ongoing Bassman build issues

Discussion in 'Amp Central Station' started by 8urchevy, Mar 13, 2019.

  1. 8urchevy

    8urchevy Tele-Meister

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    Hello again. My lazy ass finally did a complete pinout voltage check instead of just the plates. The long tailed pair phase inverter voltages are crazy and I just cannot figure out what is going on but I have a funny feeling that is where my tone issues are lying. Here is what the pins read..this is a 5f6a

    actual schmatic

    1..249 230
    2..85 22
    3..119 34
    6..276 235
    7..58 23
    8..120 34

    How can these be so off? I am going insane trying to figure out what I have done. Thanks again for all your help.
     
  2. Axis29

    Axis29 Poster Extraordinaire Ad Free Member

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    Which pins? Which tube?
     
  3. D'tar

    D'tar Friend of Leo's

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    Can you post a pic of phase inv section so that we can see resistor color codes & Carefully confirm component values
     
  4. peteb

    peteb Friend of Leo's

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    What voltages look wrong?


    Plates are ok

    Cathodes are ok, maybe a little high.

    The grids are lower than the cathodes and they should be. It may be hard to get good measurements here.
     
  5. 8urchevy

    8urchevy Tele-Meister

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    Hey pete. Thanks for responding. I was concerned with the grid and cathode voltages in particular. I agree the plates are ok. Why would the grids and cathodes be so much higher than the schematic. I am absolutely certain that the resistor values are correct.

    Even with the master volume removed it still sounds like crap when I try to turn it up. It will not try to break up until the gain is turned up to 2 o'clockish and much past that I believe I am getting blocking distortion.

    Man I have no clue.
     
  6. 8urchevy

    8urchevy Tele-Meister

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    I would but can't right now .i have checked them all and they are within 5%
     
  7. 8urchevy

    8urchevy Tele-Meister

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    Actually the grids and cathodes are 3 to 4 times higher than the schematic .
     
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  8. D'tar

    D'tar Friend of Leo's

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    Wiring point to point like that... Errors can easily be overlooked. I would recommend disconnecting the 27k nfb resistor and ohm the LTPI grids to cathodes, grids to ground and cathodes to ground to see if you get logical readings. Check the value of your presence (5K) pot also and rotate while ohming cathodes to ground.
     
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  9. 8urchevy

    8urchevy Tele-Meister

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    grid to cathodes are at 1M....grid or cathode to ground is OL...presence pot checks out at near 0 to 5K and the cathodes show 0 through the whole sweep.

    Damn. Shouldn't the cathodes show some kind of resistance? I would think so. Then again I guess that the tube is grid biased.
     
  10. D'tar

    D'tar Friend of Leo's

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    Did you disconnect nfb?
     
  11. 8urchevy

    8urchevy Tele-Meister

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    Sure did.
     
  12. D'tar

    D'tar Friend of Leo's

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    Then yes you should be seeing 10k+ to ground from the cathodes (470r+10K+presence pot 0-5k)
     
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  13. 8urchevy

    8urchevy Tele-Meister

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    Just to be clear I did the checks right on the tube socket terminals. Also, any idea where I went wrong? The amp works per se...lol
     
  14. D'tar

    D'tar Friend of Leo's

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    The cathodes are tied together right. Start at cathode pin. one probe connected to cathode other probe... work back from cathode to ground...

    cathode through 470r. check

    cathode through 470r and 10k... check

    cathode to presence pot.. check

    cathode through presence pot.. check

    presence pot to ground

    1 pot lug is grounded right?
     
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  15. D'tar

    D'tar Friend of Leo's

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    You are grounding your LTPI via connection through nfb resistor and output tranny.

    Multiple threads with dissatisfaction:(

    Hopefully this will get you where you need to be:)
     
  16. Snfoilhat

    Snfoilhat Tele-Afflicted

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    You're in good hands w/ D'tar et al. walking you through this troubleshooting process, and it's nice to have a possible explanation for the bad overdrive sound you mentioned in the other thread.

    While we are here, and if you are over feeling lazy, I'd like to pick up this bit about how to measure the voltages at the phase inverter. Only measuring the VDC at the tube socket pins is not really helpful. In addition, do this:

    Measure VDC at point a to ground and write it down. Measure VDC at point b to ground and write it down.
    5F6A_Bassman_Amp_Layout_small.png
    This is one of Robrob's layouts from https://robrobinette.com/5F6A_Modifications.htm

    a - b is the bias voltage for the phase inverter. a is just your cathode (pins 3 and 8) voltages taken at a different (and maybe more convenient) location.

    b is a much better measurement of your grid voltages (pins 2 and 7) than you can get by measuring the pins directly. You should measure pin 2 and pin 7 to make sure they are connected and functioning, but just remember to throw away those numbers and replace them with b. That's why voltages measured at the control grid pins (2 & 7) don't match the schematic even in a well functioning amp.
     
    Last edited: Mar 13, 2019
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  17. D'tar

    D'tar Friend of Leo's

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    This may be your issue here
    you have no dc reference to ground

    upload_2019-3-13_12-21-42.png

    upload_2019-3-13_12-22-4.png
     
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  18. D'tar

    D'tar Friend of Leo's

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    Connect the cap to just the wiper and ground the other lug.
     
  19. 8urchevy

    8urchevy Tele-Meister

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    I just realized that and just corrected it. I have a Dr. appt to go to now so I can't test it out. Also, I want to review the other responses and suggestions above. I can't thank you guys enough. Man I hope this wakes it up. It just acted like it had way too much negative feedback. The only way I can explain it. This should do the trick. Can't believe I did that.
     
  20. 8urchevy

    8urchevy Tele-Meister

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    Thank you so much. Makes perfect sense too
     
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