Fixed Bias 2-EL84 amp suggestions needed for tube matching

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 59Vampire, Dec 6, 2017.


  1. 59Vampire

    59Vampire Silver Supporting Member

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    I have a boatload of anos and nos el84s that I need to match. My previous matcher was an old bedrock but I blew it up a while back and stopped matching.

    Can anyone suggest an inexpensive amp that fits the bill that I can cannibalize for testing purposes?

    Thanks.
     
  2. pdf64

    pdf64 Member

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    Change any cathode biased amp to fixed by swapping the cathode resistor/s for zener/s, and then add a couple of 1 ohm cathode current sensing resistors.
    The zener/s would need a similar dissipation rating to the resistor/s they replace.
     
  3. 59Vampire

    59Vampire Silver Supporting Member

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    Thank you but I am not capable of altering anything! Using a bias probe is one thing.......
     
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  4. pdf64

    pdf64 Member

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    Pro or Blues Junior then?
     
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  5. 59Vampire

    59Vampire Silver Supporting Member

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  6. Blue Strat

    Blue Strat Member

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    ^^^ This. Unless you're willing/able to modify an amp, this is your best bet.
     
  7. 59Vampire

    59Vampire Silver Supporting Member

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    I have read that these amps run tubes very hot. Would that be an issue when matching tubes? Keep in mind I am not a tech but can read a meter with the bias rites that I have. Thanks.

    Can anyone build a testing rack?
     
  8. Blue Strat

    Blue Strat Member

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    Probably not an issue for a one minute, or less, test since these amps seem to run for 100s of hours as is.

    Of course, but I can almost guarantee you won't like the R&D, build, and test cost.
     
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  9. AA864

    AA864 Supporting Member

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    define inexpensive.
     
  10. 59Vampire

    59Vampire Silver Supporting Member

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    Inexpensive? Lol. Certainly not 500. I just need something to match tubes. I saw pro juniors going for 250ish.

    I just need a functional amp that will not crap out and cause me to spend money to have it fixed. I just need to match a large amount of nos and anos el84s I have been hoarding
     
  11. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    So would a fixed bias single-ended little power section work for you? Will you basically just be loading a tube in it, then checking where it biased out? I'm thinking something could be put together from junk drawer/spares parts that would serve for that purpose pretty cheaply. So all you'd really need would be a power supply and the output stage?
     
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  12. JJman

    JJman Member

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    Epiphone valve jr. It is SE and less than $100 used. Just use a good voltmeter and measure the cathode dc once warm for a while. Tubes with the same cathode dc in there should be dc matched in a PP amp. If you use a PP amp for matching, you have to leave one tube in side A, changing out the tubes in only side B each time. Otherwise you're changing an extra variable. Either approach should work for matching.
     
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  13. 59Vampire

    59Vampire Silver Supporting Member

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    Jeff and JJman,


    I dont think a single ended amp would work for matching. I am not a tech, but my research in the past, and in all fairness, I cannot remember the specifics, told me that in order to accurately match by plate current I would need a fixed bias 2/4 tube amp. I previously used a bedrock four el84 amp but blew it up a few years back and sold it off as a project.

    I am under the imnpression that running a single tube does not accurately reflect the measurements needed for matching.

    PLEASE correct me if i am wrong beefore i buy an amp. I have a vht with the el84 converter i can use but again, I do not think that is the way to go unless i am wrong. Which of course wouldnt be the first time!
     

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