Power tube ID help needed

Discussion in 'The Workbench' started by mkayers, Apr 10, 2020.

  1. mkayers

    mkayers New Member

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    Hey guys, did alot of googling & can't find a good answer so maybe you can help. A couple weeks ago I bought a 1984 Marshall 4212 combo. I wanted to retube it so I went to the tube depot site & when you punch in that amp it comes up with 5 ECC83s and 2 EL34s. ECC83s look right but I'm not sure on the EL34's. Ones in the amp do not have a single mark or number anywhere (and I've went over them with a magnifying glass lol) and they're what I would call "bell shaped". Would these be 6550's? upload_2020-4-10_8-40-2.png
     
  2. pedecamp

    pedecamp Well-Known Member

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    Just from the shape they look like many KT88, KT66 and some 6550 tubes. Lets see what others here have to say, but I'd be inclined to have a tech look at the amp to see what its wired for, people have been known to put the wrong tubes in amps or have them rewired for different tubes, a tech will be able to determine this. I'm gonna bet youre right and theyre 6550 tubes, many 800's came stock with those including mine which I had converted for EL34. Good luck! :yesway:
     
    Last edited: Apr 10, 2020
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  3. South Park

    South Park Well-Known Member

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    What size resisters are hanging on the tube sockets . People should label what tubes are in use in these amps
     
  4. mkayers

    mkayers New Member

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    Still trying to look it up but here is a pic
    upload_2020-4-10_11-51-32.png
     
  5. Jon Snell

    Jon Snell New Member

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    The KT66, which was widely used in the 60s and 70s has a bakelite base so my guess is KT88.
     
  6. Micky

    Micky Well-Known Member VIP Member

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    They look like old Sovtek KT66
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    The screen resistors are 1000 ohm,
    KT66 / 5881 / 6L6 uses 470 ohm usually.
    This would indicate that the amp is set up for EL34 or 6550

    Can you make a test?
    Remove output tubes.
    set meter for DC volts, connect black probe to chassis.
    Turn power on, set standby switch to "warm-up." Do not turn standby to "operate"
    Connect red probe to pin 5 of output tube socket.

    What is the DC voltage reading from pin 5 of the output tube socket?

    The voltage on pin 5 will tell you what tubes belong in this amplifier.
    If the voltage is approximately -52 to -57 VDC, then the amp is intended for 6550.
    If the voltage is approximately -42 VDC, then the amp is set up for EL34s.

    3. Bias split resistors
    If the bias split resistors are 150K this is usually a 6550 amplifier.
    If the bias split resistors are 220K this is usually an EL34 amplifier.
     
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  8. Chris-in-LA

    Chris-in-LA Well-Known Member

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    This is a pic he posted on another forum. So, R52 and R53 look to be 150k which would be correct for 6550’s, right?

    upload_2020-4-13_23-6-5.png
     
  9. mkayers

    mkayers New Member

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    Hey Chris, thanks for doing the work for me! :cheers:
     
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  10. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I’m here to help. I was going to suggest you post here. You should get more feedback here from knowledgeable Marshall folk than on the other forum.
     
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  11. ampmadscientist

    ampmadscientist Well-Known Member

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    What is the voltage on pin 5?
    Yes 150K is usually for 6550 in a Marshall amp.
     
  12. mkayers

    mkayers New Member

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    I'll check the voltage tonight after work.
     
  13. anitoli

    anitoli Well-Known Member

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    The getter flash is white, those tubes look fried.
     
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  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Those tubes are broiled well done. :lol:
     
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  15. Seanxk

    Seanxk Well-Known Member

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    Most certainly 6550, but probably a russian copy of the Tungsol original from 50's 60's and yep they're shot.
    GE made them too but they weren't coke bottle shaped.
    Here's some original Tungsol's that are still cooking... IMG_3421.jpg
     
  16. mkayers

    mkayers New Member

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    Yeah you can barely tell these guys are glowing lol.
     
  17. mkayers

    mkayers New Member

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    Getting about 53.5 volts so looks like 6550s.
     
  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    That tube on the right has the most heavily oxidized getter patch I've ever seen on a tube that didn't have a broken envelope. It's seen many thousands of hours of operation. Stick a fork in it, it's done.
     
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  19. American Viking

    American Viking Well-Known Member

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    Marshall JCM800's distributed to the United States in 1984 would have come with 6550's. Their British and European counterparts would have EL34's. It requires a minor circuit change to make the appropriate swap if desired.
    If the amp has never been recapped I'd say now is a good time. It's up to you whether to change the circuit to accept EL34's. That's what the amp was originally designed to use. The idea to use 6550's for the American market was really to mask the fact that a couple EL34's were having reliability issues at the time from what I understand.
     
  20. mkayers

    mkayers New Member

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