-electronics-forum.com/...ad.php?t=48242
                        "Take two placebos, works twice as well." Enzo

                        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                        "If you're not interested in opinions and the experience of others, why even start a thread?
                        You can't just expect consent." Helmholtz

                        Comment

                        • #13
                          Perhaps, but I know there are at least 2 other "MY JET IS TOO QUIET!" threads about J12-Ds. I started one. I just ran across the other earlier tonight, looking for who-knows-what...

                          I guess BOTH of those circuits suck; they were one right after the other, I believe (unless a Golden-Glo Jet was released?). Point being, neither lasted very long at all...

                          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 -

                          Comment

                          • #14
                            On the topic of diodes, it helps to learn to see the whole picture. In a typical tube amp, the biggest current draw is going to be the output stage. There will be an initial surge at turn-on to charge the power supply capacitors, but after that, a pair of 7591s, according to datasheet conditions, should always be under 200mA.

                            Diodes have improved since the 1960s, but they were already improving rapidly during that decade. I often replace early 60s diodes as a matter of preventive maintenance since the parts cost is negligible. In one case, I did find a 1960s diode that produced a lot of switching noise even though it worked. From the late 60s, it's a judgment call. Ampeg may have chosen a higher-current diode at the time to handle the turn-on surge, which could take out diodes of the era. 800V PIV may have been the best they could get at the time. You can replace them with 1000V PIV diodes -- and probably should. Worry about diode failures in the 60s was part of why you see Fender amps with several diodes in series.

                            Also, with two 7591s in cathode bias, run as pentodes, the highest datasheet claim is 28W, but that's with plate voltages 90V higher than in this Ampeg. Yours might be good for ~21 Watts. When someone is concerned about output power, the scientific way to do it is to hook the amp to a dummy resistive load and a scope and determine how much power it's putting out. If power is significantly below the specs for the amp, there's an amp problem. If the amp is putting out its rated power, you could have a speaker problem -- or just a speaker that was never that sensitive to begin with. If in doubt about the speaker, substitute another known-good speaker.
                            Last edited by Rhodesplyr; 01-23-2020, 04:20 PM.

                            Comment

                            • #15
                              Worry about diode failures in the 60s was part of why you see Fender amps with several diodes in series.
                              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.
                              Education is what you're left with after you have forgotten what you have learned.

                              Comment

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