Are These Tube Close Enough?

Discussion in 'The Workbench' started by BowerR64, Mar 10, 2019.

  1. BowerR64

    BowerR64 Well-Known Member

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    These came in a used JetCity Amelia i got friday.

    unmatched pair.jpg

    This is what the lowest one was set at when i got it.

    jdt city 1.jpg
     
  2. 67Mopar

    67Mopar Well-Known Member

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    No more than 8mA variance is recommended, but they're fine. I like mine a bit out of spec, as it seems to add a certain something.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Looks like the amp has something wrong with it.
    Screen resistor blown?
    Why is there 0 plate volts?
    0 plate volts indicates the OT is bad.
     
    anitoli likes this.
  4. anitoli

    anitoli Well-Known Member

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    Why does the first pic show no plate voltage?
     
  5. 67Mopar

    67Mopar Well-Known Member

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    Hey, AMS... Wouldn't the plate voltage be the same on both?
     
  6. BowerR64

    BowerR64 Well-Known Member

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    nvm now i know i need to replace em. i dropped the one reading 14. and now its at 30. the dang probe box slid off the cabinet and pulled the tube and it hit the carpet still warm now its double what it was reading

    its just a matter of time now right?

    new bias.jpg
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    OT bad?
     
  8. 67Mopar

    67Mopar Well-Known Member

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    Oops! Dropping hot tubes is a no-no. Well, dropping tube is a no-no...
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Unless the OT is toast...
     
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  10. BowerR64

    BowerR64 Well-Known Member

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    You know ive had this probe do that before in the past when taking pictures of the screen. Im thinking that it shows the display in pulses or something and i just happen to snap the image when the display is pulsing off.

    Ive wiggled the wires and it has no shorts its just sometimes when taking a picture the voltage will show 000 like its rechecking the voltage. Its never showed 000 with the naked eye.
     
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  11. 67Mopar

    67Mopar Well-Known Member

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    I ran tubes with a 10mA variance for a couple of years, and never had an issue. The amp actually sounded better than when I used a closely matched set. Both sets were =C= brand.
     
  12. charveldan

    charveldan Well-Known Member

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    With 468 plate voltage your bias should be 37ma.
     
  13. BowerR64

    BowerR64 Well-Known Member

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    These are the stock jet city china tubes they say "electron tube" EL34-B

    Damn i cant believe i dropped it, what a bone head move.

    Oh well i found a matched set so ill put em in when i get em.

    This is the second JetCIty ive had the first one was a JCA50H and it was about as funky as this one.

    The way the pots taper feel weird on these amps. Some of the pots have a slow taper and dont seem to do anything in some positions and some work like what ide expect.
     
  14. MarshallDog

    MarshallDog Well-Known Member

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    And...That’s what I was wondering?

    That’s very strange!
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Then you should have returned them.

    As said, something is very wrong, the probes and maybe the power tubes if not more.
     
  16. BowerR64

    BowerR64 Well-Known Member

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    Let me explain again.

    When i look at the bias probe with the naked eye i always ALWAYS see a plate voltage its only when im taking a picture of the probe i seem to get a no plate voltage reading. I went round and round with you guys over this shit one other time with a different amp and you guys swore up and down something was wrong with that amp too, then it was the probe, then it was the website i got it from or something.

    It has something to do with the refresh rate of the display or something with the flash. Like i said ive wiggled the wire ON THE PROBE and it never goes blank like that it just seems if i time it just right when taking the picture it shows 000

    Some times the display does this also, but it didnt look that way when i look at it with the naked eye.

    This is digital shit, digital camera, digital display, flash, 60 cycles hell i dono

    This is one probe and 2 different images cropped together the one with no plate voltage was just bad timing. (the first image in the original post to clarify so there is no more confusion)



    scrambeled display.jpg
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    The tubes should be about 3 ma apart or less really if the tubes are matched.
    We think something is wrong based on the readings which is understandable considering what we see on the meter.
    If the readings show a big difference check the screen grid resistors.
     
  18. BowerR64

    BowerR64 Well-Known Member

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    i got the new ones today im done messing with it

    New tubes.jpg

    The higher one is pictured the other one is slightly lower at 31.1mA-DC
     
    Last edited: Mar 14, 2019
  19. charveldan

    charveldan Well-Known Member

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    Seems you need to change the bias resistor.
    Did the amp originally have 6L6's or something other than EL-34's ??
     
  20. BowerR64

    BowerR64 Well-Known Member

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    now why would you say that?

    i already posted what it had above
     

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