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EL34 choice for Dynacord Eminent II - 740V plate voltage

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  • EL34 choice for Dynacord Eminent II - 740V plate voltage

    I've recently picked up a late 70's Dynacord Eminent II amp, schematic here: http://www.peel.dk/Dynacord/pdf/Emin...ision) (2).pdf

    The amp has no major issues, so I'm just going to renew the filter caps and bias trimmers and see if I can get some use out of it for low volume bass guitar gigs. The amp is a class AB2 circuit with 740V on the plates and 420V on the screens of the EL34s, claiming 80 watt RMS and 110 watt peak output.
    It's running a pair of Hoges EL34 at the moment, which I gather are Siemens/RFT under another name and probably original to the amp. I'll try running it with the Hoges initially, though they may be quite well worn. If I want to replace those, which current production EL34 would be dependable at the voltage? Some recommend JJ, while others warn against them. I think the supplier I tend to use has Shuguang, EH, JJ or Winged C/Svetlana, though the Winged C are more expensive than the others. NOS are almost certainly more than I want to spend.

  • #2
    Can't offer any brand suggestions, but I will offer this.
    Check what those old tubes are biased at (that have performed well for so long) and be sure you don't bias the new ones any hotter than that.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      The =C= are more expensive because any of them are also now NOS. SED discontinued production a few years ago. They may well be the best option of new(ish) production tubes... some report problems with them going microphonic early, but they're usually rocked pretty hard, too. My 6L6GCs did, but the tone is worth it. And they didn't go ringy, they make a "phhphphshahshph" sound. You only notice it if you bop the cabinet pretty good. I'd gladly spend the extra money again, if I have it.

      Justin
      "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
      "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
      "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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      • #4
        Checking the bias on the old EL34s seems like a good idea. Dynacord thoughtfully provide a pair of 10 ohm resistors at the cathodes to measure bias, and the schematic shows the voltage across these at the factory bias setting.
        Looking into it, there are a few owners of these amps who report good results with the JJ EL34 or E34L. The winged C look promising, but I have to decide whether it's worth the expense when a pair would be almost half what I paid for the amp.

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        • #5
          A little background on why I suggested checking the stock bias. Many amps that run the tubes at voltages we expect to be "brutally punishing", don't burn through tubes like we think they would. They are often biased very cold, like 50% or even less, but no one spends much time documenting this data (or the fact that they sound fine like this).
          Then these amps sometimes get in the hands of people who think they must have adjustable bias installed, and biased to the internet "rule" of 60 to 70%. That's often where the "burning through tubes" begins. Of course I don't think modern tubes are as robust or reliable as older manufacture were, but running them hotter really exaggerates the issue.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #6
            Regardless of the high + Ua (740V) due to the lower + UG2 (420V) EL34 relatively well tolerated high anode voltage. It would be better to use tubes with a larger balloon. The bigger the balloon allows better cooling tube. From experience 6CA7 is tube of choice.

            The anode in such tubes is robust, the greater the distance between the anode and cathode, thus the possibility to spark quite reduced.

            Tip: Whenever you replace the output tubes must be sure to adjust the bias.
            Bias is adjusted by measuring the cathode current value of 0.25 to 0.30 mA measured at the resistors R418 / R419 (10Ω), with trimmer potentiometer R412 / R413 (25kΩ).


            http://www.thetubestore.com/Tubes/EL34-6CA7-Tube-Types
            It's All Over Now

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