Jtm 45 fuse keeps blowing

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

  1. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    2,652
    Likes Received:
    756
    Location:
    flo rida
    I'm really thinking it's an issue with leads soldered to the wrong pins as was mentioned. pins 2 and 7 are the heaters, pins 1 and 8 essentially both go to ground. one pin off in either direction shorts the heater winding to ground with a tube inserted if I'm imagining this correctly
     
    South Park likes this.
  2. william vogel

    william vogel Well-Known Member

    Joined:
    Dec 18, 2016
    Messages:
    315
    Likes Received:
    254
    Your bias pot is correct. The pot has two wings under the bottom that solder to the two turrets at the positive ends of the two capacitors, these wings are one terminal of the pot and serve as a bridge between the caps. The other two legs of the pot are connected as you have them. One leg goes to the resistor (47k I think) the other leg bends back to the positive lead of the second cap.
     
  3. Travis398

    Travis398 Well-Known Member

    Joined:
    Dec 7, 2011
    Messages:
    784
    Likes Received:
    1,114
    Location:
    USA
    Standby switch to pin 8 should look like this,

    rectifi.png
     
  4. Travis398

    Travis398 Well-Known Member

    Joined:
    Dec 7, 2011
    Messages:
    784
    Likes Received:
    1,114
    Location:
    USA
    I don't see it here


    jtm build.jpeg
     
  5. Travis398

    Travis398 Well-Known Member

    Joined:
    Dec 7, 2011
    Messages:
    784
    Likes Received:
    1,114
    Location:
    USA
    Probably should cap off these wires too

    cut wires.jpeg
     
  6. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    Shouldn't a badly wired rectifier short the circuit even without the power tubes installed? All my voltages on the rectifier are correct

    Edit:
    The red wire from the standby switch is attached to pin 2 of the rectifier socket, I'll change it to pin 8
     
    Last edited: Aug 23, 2020
  7. pleximaster

    pleximaster Well-Known Member

    Joined:
    Feb 6, 2012
    Messages:
    1,636
    Likes Received:
    1,896
    Travis is probably on to something here! So check the wiring and the mains caps! if you have had a wrong wiring here the caps might have short too...

    plexi
     
  8. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    4,709
    Likes Received:
    3,115
    Good morning. Lots of traffic looking at this - excellent, all really helpful.

    First thing to do is cap those cut/exposed wires on the PT off with some shrink wrap.

    I do not want to confuse you but which PT is it? It looks like a Heyboer/Metro type but the primary colours are not familiar to me (Orange is typically common - yours is cut? This is the one that goes to the fuse and the live and blue is 220v (the others would be red 120v, violet 240v) but I see white, grey, orange all cut?

    Where in the world are you that has those mains colours (yellow for ground, blue and red - or is that brown? for live and neutral)

    That is probably not too much of an issue given your voltage chart - unless you have moved stuff around whilst problem solving?

    The GZ34 rectified HT comes off the heater which is 2 and 8 and 8 is normal HT out (I do not know of any subtle differences inside a GZ34 that would give normal output except under load - anyone?).

    Looking over your wiring I see the caps and choke wired up correctly. Still cannot really see the cut-out on V4 for orientation but V5 looks correct although shadows are tricky.

    I presume you are using 500mA T rated HT fuse (aka SLO BLO) type on the HT? I also presume that you have the amp plugged into a load (speaker) when you have the output tubes in?

    Sorry for the questions but it looks generally ok apart from points the guys made above.
     
    thetragichero likes this.
  9. william vogel

    william vogel Well-Known Member

    Joined:
    Dec 18, 2016
    Messages:
    315
    Likes Received:
    254
    It’s a Heyboer 1202-55 from Amp Parts Direct and apparently Heyboer uses different colors for primary on that line card. I also don’t know if using pin 2 on a GZ34 matters except the data sheet instructs pin 8 for rectified output??? I’d just move it over to 8. I don’t see how that would hurt anything past the rectifier tube. The 5 volts is just floating. He has bias voltage on both output tubes, I really think a current limiter is needed here. They are easy to build and a real life saver for first startup.
     
  10. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    It's indeed a amp parts direct Transformer, even though I ordered it through the MetroAmp store.
    I already contacted heyboer for the color coding and connected all the leads according to their information.
    I'm using a 500mA fuse
     
  11. coolidge56

    coolidge56 Well-Known Member Double Platinum Supporting Member

    Joined:
    Jan 30, 2019
    Messages:
    1,002
    Likes Received:
    2,149
    Location:
    WA state hinterlands
    I checked a bunch of your wiring and resistor values it looks good, your voltages are good. Are you sure you are using a SLOW BLO fuse?
     
  12. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    Its a regular 500mA fuse I guess. The two slow fuses that came the kit blew as well.

    When I had the slow blow installed I even got some arching at the v4 socket.
    I also had a bad tube with a vacuum leak that I had to replace prior to opening this thread
     
    Last edited: Aug 23, 2020
  13. Guitar-Rocker

    Guitar-Rocker Well-Known Member

    Joined:
    Mar 5, 2012
    Messages:
    2,831
    Likes Received:
    2,475
    Location:
    Indianapolis, Indiana
    Arching at V4 needs to be looked into. Make sure the pins aren't arched across to the filament pins
     
  14. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    What do you mean by that exactly?
     
  15. william vogel

    william vogel Well-Known Member

    Joined:
    Dec 18, 2016
    Messages:
    315
    Likes Received:
    254
    He means carbon tracked. You replaced the socket. After the socket replacement has it arced again or just blows the fuse? Are you using the standby switch? You should flip on both switches at the same time with cold tubes.
     
  16. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    With the replaced socket I simply got a blown fuse but I don't know if it's because I used a regular fuse this time, I only had 2 slow blow fuses. I'm letting the tubes preheat and the turn off the standby.

    There are carbon traces between pin 2 and 3 of the v4 socket
     
    Last edited: Aug 23, 2020
  17. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    4,709
    Likes Received:
    3,115
    Arcing usually happens with old and dirty sockets (shorts plates to heaters) not crisp new builds and on poor quality sockets, none of which should apply in your case. The Beltons as supplied are decent and I use in new builds.
    Once you have carbon tracking that socket is toast and cannot be trusted - throw it away and replace. Is the valve itself charred too?
    With a GZ34 you can switch on both mains and standby simultaneously (the build up on the rectifier gives a soft start.
     
  18. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    I'll try to get my hands on some slow blow fuses and will install the new socket. Maybe it will work this time.

    Could a bad tube (vacuum leak) be the cause of arching?
     
  19. Marcelcar

    Marcelcar Member

    Joined:
    Aug 22, 2020
    Messages:
    79
    Likes Received:
    9
    I installed a new tube socket and got some high quality fuses but again got the same issue : the fuse blows after a few seconds of warmup...
    I didn't see any arching this time tho
     
  20. neikeel

    neikeel Well-Known Member

    Joined:
    Feb 12, 2014
    Messages:
    4,709
    Likes Received:
    3,115
    A few things to check (given the results it's going to be something weird) is to check for shorts to ground on each tube pin (of course 1 and 8 will) and that both screen resistors are in spec.
    I think that any other tests would merit using a current limiter - make up a light bulb to clip in circuit in lieu of the HT fuse.
    Things I would want to test would be shorted filter cap, correct choke measurements.
    Can you take a photograph to show the keyway in your sockets.
     

Share This Page