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"Half Power" switch problem

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  • #16
    All this regarding pin 6. I don't get why it's wired as shown. Since pin 6 isn't internally connected and is typically used as a mounting post for the screen resistor. There's no reason the shared lead to the #6 pins and the switch couldn't just go to the switch. And in fact the shared lead between the #6 pins seems like a vestigial appendage.?. That's a hell of a clumsy wiring scheme and lack of proper circuit knowledge for a $2600 amp. That being the case I have to wonder what else might be weird, and possibly wrong.
    "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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    • #17
      All this regarding pin 6. I don't get why it's wired as shown...#6 pins seems like a vestigial appendage.

      How about this: perhaps the original amp didn't have the triode switch. The two 820 ohm resistors were soldered pin 4 to pin 6 on each socket the way a zillion Fender amps are wired. They added the switch, moved the resistors and just never removed the wire between the tubes. I'd buy that.
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        Originally posted by Enzo View Post
        How about this: perhaps the original amp didn't have the triode switch. The two 820 ohm resistors were soldered pin 4 to pin 6 on each socket the way a zillion Fender amps are wired. They added the switch, moved the resistors and just never removed the wire between the tubes. I'd buy that.
        Just heard from the amp company-- the switch is not factory

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        • #19
          You said you tried other power tubes, but have you tried another rectifier tube?
          "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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          • #20
            Originally posted by Valvehead View Post
            Just heard from the amp company-- the switch is not factory
            In that case, since you can't find what's wrong in the wiring or component functions you could just remove the triode switch, wire the amp stock and then test it. If it works you can wire up a triode switch. That way you'll be sure of the wiring and components. It's the long way around, but if you can't see or otherwise detect the problem now then this should at least work, if not as quickly.
            "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

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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            • #21
              I'd also check bias voltage and idle current. Maybe bias is hot and flipping the switch sends it over the edge of the fuse rating.
              "Yeah, well, you know, that's just, like, your opinion, man."

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              • #22
                Originally posted by Chuck H View Post
                In that case, since you can't find what's wrong in the wiring or component functions you could just remove the triode switch, wire the amp stock and then test it. If it works you can wire up a triode switch. That way you'll be sure of the wiring and components. It's the long way around, but if you can't see or otherwise detect the problem now then this should at least work, if not as quickly.
                ok thanks . I looked at it briefly today . Im out of 3 and 4A fuses , so i tried a 5A in it temporarily . Powers up on the limiter...and then off the limiter fine . It will not pop that one. Chopsticking ..tapping on all tubes ...triode/pent. mode . Did that for at least 20 minutes . I dont have a spare gz34 here ...but..its acting ok now ..so...Ive got fuses on the way --im sure this takes a 3a or 4a main fuse (i did email the amp maker)

                I do have a new Sovtek gz34 ordered
                Last edited by Valvehead; 05-29-2020, 07:22 AM.

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                • #23
                  Like Enzo said, measure the current through the fuse, both when running and during the power-up surge.

                  JTM style amp with a pair of KT66 should not require any more than 3A slow-blow fuse.
                  "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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                  • #24
                    Originally posted by Valvehead View Post
                    Would the rectifier tube have any play in this ? I dont have a spare here . B+ is 474v
                    ...
                    I do have a new Sovtek gz34 ordered
                    With GZ34 and 474V I would try to find out if the 5V winding is capable of supplying 3A to install a 5U4 and get around 440V. On a JTM45 with KT66 it is an ideal voltage.

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                    • #25
                      There is a possibility that the person who installed the switch, reversed the OT primary leads on the switch, which might give rise to high current draw in pentode mode caused by oscillation. Oscillation in pentode mode is more likely than in triode mode because of more gain.
                      - Own Opinions Only -

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                      • #26
                        I might be stating the obvious, but did you check the fuse speeds and that they are indeed correct for the amp? Useless using a fast acting fuse in the place of a slow blow....

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                        • #27
                          Originally posted by g1 View Post
                          Like Enzo said, measure the current through the fuse, both when running and during the power-up surge.

                          JTM style amp with a pair of KT66 should not require any more than 3A slow-blow fuse.
                          dc Amps inline with fuse-- Not getting a reading on either of my meters

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                          • #28
                            Originally posted by diydidi View Post
                            I might be stating the obvious, but did you check the fuse speeds and that they are indeed correct for the amp? Useless using a fast acting fuse in the place of a slow blow....
                            yes , slo-blo

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                            • #29
                              Originally posted by Valvehead View Post
                              dc Amps inline with fuse-- Not getting a reading on either of my meters
                              Mains fuse is AC current.
                              "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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                              • #30
                                Originally posted by g1 View Post
                                Mains fuse is AC current.

                                ya sorry...brainfart....im used to testing current draw on my car ...12vdc

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