New JTM45 build has no sound no - bias except on the + diode

Discussion in 'Shock Brother's DIY Amps' started by Riffscaster, Aug 4, 2020.

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  1. Riffscaster

    Riffscaster TDPRI Member

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    Hi, this hoping is going to get my second scratch build off the ground with some help here on the Tdpri forum.


    1.The + side of the diode on the Bias reads -35 without tubes installed

    2. I cannot get any bias reading on the pins 1 & 8 of the Kt66 with tubes installed.


    Only saw one resistor burn up and had replaced it right away, so All the components seem ok.
    I've pulled tubes and the hum does not go away unless I remove power tubes.

    Checked for Stray/crossed wires on the tube sockets as well as doing continuity wiring tests but all checked out ok, So have decided I need to ask some opinion on my favourite forum.

    The Ceriatone Layout is a Layout that closely resembles my build.
     

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  2. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    But why did it burn up? It's not a good sign...
     
  3. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    E seems connected to ground and the bias point between the 220k's. If that's really the case, then your output tubes are running way, way, way too hot. I wouldn't put tubes in before I got a good bias voltage at the tube sockets.

    D seems to be the output of the bias circuit. It is used at the intensity pot. My JTM45 doesn't have tremelo.

    Could we have pictures? And a schematic and layout of your amp would be nice too. A non working amp mostly closely resembles a working amp too. The error is the only difference...
     
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  4. Wally

    Wally Telefied Ad Free Member

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    What resistor burned up? How far have you traced that bias voltage? Do you have ‘good’ bias voltage at the tube sockets without the power tubes installed?
    I don’t like how they drew the control grid stoppers in that layout. It makes me ask if you might have mistakenly used pin 1 as a connection point for those stoppers? If so, that would be a problem if you are using EL34s.
     
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  5. D'tar

    D'tar Friend of Leo's

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    Pin 1 and 8 should read 0vdc as they are direct path to ground/chassis unless you have 1ohm 1%bias resistors installed to aid in measuring current.

    Your bias voltage reading should be on pin 5.
     
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  6. Riffscaster

    Riffscaster TDPRI Member

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    \

    the 100k V2 Cathode resistor.
     
  7. Riffscaster

    Riffscaster TDPRI Member

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    I have the 1ohm resistor on pins 1-8 for bias reading
     
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  8. Riffscaster

    Riffscaster TDPRI Member

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    The "E'' looks as if its going to ground through the .047 capacitor on the middle pin of the intensity VR. This is a bit confusing to me also, maybe this is why the bias is getting so hot.
     
  9. Riffscaster

    Riffscaster TDPRI Member

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    I had put the 1 Ohm resistor to 1 and 8 the output tubes, they get very hot readings and they are NOS tubes I had for this project. Have removed them as It was buzz/humming like crazy.
     
  10. Riffscaster

    Riffscaster TDPRI Member

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    IMG_20200810_192225.jpg IMG_20200810_192437.jpg IMG_20200810_192611.jpg
     
    Last edited: Aug 11, 2020
  11. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    The rectifier pin4 and pin6 should have the high voltage from the PT connected.
    Pin2 and pin8 should have the 5V heater wires connected.

    Rectifier pin8 on the Ceriatone layout goes to the standby switch and then to the positive side of the first filter cap. If the standby switch is not connected like the layout, then pin8 should be connected to the positive side of the first filter cap.

    The Ceriatone layout shows pin6 going to the bias resistor on the board. The other end of that resistor goes to the striped end of the diode.

    EDIT: The OP removed the questions this post answered.
     
    Last edited: Aug 11, 2020
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  12. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Possibly the negative feedback is positive. You may need to swap the OT primary wires. (Pin3 on the KT66.)

    Make sure the bias pot is at maximum resistance.

    Where are the pictures of this build?
     
    Last edited: Aug 11, 2020
  13. Riffscaster

    Riffscaster TDPRI Member

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    Schematic on a JTM45 with tremolo
     

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  14. Riffscaster

    Riffscaster TDPRI Member

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    The 100k V2 cathode burned up, replaced it with a 1/4 watt seems to be holding up under start up?

    The bias voltage was not going further down stream than the + side of the diode ?

    Have no negative bias voltage on the tube sockets (Pin 1 and 8 to ground)
     
  15. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Do you have a jumper under the board here:

    Screenshot_20200811-085401.png
     
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  16. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    What resistance do you get from the *no voltage side* of the diode to ground?

    What diode are you using? 1n4007?
     
  17. Riffscaster

    Riffscaster TDPRI Member

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    IMG_20200810_192337.jpg IMG_20200810_192632-2.jpg
     
  18. Riffscaster

    Riffscaster TDPRI Member

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    Yes there's a jumper between the circled components.
     
    Last edited: Aug 11, 2020
  19. Riffscaster

    Riffscaster TDPRI Member

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    -65 MA to ground on the no volt, yeah its a 1n4007
     
  20. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I guess that is voltage. @Lowerleftcoast asked for the resistance.

    Have you removed the output tubes? As long as your bias is not ok, you should.
     
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