JTM 45 build with no voltage on V1,2,3

Discussion in 'Amp Central Station' started by tomward, Nov 23, 2021.

  1. tomward

    tomward TDPRI Member

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    Been screwing around for three days now building this amp using the Metro Amp JTM45 Kit instructions. It’s built and has -45 v on the output tubes at pin 5 but nothing on V1,2,3.
    Of course I got interrupted and confused along the way so I took my time,went back in to pin one of valve one and rebuilt it on paper connection by connection. Thinking somewhere I missed a ground wire or whatever.Everything looks good.
    Anyway …here I am.You probably already see or know the issue.
    Can you tell me how to find it.
    Please help me.

    here are the instructions I used:
    http://www.valvestorm.com/sites/default/files/JTM_45_KIT_V2.1.pdf

    There are several Ceriatone layout sheets out there I used the 01 Dec 2020 version but then I found the 30 July 2009 and find it a bit less complex to follow.I would have used this one if I had seen it available.
     
  2. nathan5782

    nathan5782 Tele-Meister

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    Are you at least getting any voltages on the heater wires? There's a voltage chart on page 20 of the PDF you linked, I built a Mojotone kit but used the same instructions for help since the Mojo kit didn't really have instructions. I would start looking at the big cap 16uF 450V near the middle of the board, it has a 10K 2W resistor next to it.Drain all the caps and start checking continuity.

    http://www.ceriatone.com/ceriatone/wp-content/uploads/2020/11/JTM45-1-Dec-2020.jpg
     
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  3. dan40

    dan40 Friend of Leo's

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    If you set your meter to AC voltage and measure across pins 4/5 and 9 of the preamp sockets, what voltage reading do you see? Now set the meter to DC voltage and measure pins 1 and 6 on each of the preamp sockets. Is there any voltage at these points?
     
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  4. tomward

    tomward TDPRI Member

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    Pins 4,5,9 gets 3.54.
    But 1 and 6 gets zero.
     
  5. tomward

    tomward TDPRI Member

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    O.k yes heaters are good.
     
  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm no big amp expert, but I do better with pictures than words. *If* I'm right, and if the heaters all measure good and the tubes light up (do they?), I might check for continuity in the power rail -- that is, continuity between all the red circles here, and continuity between both blue circles, and about 10K resistance between red and blue. Of course you can also check for continuity from the left blue circle to that pin 6, and for 100K resistance between the right blue circle and the two pins it feeds...

    This is extra-relevant following your 0V answer to the good question from @dan40 . This is pursuing the lead from @nathan5782 . As he says, you can do this one with amp off, caps drained...

    JTM.jpg
     
    Last edited: Nov 24, 2021
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  7. tomward

    tomward TDPRI Member

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    Oh Boy thank you for this heads up.I will report back.Thank you KIng.
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Go for it; I edited my post to note I was just following up on the smart posts before mine...
     
  9. tomward

    tomward TDPRI Member

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    Continuity is good.
    But zero ohms tween the 100k's and their pins.

    Maybe the V1 socket is faulty?
    It does appear a bit loose...hmmm.

    I will investigate.
     
  10. wabashslim

    wabashslim Friend of Leo's

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    To clarify - do you mean zero ohms as in "no resistance - perfect continuity" or "open - no connection"? if there's no reading, that's the opposite of "zero ohms."
     
  11. tomward

    tomward TDPRI Member

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    OK good point and I thank you for that clarification.The meter reads 0.002k ohms.
     
  12. tomward

    tomward TDPRI Member

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    Well I changed the V1socket and noticed a “scorch” mark at pin 7.Rewired and same symptoms. The tube lights so voltage is there but not on the 1,3,6 and 8 pins.

    Something is not connected.
    I will get back over it again tomorrow I guess and maybe I can find the error.

    Thank you for your help guys.
     

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    Last edited: Nov 24, 2021
  13. dan40

    dan40 Friend of Leo's

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    Mixing the Ceriatone layout and using the Metroamp instructions may be the cause of your "no voltage" problem. Even though they are the same circuit, there are normally some subtle differences between different companies layouts. Looking at the Ceriatone layout you linked to, there is a red, dashed line that leads from the 16 uf capacitor on the chassis and connects to the top left of the board. This is part of the B+ rail that powers the preamp. Checking the DC voltage at the 16 uf chassis cap, you should be seeing several hundred volts at the positive lug on this cap. Follow that red wire to the board and check every connection between there and the pair of 100k resistors at the far right end of the board. When you find the spot that you don't show a voltage reading, you have found your problem. You may have a jumper wire that is not making good contact at one of the turrets. Many builders will run these under the board but it is very easy for them to fall out when you heat a turret to solder a component on the topside of the board. For this reason, I like to run my power jumpers on the top of the board. It makes troubleshooting like this much easier and there's no chance of them falling out when soldering either.
     
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  14. tomward

    tomward TDPRI Member

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    Oh man! I love it! Hope you are correct.
    Going to try this in the morning! Thank you Dan.
     
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  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm a bit confused. There *should* be 0 ohms = no resistance = continuity between the *socket* end of the 100K and their pins -- that's a wire. And yet, if there's not 100K resistance from the *top* of the 100Ks to their pins, then it suggests that wire goes right around them -- which seems unlikely.

    Your report that continuity is good referred to my red and blue circles? What was the resistance from the last red to the first blue?

    I really like the note from @dan40 that layouts differ slightly. To use a relevant layout, here's the *board* (no wires??? dudes...) from the Metropoulos manual -- NOTE this is now in Marshall orientation, tubes at the top. Do his voltage measurements. If I'm flipping the board right and seeing straight, I think you especially want to measure DC voltages at the five green squares; all points on the "power rail" -- the wiring string that feeds the plates on your unpowered V1, 2, and 3.

    BTW, for safety, when you measure voltages, I hope you have a clip-on probe for at least the COM/ground/chassis lead. You want to do this with one hand behind your back, right?

    Also, we shoulda asked you for pics of the board and the socket connections right at the start. You can post 'em full width if you drag JPG files to the compose window, or use the Upload a File button, then just hit Full Image.

    JTM2.jpg
     
    Last edited: Nov 24, 2021
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  16. dan40

    dan40 Friend of Leo's

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    @tomward Good luck with the troubleshooting! You should be seeing high voltage at the 15k 3 watt resistor, then down to the 100k/82k pair of plate resistors in the PI. From there it travels to the 10k/2 watt on top of the 16 uf capacitor and then finally on to the pair of 100k plate resistors on v1.

    You should also see around 400vdc at point "G" on the main 32/32 uf cap can. That feeds the board at the 1k/5watt resistor. Make sure you have voltage at all of these points. As King Fan noted, posting some good, clear pics will help us also.
     
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  17. tomward

    tomward TDPRI Member

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    King and Dan I totally appreciate your following up on me here.
    Of course today is consumed with the greatness of Thanksgiving so I must defer to culture but I could still get it done...maybe.
    Thank you so much for helping me.
     
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  18. tomward

    tomward TDPRI Member

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    IT’S DONE! Thank you guys for helping me think about this amp.
    I had some wires in the wrong place but no damage,no smoke.
    OT transformers squealed so I swapped them.
    Sounds o.k.so far.
    I’m no guitar player but I do love playing guitar.

    What can you advise me about a breakin period?
    Can I leave the amp on and that counts toward breaking in?

    As far as biasing … if I have -44 and - 45 ish is that done?
     
    Last edited: Nov 27, 2021 at 7:47 PM
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  19. dan40

    dan40 Friend of Leo's

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    You really can't bias the power tubes properly by setting the bias voltage to an arbitrary number. Every pair of power tubes will bias up into a different range so one pair may run hot at -44/-45 and another pair may be running cold. You need to use a bias meter that plugs into the socket or add a couple 1 ohm resistors between pin 8 and ground on the socket for you to measure the current. There is also the "transformer resistance method" that works well. Read through this page and it will describe some of the methods.

    https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
     
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  20. tomward

    tomward TDPRI Member

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    Thank you Dam.
    That will be today’s work.
     
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