New tubes rebiased sounds like crap. Please help

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

  1. thetragichero

    thetragichero Well-Known Member

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    oh all you have is a fluke meter. you poor thing :drool:
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes the standby will give you an indication of all 4 tubes current at the same time.
    Set the meter for: "AMPS."
    This is a common method of setting bias including for Marshall amps.
    But it does not read individual tubes one at a time.
     
    Last edited: Feb 5, 2020
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    At pin 5 will be a negative DC voltage on each output tube socket.
    Take out the power tubes.
    Turn the power on.
    Set standby switch to "warm-up." Do not turn standby to operate yet.
    Set meter for DC volts, connect black probe to chassis.
    Test each socket pin 5 with the red probe.
    On each output socket pin 5 should be approx -42 VDC for EL34 output tubes. Confirm this voltage on all output sockets.
    Report your readings to the forum.

    * For 6L6, 5881, 6550, should be approx -52 volts DC on each socket pin 5.
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes it does you are right about that.
     
  5. 12AX7

    12AX7 Member

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    I got 41.7 on all 4 tube sockets selector switch was set for EL34 tubes and not the 6L6 option.
     
  6. 12AX7

    12AX7 Member

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    Now if I can just learn how to use the thing.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    It looks like the bias is set correctly for EL34s.
    Put the old preamp tubes back in and see if it sounds better. You might have a bunk new preamp tube.
     
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  8. 12AX7

    12AX7 Member

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    ok I'll do that in about and hour and report back. I have to run to the store for a few mins.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Get the schematic and check the low voltage plus and minus power supplies.
     
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  10. ampmadscientist

    ampmadscientist Well-Known Member

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    Somebody has to suffer, somewhere.
     
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  11. Matt_Krush

    Matt_Krush Well-Known Member

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    I read it...
    That's about what I thought.
    The standby switch method is great if:
    a) All tubes are matched pretty close say within 6%
    b) There are no problems with the pins/sockets
    c) The voltage dropping resistor isn't faulty and vary with heat/current flow.

    This method you could still have the push side very cold biased and the pull side very hot biased and you could still measure the '190' ma or so.
    If a tube or two is way out of spec...you don't know which one might be under or over drawing current.
    If it were my amp...I'd want to know what each half of the push pull circuit is doing...I'd want to make sure its close to balanced.
     
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  12. 12AX7

    12AX7 Member

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    https://www.thetubestore.com/lib/th...arvin/Carvin-Legacy-Vl100-Vl212-Schematic.pdf There's the link to the schematic I have. I'm not sure if there is a better one available or not maybe easier to follow this one is spit in two. You'll have to help me out on what the low voltage plus and minus power supplies are. Leads from power supply maybe? I put the old pre amp tubes back in and tested it and the clean channel still has a fuzzy distortion to it. i dont think its as bad now but that could just be my mind playing tricks on me.
     
  13. 12AX7

    12AX7 Member

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    I have a new match quad of GT in my personal stash I'm going to try them. It would not surprise me if these new JJs he gave me with the amp were not matched.
     
  14. 12AX7

    12AX7 Member

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    Well I put my set of matched grove tubes in it, took all the measurements again, recalculated the bias and set it to 175mA. It did sound better but the clean channel has a hint of overdrive in it and its a lot more noticeable at around half volume. This test was done with the old set of preamp tubes.
     
  15. 12AX7

    12AX7 Member

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    https://www.yumpu.com/en/document/read/26550634/legacy-vl100-vl212-carvinservicecom I found another bias instruction manual for this amp it appears to be by Carvin and it says bias it across the standby switch to 100mA just like they do it in the factory. What do y'all make of this? 100mA vs 180mA is a big difference. Also when I installed my new matched set of groove tubes in it the plate voltage went up some more to 490vdc. Could that plate voltage being too high case this issue the book said it should be around 450vdc?
     
    Last edited: Feb 6, 2020
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    The plate voltage goes up or down depending on the bias setting.
    Install the tubes, set the bias, then measure the plate volts.
    100 ma is way too low, you had it right when there was -41.7 volts on pin 5.
    180 ma should be pretty close to correct.*

    We need to check some other stuff if the amp still doesn't sound right.

    *Because we are measuring plate + screen + preamp tubes, the 180ma is not the total plate current. The plate current is about 20-25 ma lower than what we are seeing at the standby switch.
     
    Last edited: Feb 6, 2020
  17. Matt_Krush

    Matt_Krush Well-Known Member

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    seriously...get a bias king or some other method to learn to read the current/voltage at each tube so you can determine if there are no other issues.
    The Carvin cumulative bias you are doing is only good if you know everything is perfect.
     
  18. ampmadscientist

    ampmadscientist Well-Known Member

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    Yes that is correct. But it's not a "wrong" method.
    It's a method that is used widely, even with Marshall amps.

    Does the OP have a bias probe? Or, is the OP able to install 1 ohm resistors on the output sockets?

    Even with a bias probe, you are not reading plate current "only."
    Even with 1 ohm resistors you are not reading plate current "only" either.
    So any of these methods (including probe or resistors) is inaccurate.

    The only way to measure plate current is to put the current meter on the plate wire.
    And- that does not tell you "watts" either.
     
  19. Gunner64

    Gunner64 Well-Known Member Premium Member

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    It got better after you changed preamp tubes? Maybe one of them just has a dirty socket, or a solder joint that needs attention. You said volume was coming and going before the tube swap and now its distorted on the clean channel only? I would look at the sockets on the 12ax7's.

    I would fire it up and gently nudge each tube. This usually will tell you If its a dirty socket or something related to the socket not making good contact. I'm not saying this is the problem, but this should confirm, or rule it out as the cause.
     
    Last edited: Feb 6, 2020
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  20. tschrama

    tschrama Well-Known Member

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    OMG, just stop giving this guy wrong info!

    180mA is fine.

    The problem is something else.
     

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