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"Half Power" switch problem

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  • "Half Power" switch problem

    Have a JTM Marshall clone with 2 -KT66 / GZ34 that is popping the Mains fuse . This amp doesnt have a HT fuse. It works great in Triode mode , but when switched to Pentode it fries the mains fuse instantly . Ive swapped out the power tubes and it still blew in Pentode mode.

    this one has 2 , 820 ohm between the switch and grids . Those test good . I cant find any of the pins or switch contacts shorting to ground . Would the rectifier tube have any play in this ? I dont have a spare here . B+ is 474v

    Fired it up with all tubes in on my bulb limiter and B+ is 223v and the bulb doesnt show a problem in either mode .

    Any ideas ?

  • #2
    Schematic?
    "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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    • #3
      Does the switch do this with ALL power tubes removed?
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Dont have a scem
        .....its a boutique amp.....JTM 45 Bluesbreaker clone ..should be the same besides the preamp as a Marshall....made by Louis Electric

        probably very close to this ---
        https://drtube.com/schematics/fender...-schematic.gif

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        • #5
          Originally posted by Enzo View Post
          Does the switch do this with ALL power tubes removed?
          no--only with power tubes in...and it wont act up on the bulb limiter

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          • #6
            Did the amp ever work, or is it a new build? In other words, is it possible something could be wired incorrectly?
            "Yeah, well, you know, that's just, like, your opinion, man."

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            • #7
              The part of the schematic I'm interested in is the half power switch. If there is no schematic, you need to know if it's wired up right, or if it ever worked right, like The Dude mentioned.
              "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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              • #8
                ill have to ask the owner, he bought it used . Not sure if the switch is factory

                I can draw up a diagram here in a minute

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                • #9
                  I agree, g1. Since the amp works in triode mode, presumably with screens connected to plate, I can't think of a reason you would draw more current in pentode mode with screen resistors in circuit and a screen supply. If the screen supply was missing or low in that mode, there would be less current- not more. That is, unless there is a wiring error.
                  Last edited by The Dude; 05-28-2020, 05:06 AM.
                  "Yeah, well, you know, that's just, like, your opinion, man."

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                  • #10
                    crude drawing.....
                    Attached Files

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                    • #11
                      Are you switching pentode/triode mode with the amp running? What is the mains fuse value?
                      "Yeah, well, you know, that's just, like, your opinion, man."

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                      • #12
                        Originally posted by The Dude View Post
                        Are you switching pentode/triode mode with the amp running? What is the mains fuse value?
                        Amp off when switching . It came to me with a blown 8 amp...i assume it takes a 3 or 4 amp being a 2 power tube Marshall type ??? so thats what i put in ...a 3a slo blo. Blew that one. Im out of 4a fuses . Popped 2 of the 3A slo-blo .

                        I just tried a 5a fast blo and the amp isnt popping it...either mode . Tried several times . Is this maybe an in-rush current thing ? No standby ....Or it needs a 4 or 5 amp fuse ? The chassis is not labeled (its a $2600 amp...fuse rating not labeled)

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                        • #13
                          Originally posted by Valvehead View Post
                          crude drawing.....
                          I assume the line that's common to the unused Pin 6's of the two KT66 power tubes go to a power supply tap on the PCB, probably after a choke where one would usually find the screens being fed from. In the Triode Mode, those 820 ohm resistors connect to the plates of each tube's plate connection, similar to that stated by Enzo, but with the resistors. I don't see anything obvious here, unless the PCB connection is other than what I assumed here. It only works in 'Triode' mode, right?
                          Logic is an organized way of going wrong with confidence

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                          • #14
                            So put the unit on a current meter. What is the mains current draw. Is it a steady highish current just waiting for a "straw that broke the camel's back" to pop a fuse? or is it sitting at a reasonable common lowish reading and spikes hard when switch is thrown.

                            COuld be something about the switch. It may not MEASURE shorted, but it could have a flake of something inside so it arcs in one position. You could take the wires off the switch and either tack in a different switch or just simply tack solder the resistors to the pin 6 wire to hard wire pentode mode. Does it still behave this way?
                            Education is what you're left with after you have forgotten what you have learned.

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                            • #15
                              I also wonder where pin 6 goes.
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