What could be the reason for too high voltage from filter caps?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by O RUde, Jun 21, 2019.

  1. VICOwner

    VICOwner Supporting Member

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    Measure the a/c voltage on the red pair of wires coming out of the power transformer. The layout shows 465 volts b+. You should see about 660 volts a/c between the two red leads or about 330 volts a/c between one red lead and chassis. Is this what you get?
     
  2. VICOwner

    VICOwner Supporting Member

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    The schematic says 340v ct @ 450 mA. Are you using the international taps transformer? Are you here in US or elsewhere. What is your wall a/c voltage?
     
  3. ironman28

    ironman28 Member

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    What are the voltages for B1 B2 B3 B4 B5 with all the tubes installed and the power and standby by on?
    Plug your guitar in to the fx return and see if you get sound to the speaker with the amp power and standby on.
    The voltages on B4 and B5 depend on V1 and V2 drawing current through the 22k and 2.1k resistors to create a voltage drop.
    Both the layout and the schematic do not show the heater supply connections for V1 V2 and V3. Check the heater voltage on pins 4/5 and 9 on each preamp tube if you have wired the htr1 and htr2 to the 9 pin sockets. It should be around 6.3 vac.

    Double check the 22k and the 2.1k resistors with the power off and the caps discharged.
    That layout should show the heater wiring.
     
  4. pdf64

    pdf64 Member

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    If there’s 21.75Vdc between the cathodes and 0V, then something is very wrong, hence an open circuit there seems likely.
     
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  5. HotBluePlates

    HotBluePlates Member

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    Either that, or there are 33kΩ and 22kΩ resistors where there should be 3.3kΩ and 2.2kΩ resistors.

    Okay, at this point I'd say measure pins 1, 3, 6, and 8 of V1 and V2. We're gonna have to figure out what is not right.

    Being a new build, you will find the problem will be wrong part value, a missing wire/connection, a wire/connection in the wrong place, an unintentional short-circuit (solder blob or stray wire strand) and/or a poor solder joint. Everything to be checked will need to be checked with a meter, measuring voltage or resistance.
     
  6. O RUde

    O RUde Member

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    It is 462v
    AC to rectifier: 658ac
    From rectifier 457,
    B5 436
    B4 438
    B3 450
    B2 455
     
  7. O RUde

    O RUde Member

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    I use 240v transformer, it's a Hammond 290FEX.
     
  8. O RUde

    O RUde Member

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    12ax7 measurements:
    V3 V2 V1
    1:252v 430 432
    2:19.86 19.80
    3: 42 21 21.72
    6: 448 410 429
    7: 5.85 16 21
    8: 42 21.90 22v
     
  9. VICOwner

    VICOwner Supporting Member

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    Take a good picture of your board, it sounds like you’re resistors are mixed up. I may be able to read them from the picture and help sort it out. I’ve built a Dumble amp.
     
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  10. O RUde

    O RUde Member

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    I wonder if it has something to do with the potentiometers on the board itself, I rotate it a bit, and suddenly I had some scratching sounds on the parts that follows after the B+3. I also have scratching sounds from the V1 and more. Could the wiper be placed on the wrong point? It looks to me like the 10k PM goes to B+3 and the 100k PM goes to the 68k from the tube pin 7?
     
  11. O RUde

    O RUde Member

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    So the story evolves. I have now gotten to the point where it sound out, but it's some uncontrollable noise that in the end blows the fuse?

    The input of the power amp on the backside respond to the jack.
     
    Last edited: Jun 24, 2019
  12. Tron Pesto

    Tron Pesto Member

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    Whoa - dude, all due respect, you might want to put down the solder iron for a bit. Internal pots are typically bias pots - you can't just go arbitrarily cranking on those for the hell of it. You can create a situation where you will melt-down tubes, possibly transformers and more - the best worst-case scenario is fuses start to blow before you immolate the entire amp.

    You need to do study tube amps design and electronics in general. Bone up, then head back in.
     
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  13. Tone_Terrific

    Tone_Terrific Supporting Member

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    Did you get the voltages corrected?
    If B+1 and b+2 are close to spec, the power supply is basically fine, and if you have good voltages in the power section go back and check each preamp tube for wiring, component values, and voltages. It will take a while.
     
  14. VICOwner

    VICOwner Supporting Member

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    I can't read any of the red resistors, I was hoping you used banded resistors. What I know is that the cathode voltages on v1 and v2 should be less than 3 volts typically around 1.5 on the input. In the Dumble circuit the cathodes with the 3.3k and 2.2k will be above 1.5 volts but less than 4 volts. With all the cathode voltages above 15 volts the tubes are biased so far into cutoff that they may not amplify. Are you sure that you have the grounds connected in the centers of the cathode resistors?
     
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  15. HotBluePlates

    HotBluePlates Member

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    There is almost zero current through the tube, causing both the cathode & plate voltages to be high. Either your cathode resistors are ~470kΩ, or the grounds are not good for the cathode resistors.

    There are also voltages present on V1 & V2 pins 2 & 7 where voltage should be 0v.

    Something is way-wrong with the preamp wiring.

    There are 2 internal trim pots, one that is an interstage voltage divider and another that balances the phase inverter. The latter can/should cause some scratchy noise when adjusted, as it has d.c. volts across it.
     
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  16. O RUde

    O RUde Member

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    In this build there are 5 grounding points. No 1 close to the jack inputs, the second and third between the El34's, fourth on the PT screw and the fifth by the voltage regulator for the 12v relay transformer. The groundings alway seem to be an issue, but I have checked, and the ScR looks to be good. 2.2k and 3.3k on both, 4.7uF on the caps.
     
  17. pdf64

    pdf64 Member

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    Not sure what 'ScR' is, but however things look, something is apparently wrong, ie the resistances between the cathodes and chassis 0V must be a lot higher than the nominal value of the resistors on the board.

    However, as you have blown a fuse, your first job should be to build a light bulb limiter and only power the amp through that until all such issues have been resolved.
    I appreciate that incandescent light bulbs may be more difficult to buy than they were a few years ago but halogen types are fine, also consider asking friends and relatives if they have any spare old bulbs at the back of a cupboard etc.
     
    Last edited: Jun 25, 2019
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  18. O RUde

    O RUde Member

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    Last edited: Jun 25, 2019
  19. Roe

    Roe Member

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    PT and caps seem fine, but its not drawing much current. check wiring of the power supply, including plate resistors and try with preamp tubes before you add the 6l6s
     

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