ongoing Bassman build issues

Discussion in 'Amp Central Station' started by 8urchevy, Mar 13, 2019 at 9:28 AM.

  1. D'tar

    D'tar Tele-Afflicted

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    Oh man! you better take tomorrow off so you can test drive your new bassman!!!
     
  2. 8urchevy

    8urchevy TDPRI Member

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    Can't wait to get home to try it out. Didn't think about the fact that capacitors don't work that way. Thought I was being clever I guess. Lol. Wrong
     
  3. 8urchevy

    8urchevy TDPRI Member

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    I did already actually .lol
     
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  4. peteb

    peteb Friend of Leo's

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    Why are the cathode voltages high?


    OK, I see that they are high for the 5F6-A Bassman schematic. other similar amps, like the AA864 Bassman have 95 volts on the cathodes. that was why I said that 120 is OK. It is OK for LTPI's in similar circuits.
     
  5. 8urchevy

    8urchevy TDPRI Member

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    I think that the ground reference was no good due to how I wired the presence circuit .
     
  6. peteb

    peteb Friend of Leo's

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    that may be. If the cathode was not connected to ground, there would be no cathode current and no cathode voltage.


    If the cathode DC voltage is 3-4 times what it should be, then the cathode current is 3-4 times what IT should be. I would look there.
     
  7. 8urchevy

    8urchevy TDPRI Member

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    Thank you for all your help through this .
     
  8. 8urchevy

    8urchevy TDPRI Member

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    A bit bummed. It does sound better, but it still isn't breaking up right . could it be weak output tubes? They are from a 1961 pa so i guess it's not out of the question.
     
  9. D'tar

    D'tar Tele-Afflicted

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    Not sure what all you have modified in attempt to resolve your issue but perhaps you should return the circuit to stock. Definitely try a new set of tubes if you suspect they are past their prime.
     
  10. 8urchevy

    8urchevy TDPRI Member

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    Well, I only have one input and no 50v tap on power transformer, so the only other thing I did was add in the second half of the V1 on a switch to "jumper" the two channels. The same result happens with just one half of V1 as well just not as loud obviously.

    I forgot as I corrected the presence circuit that I removed the negative feedback loop and ground the cap and resistor to the preamp ground point before this post and same result happened.

    The output tubes are cathode biased at 63% Oh and the filter cap for the V1 and 2 tubes is a 20uf not the 8uf which I can't see how that would keep it from breaking up naturally.

    One last thing, the same result happens even with the master volume removed. It will break up, but very poorly.
     
  11. D'tar

    D'tar Tele-Afflicted

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    Cathode bias at 63% is pretty cold. Can you get that up to 90-100%
     
  12. D'tar

    D'tar Tele-Afflicted

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  13. 8urchevy

    8urchevy TDPRI Member

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    I would have to figure out the resistor value and order a replacement. It is currently at 250 ohms.

    Or do that link you posted. Rob had shown me that as well. I think I will order a replacement tube set anyway. Worst case scenario I have an excuse to build another amp, but this time more traditionally...lol I was quite happy the damn thing worked as this is my first build. Would have been a whole lot simpler with a tag board.
     
  14. D'tar

    D'tar Tele-Afflicted

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    Are you using 6l6 and 250r with bypass cap?
     
  15. 8urchevy

    8urchevy TDPRI Member

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    6l6gc and yes 250 with bypass 25uf
     
  16. dan40

    dan40 Tele-Afflicted Silver Supporter

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    If it has not been mentioned yet, be sure the 470r resistor between the two 1m's is definitely 470r and not 470k. Many new builds have had this mistake and the main symptom is a severely neutered sound from the amp.
     
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  17. peteb

    peteb Friend of Leo's

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    That’s right, if the cathode voltage is wrong, either the cathode resistors are wrong or if the resistors are right, then the current is wrong. If the resistors are right, and the current is wrong, I would think, the only reason could be the tube is off.


    I thought the resistors had been verified.

    Either way, the DC voltage on the cathode should not be affected by the rest of the circuit, only the ground connection, the values of the resistors, the plate voltage and the tube itself.
     
  18. D'tar

    D'tar Tele-Afflicted

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    Pete... There was a missing ground at the presence pot. I beleive he has fixed.

    How are your numbers now?

    I would like to see a complete voltage chart.
     
  19. peteb

    peteb Friend of Leo's

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    Ok, I see, I thought it was the cathode ground. Thanks





    But that is the cathode ground.
     
  20. peteb

    peteb Friend of Leo's

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    These are my voltages for an LTPI on a Bassman (AA165)


    Pin 1 plate 233
    Pin 2 grid loud pop
    Pin 3 cathode 104

    Pon 6 plate 222
    Pin 7 grid 68
    Pin 8 cathode 104




    If the tube is looked at, not the particular circuit, I think the voltages look ok.
     
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