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Where would you bias TungSol 7581?

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  • Where would you bias TungSol 7581?

    Customer brings me a Ceriatone Dumble clone he has never powered up, and a pair of TungSol 7581A to install and bias. This is my first encounter with this tube. I'm wondering where to bias them. The datasheet says it is a 35 watt plate dissapation "direct replacement for 6L6GC, but features additional controlled zero bias characteristics and a low loss base."

    What does that even mean?

    https://frank.pocnet.net/sheets/127/7/7581A.pdf
    Last edited by Randall; 06-12-2019, 03:48 AM.
    It's weird, because it WAS working fine.....

  • #2
    For me, firstly I'd want to know how the Dumble that the clone is based on is biased. Then I would treat this tube the same as a 6L6GC in that circuit.
    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
      Agree^^^^

      Remember, this is just a guitar amp, not a NASA laboratory thingie. Read "direct replacement for 6L6" and stop. Treat it like any 6L6.
      Education is what you're left with after you have forgotten what you have learned.

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      • #4
        Originally posted by Randall View Post
        low loss base.

        What does that even mean?
        I'm pretty sure that 'low loss base' pertains to radio transmitter use. Unless there's something very unusual about that Ceria amp I doubt it will make any difference at all.
        This isn't the future I signed up for.

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        • #5
          Originally posted by Enzo View Post
          Agree^^^^

          Remember, this is just a guitar amp, not a NASA laboratory thingie. Read "direct replacement for 6L6" and stop. Treat it like any 6L6.
          Correct. Typical bias dissipation is somewhere between 50 -70% and I'd treat these like a KT66 or KT88 or 6550 and bias them pretty hot. I'd start at 70% and evaluate the tone and feel of the amp then adjust accordingly. The 7581 sounds best biased a little hotter, in my experience.

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          • #6
            FWIW, the 7581 is a 30 watt tube, but the New Sensor Tung Sol 7581A is a 35 watt tube.

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            • #7
              I bounced it off Ceria, and he responded straight away. He doesn't know this tube, and is not sure if it really is a 35 watt, but he seems to be a 70% is plenty kinda guy. 65% is what he suggests, and that is usually about where I park 6L6s on most amps unless I know it is supposed to be cold like a Twin or that monster Peavey EVH.
              It's weird, because it WAS working fine.....

              Comment


              • #8
                What I've noticed:

                The data sheet you provided for a 7581 shows a 30W tube. There is also a TS data sheet for the 7581A, which shows a 35W tube.

                Here's my problem with all of these "reissues..." Well, obe of my quibbles. New Sensor didn't manufacture the tube and run it through the tests to truly determine specs. Seems to me they are just plastering their name onto a scan of an old vi tage TS 7581 data sheet. But if they're marketing these as 7581A, then we might have some confusion, huh? What if they ARE 7581, and not 7581A, but are marketed & branded wrong? Maybe for those who bias conservatively it's not an issue; but thanks to internet lore everyone just wants to drive the piss out of everything... Might induce some meltdown for those who live on the edge without understanding the consequences of actions. I could see a scenario where they issue a 6L6GC data sheet copy & give it away with a 6L6GB tube "reissue." Most people wouldn't be careful enough to cross-check specs & woukd blow some stuff up...

                Myself? I don't trust these "reissues." Especially if they can't come up with their own legit data sheets. I'd start by biasing them as vold as possible & gradually warm them up until they start to mekt down, then back off again. I'll trust my own experience more than I'll trust a rebranded data sheet. At least JJ has the decency to make their own.

                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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                • #9
                  Originally posted by Randall View Post
                  I bounced it off Ceria, and he responded straight away. He doesn't know this tube, and is not sure if it really is a 35 watt, but he seems to be a 70% is plenty kinda guy. 65% is what he suggests, and that is usually about where I park 6L6s on most amps unless I know it is supposed to be cold like a Twin or that monster Peavey EVH.
                  I use the 7581A regularly, treat it like a 35 watt tube, and crank up the current. This is one of those tubes that sound better when ran hotter.

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                  • #10
                    To clarify, I am talking about the 35 watt 7581A, but I mistakenly posted the 30 watt 7581 datasheet. I have fixed that.

                    With the exception of Tone Meister, does anyone really know if this is a 35 watt tube? If I can't get this resolved, I am inclined to run it hot for a 30 watt and not so hot for a 35 watt, and call it good.
                    It's weird, because it WAS working fine.....

                    Comment


                    • #11
                      I don't see any point in idling them any hotter than any other 6L6 type tube in a BF type power amp, ie 30mA cathode current / 6L6. All such tubes have the same bogey characteristics.
                      Why should those with a beefier plate require a wider conduction angle?
                      My band:- http://www.youtube.com/user/RedwingBand

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                      • #12
                        Originally posted by Randall View Post
                        To clarify, I am talking about the 35 watt 7581A, but I mistakenly posted the 30 watt 7581 datasheet. I have fixed that.

                        With the exception of Tone Meister, does anyone really know if this is a 35 watt tube? If I can't get this resolved, I am inclined to run it hot for a 30 watt and not so hot for a 35 watt, and call it good.

                        https://www.thetubestore.com/tung-sol-7581a

                        I tour with a Dumble-circuit amp and have used these tubes for about 5 years. I set idle bias at 25 watts and the power tubes last a couple years. This type sounds and feels better ran hotter than a typical 6L6 to my ears.

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                        • #13
                          Have you tried 'biasing' the tubes until the crossover notch just disappears?

                          I use a lowish output voltage for the test.
                          Forget about the slight notch that appears when approaching full power!

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                          • #14
                            I ended up at 22.5 watts, which is about where Ceriatone guy advised.
                            It's weird, because it WAS working fine.....

                            Comment


                            • #15
                              FWIW, the original 1962 Tung-Sol 7581A datasheet in the link specifies a max. plate dissipation of 35W. The design example PP AB1, Vp=450V, Vs=400V biases the tubes at 58mA corresponding to 26.1W or 75% (of 35W).
                              - Own Opinions Only -

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