New tubes rebiased sounds like crap. Please help

Discussion in 'The Workbench' started by 12AX7, Feb 4, 2020.

  1. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Who are you refering to?. My advice is not wrong. It could absolutely very well be a dirty preamp socket.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Yeah I think the plates alone are more like 160 if it's reading 180. But that should be just about fine. No doubt there is an advantage to each tube measured separately...

    We need to go through and get the voltage readings.
     
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  3. tschrama

    tschrama Well-Known Member

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    Tab the relays. One might be stuck.
     
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  4. 12AX7

    12AX7 Member

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    I've cleaned all the sockets with cleaner. Ive checked solder connections on the sockets to the board for cracks with a magnifying glass and see none. I've taped on all the tubes with a pencil eraser and got no noise.
     
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  5. 12AX7

    12AX7 Member

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    Ok this is what I've done today as far as testing. I still have my set of matched tubes installed and I adjusted the bias setting until it brought the pin 5 reading up to -42.3. this left my bias reading across the standyby switch reading 113mA. It changed my plate voltage on pin 3 to 485vdc. Believe it or not the amp sounds much better on the clean channel now but its still not quite right.
     
    Last edited: Feb 6, 2020
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    When you measure 113 ma, what is the meter set to?
    Is the meter set to "current?"
    Are the probes plugged into the current jacks?
     
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  7. tschrama

    tschrama Well-Known Member

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    try an even lower bias mA... the D+, also feeding de fxloop driver might need a higher V+. just try it out at 90mA.
     
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  8. 12AX7

    12AX7 Member

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    Meter is set on the mA setting red plug is plugged into the A hole here's a pic. https://www.dropbox.com/s/4d594avafcr5e20/20200206_152723.jpg?dl=0 The meter dosent seem to read right if I put the red plug into the mA hole i only get a reading of 0009.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Cause the internal ma fuse is blown?
    Also make sure the battery is fresh.
     
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  10. 12AX7

    12AX7 Member

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    Bat. is new not sure about the fuse the meter isn't mine its on loan but it is reading mA in that plug you can see the mA in the meter screen if you zoom in. the screen protector is a little ruffed up but it says mA . Ok guys y'all ain't gonna believe this but I moved the bias across the standby switch to 101mA and that changed my plate voltage on pin 3 to 491vdc pin 5 now reads -43.7. Measurements at the standby switch are being taking with the amp in standby mode. The amp now sounds normal. WTH?
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    It might be that the meter is out of calibration...
    But if 101ma makes the amp sound normal then it's not a bad thing.
    Usually colder bias sounds worse not better.
     
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  12. 12AX7

    12AX7 Member

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    Ok I'm going take my tubes out and put his set back in and see if I can get the same results. If not should I assume his set aren't matched?
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    If one tube is hotter / colder than the others then it's not matched.
    Temperature is an easy way to tell (besides measuring).
    The current of each tube can be tested separately using numerous methods.
    The most common method being: a 1 ohm resistor between pin 8 and ground of each tube.
     
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  14. 12AX7

    12AX7 Member

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    I assume this is how you go about this current test on pin 8. Solder in a 1 ohm resistor to pin 8 at the circuit board and clip the other end to ground then measure for current on the grounded side of the resistor with the red probe and black prob attached to ground or do I put the red probe on one side of the resistor and the black probe on the other? I don't have a 1 ohm resistor on hand closest I have is a 0 ohm 1/4 watt but I would like to learn how this is done for future reference. I reinstalled his tubes and rebiased them to 100mA, pin 3 is now 501vdc, pin 3 is now -46.4vdc. The amp sounds good now. Can I call the owner and have him pick it up or are these test results to high? Is there any other testing I should do to be sure?
     
  15. 12AX7

    12AX7 Member

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    I just took the meter apart tested bat and its good. I did find one fuse https://www.dropbox.com/s/3rb6cz0i50vucnn/20200206_174401.jpg?dl=0 that will not pass conductance test (will not buzz) and no change on all the ohm test settings
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    I assume this is how you go about this current test on pin 8. Solder in a 1 ohm resistor to pin 8 at the circuit board and clip the other end to ground then measure for current on the grounded side of the resistor with the red probe and black prob attached to ground or do I put the red probe on one side of the resistor and the black probe on the other?

    You can put the red black probes on any side across the 1 ohm doesn't matter.
    The only way I could tell if it was working right is with a scope.
    The bias seems too cold but you say it sounds good so...
     
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  17. 12AX7

    12AX7 Member

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    Thanks for all the help everybody. If anyone can think of anymore test I should run on this thing to confirm its running right let me know I'll have the amp until tomorrow afternoon sometime. I'd like to test the current of pin 8 but since I don't have a 1 ohm resistor or a bias probe I don't know how else I can test it.
     
  18. 12AX7

    12AX7 Member

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    I played the amp some more and do agree with you the bias is to low. I did some more research and found this info about Carvin amps. https://www.eurotubes.com/store/pc/carvin amp page.htm
    It says near the bottom that Carvin bias their Legacy, MTS and V3 models cold at 100mA and that they recommend to bias between 150mA-180mA. So I tried to do what I did when I had my set of tubes in it adjusting the bias until I got -42vdc on pin 5. Well that brought my pin 3 voltage down to 490vdc and gave me a reading across the standby switch of 158mA. The clean channel still sounds clean and the distortion channel sounds great now. I'm leaving it right here. I don't know why but it sounded like crap at 180mA with both sets of tubes while this set sounds good set at 158mA the other set didn't sound good at 160mA. I think it's righ now guys. Thanks again!
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    they recommend to bias between 150mA-180mA.

    Yes I figured that.
    If the plate volts is 500 volts DC it means that the power tubes are shut off.
    The voltage should drop quite a ways when the output is biased 70%.
    The tubes should get nice and hot, that is normal.
    If the tubes are only "warm," it won't sound good.
     
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  20. 12AX7

    12AX7 Member

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    Just to be clear it's Eurotubes that recommends bias setting of 150-180mA From all the research I've did and I can find Carvin still recommends 100mA.
     

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