Problem with Mojotone 5F1 build

Discussion in 'Amp Tech Center' started by gdzero, Nov 15, 2019.

  1. gdzero

    gdzero TDPRI Member

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    I have had this 5F1 kit for 2 years and this is the 3rd time I have stripped it out and started from scratch. I have completed the build and everything checks out up to installing the 6V6 power tube. There is no filament glow and the DC voltage on pin 5 of the power tube is 29 VDC. According to the manual it should be 0 volts. All other voltages are checking a little high. I would appreciate any help and suggestions as to where to look for the problem.[​IMG]

    upload_2019-11-15_21-42-15.jpeg
     
    Last edited: Nov 15, 2019
  2. Wally

    Wally Telefied Ad Free Member

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    Do you have a good picture of the actual circuit? That might be helpful.
    I don’t like the way the AC is wired in. Best and safest is to take the white neutral directly to one end of the PT primary. The Black hot should run through the fuse, to the switch and to the other end of the PT primary. That won’t have anything to do with your problem ,though.
    The layout drawn there does not show that the cathode on that 6V6 needs to go to ground....whether or not it goes to ground while,passing through that empty pin 1 is of no importance, but pin 8 needs to make ground connection.
     
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  3. RottenTheCat

    RottenTheCat Tele-Holic

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    It does. Looks like a wire underboard.

    I'm wondering about the filament wires... 6.8v at light, and nada at the socket? Gotta be a bad connection, or busted wire. Fix that and it oughta work. Wondering what the exact voltage is on pins 7 and 2 on the 6V6. Shows 19v on the cathode, but... not sure if that should be, if there is no filament voltage.

    Where is the other speaker lead? I see the hot side, not the ground side.
     
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  4. Wally

    Wally Telefied Ad Free Member

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    My apology for my post above....I was comparing the Fender layout to the layout above and confused two different approaches. The laughs on me! The layout above has the socket rotated clockwise in comparison to the Fender...and the Fender ties pin1 to pin 2 heater to ground. Duh......

    Yes, the lack of heater voltage to the tubes is a major concern. That should be able to traced out easily.
     
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  5. Junior Little

    Junior Little Tele-Meister

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    Is that 470VDC on pin 8 of the 6V6?
     
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  6. gdzero

    gdzero TDPRI Member

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    Yes, on pins 8 & 3.
     
  7. gdzero

    gdzero TDPRI Member

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    I'm not an electrician nor do I have any experience working on tube amps so I'm not real familiar with the terminology. Where would I check for the heater voltage? What about the 29 VDC on pin 5? The manual says it should be 0 volts.
    Thank-you
     
  8. dan40

    dan40 Tele-Afflicted

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    With you meter set for AC voltage, probe every connection along each of the green filament wires from PT to pilot light to each socket. When measuring each individual wire with you black meter lead to ground, you should be seeing 3.15vac at each connection. Wherever you stop seeing the voltage along the wires path would be you source of trouble. Most likely a bad solder joint causing it.
     
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  9. dan40

    dan40 Tele-Afflicted

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    For you to have 470vdc on pin 8, there would have to be a wiring error. Was that pin 8 measurement taken with the 6v6 in the socket or not?
     
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  10. gdzero

    gdzero TDPRI Member

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    Yes there is 6.8 VAC at the socket...pins 2&7 on 6V6 and pins 4/5&9 on12AX7.
    The speaker socket grounds to the frame.
    Thank-you
     
  11. gdzero

    gdzero TDPRI Member

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    I must have written this down wrong. It now checks 31.5 VDC...tube in and out.
     
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  12. gdzero

    gdzero TDPRI Member

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    These points all check good.
    Thank-you
     
  13. dan40

    dan40 Tele-Afflicted

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    Does the pilot light light up? Is it only the 6v6 that does not light up or is it both tubes?
     
  14. gdzero

    gdzero TDPRI Member

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    Just the 6V6, pilot is lit.
     
  15. gdzero

    gdzero TDPRI Member

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    Just updated the image to reflect edited voltages.
     
  16. gdzero

    gdzero TDPRI Member

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    Here is a picture of the amp. I realize there are several things that need changed and I will but for now I'm concentrating on getting it to work.
    Thank-you

    Haven't been able to add photo.
     
    Last edited: Nov 16, 2019
  17. dan40

    dan40 Tele-Afflicted

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    I am assuming that you have tried a new tube to rule that out. I did have a build recently with a very similar problem. I had good filament voltages but the 12ax7 would not heat up. Turned out to be a bad solder joint on pin 9. I had good voltage readings at the solder joint on pin 9, but it was not making good electrical contact with the pin itself. I found the issue by doing a voltage check from the tube side of the socket. Once I saw that I had no voltage going to the tube, I reflowed the connection and the amp fired right up.
     
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  18. gdzero

    gdzero TDPRI Member

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    No i haven't tried a new tube, I'll have to order one. I will test the socket from the tube side.
    Thank-you
     
  19. zook

    zook Friend of Leo's

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    Is the tube properly aligned in the socket? Some tubes have a small keyway pin and it's possible that the tube is incorrectly aligned.
     
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  20. gdzero

    gdzero TDPRI Member

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    The tube and socket have 1 key each, it can only go in one way.
    Thank-you
     
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