Jtm 45 fuse keeps blowing

Discussion in 'Building the Classics' started by Marcelcar, Aug 22, 2020.

  1. Marcelcar

    Marcelcar Member

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    Hi,

    I just started my jtm45 build using the valvestorm parts and instructions.
    Everything went fine until I got to the testing phase.

    With the preamp and rectifier tubes installed everything seems fine but as soon as I install the output tubes my fuse blows.
    I have already swapped a tube because one of them had a vacuum leak and I replaced a tube socket just to be sure that it doesn't cause the problem.

    Here are my voltage measurements without the output tubes and only preamp and rectifier installed.

    V1
    Pin1 239V
    Pin3 1,9V
    Pin4/5 3,3V AC
    Pin6 234V
    Pin8 1,9V
    Pin9 3,3V AC

    V2
    Pin1 205V
    Pin3 1,2V
    Pin4/5 3,3V AC
    PIN6 335V
    Pin7 205V
    Pin8 206V
    Pin9 3,3V AC

    V3
    Pin1 275V
    Pin3 48V
    Pin4/5 3,3V AC
    Pin6 277V
    Pin8 48V
    Pin9 3,3V AC

    V4
    Pin2 3,3V AC
    Pin3 491V
    Pin4 490V
    Pin5 - 54V
    Pin6 490V
    Pin7 3,3V AC

    V5 is identical to V4.


    Can someone help me figuring
    out what's wrong with my amp?
     
  2. Kinkless Tetrode

    Kinkless Tetrode Well-Known Member

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    Is this HT fuse or the mains fuse?
     
  3. Travis398

    Travis398 Well-Known Member

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    if it takes a few seconds to happen maybe the tubes are red plating
     
  4. Marcelcar

    Marcelcar Member

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    It was the HT fuse and happens immediately after turning of the standby switch
     
  5. thetragichero

    thetragichero Well-Known Member

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    have you tried another pair of output tubes to rule out shorted tubes?
     
  6. pleximaster

    pleximaster Well-Known Member

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    Check the main capacitors and the wiring of them...

    plexi
     
  7. Marcelcar

    Marcelcar Member

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    I have tried out a new pair of tubes and still got the same problem
     
  8. South Park

    South Park Well-Known Member

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    That 54 v on the bias seams high to me turn down the bias to the lowest setting And power up one tube at a time
     
  9. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Pull all tubes out, put good fuses in, fire it up and turn off of standby. Does the fuse now hold? If so, try the sweep on your bias pot to see if the range is acceptable. No shorts with the tubes out and adjustable bias is the first step
     
    Far Rider likes this.
  10. neikeel

    neikeel Well-Known Member

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    Remember that -54v is the negative grid voltage, -54 is fine to start cold and then bring it up as needed, I see no issue there but do as Guitar-Rocker suggests first.

    Your voltage chart is correct for an unloaded amp (maybe a smidge high for B+ in a JTM45 so I impart from that you are using Classictone transformer) loaded and biased up your target would be 450 to 460v on the plates.

    Have you built many amps (I've seen output tube sockets wired one pin off all the way round (ie pin 8 as pin 1 and the others in same sequence) so when the tubes go in they are very unhappy!

    Can you post some very clear well lit pictures of your output tube sockets, filter cans etc?

    Once we have this you can be sorted pretty quickly.:applause:
     
  11. Marcelcar

    Marcelcar Member

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    It's my first build..
    The V4 socket looks different because I changed it to rule out a socket failure
    Here are some pictures :
     

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  12. South Park

    South Park Well-Known Member

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    Check the power tube sockets for correct wire location to pin socket . Put some tape down and number the pins to match things up .
     
  13. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Hard to see and not knocking your skill set, but in the photos it sort of looks like on the beige Belton socket that the Ohmite 470R is going to pin #3, not pin #4 as it should be. Pin 3 should be the OT plate wire, Blue? Pin 4 should be the screen wire (470R in your photo)
     
  14. Marcelcar

    Marcelcar Member

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    The resistor is going to pin 4 and doesn't contact any surrounding pins
     
  15. Far Rider

    Far Rider Hendrixian VIP Member

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    Maybe it's just the angle of the photo, but do you have all legs of that bias pot connected? I think that one leg should be free if I'm not mistaken.
     
  16. Marcelcar

    Marcelcar Member

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    I have installed the bias pot according to the instructions.
     

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  17. Far Rider

    Far Rider Hendrixian VIP Member

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    So the bias pot only has 2 conductors?
     
  18. Marcelcar

    Marcelcar Member

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    I don't know really know a lot about amp building yet, it's my first build. I don't know if you count the upper leads as separate conductors or not
     
  19. South Park

    South Park Well-Known Member

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    The wire layout shows two connections but the schematic shows three connections two going to ground. Check all the pins on the power tubes some thing is going to ground if blowing fuse . This kind of problems is how you learn about tubes
     
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  20. Far Rider

    Far Rider Hendrixian VIP Member

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    You're doing just fine. Your first build looks a lot better than mine did. Are there 3 pins on the bias pot? 2 at the bottom and 1 at the top?
     

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