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  • Used power tubes

    Someone is offering me around 50 or so used power tubes...USA made mostly 6L6, 6V6, EL34. Many others. They are clearly used. I don't presently have a tube tester. With or without a tester, is ther any way to get a qualitative assessment of the tubes? I woul prob offer him a buck or two each.

  • #2
    I've always gone about this by sticking them in an amp and seeing what happens. I've usually been pleasantly surprised, apart from one set of metal can 6L6s that shorted and took out my HT fuse.

    Of course you have to watch that what you're buying isn't the "suspect tube bucket" from some old TV repair shop that was too stingy to throw them away. A heavily used tube will often lose the shine on its getters, and develop patches of discolouration on the glass near holes in the plate structure that allow the glass to be bombarded by particles from the cathode.

    In a lightly used tube with a good vacuum, this bombardment causes a dim blue glow that seems to come from the glass envelope itself, in the places where these patches will appear later.
    Last edited by Steve Conner; 07-28-2013, 01:10 PM.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      For a buck or two each, you might get your money's worth. It's hard to test output tubes rigorously with most tube testers. They will spot serious problems, but, often, output tubes will test good in a tester even when they no longer sound good in an amp. If you need a matched pair, it can be a crap shoot trying to match unmatched singles.

      I have two vintage Tung Sol 6550s that both test fine in a tester, but, in an amp, as a pair, they sound muddy and muffled.

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      • #4
        Guys. Thanks for the advice. Just have me wondering what parameter degrades and leads to the drop off in sound quality over time.

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        • #5
          What I have seen on batches of old output tubes is the output voltage drops.

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          • #6
            Originally posted by fredcapo View Post
            Guys. Thanks for the advice. Just have me wondering what parameter degrades and leads to the drop off in sound quality over time.
            That's a very good question, and I wish I understood more about it. The bottom line is that tube testers present a narrow snapshot of tube performance under conditions rather different from what they encounter in an actual output stage. In the end, the best tube tester is the circuit in which you intend to use the tube.

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            • #7
              Power tubes die in two ways. Either they develop excessive grid current and go into thermal runaway, or they lose emission and can't deliver enough peak current to make rated power. I once tried some old Sovtek 6550s that failed in both of these ways at once.

              Tubes with low emission might well sound "muddy and muffled" when overdriven. The clipping due to running out of emission is softer and more rounded than the usual clipping due to grid current.
              "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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              • #8
                'I've always gone about this by sticking them in an amp and seeing what happens. I've usually been pleasantly surprised, apart from one set of metal can 6L6s that shorted and took out my HT fuse'

                As a word of warning for this method, many amps have minimal fusing, often just a line fuse on the power transformer primary.
                Unless there's a gross fault, eg equivalent to a major short across a secondary, they may take quite a while to blow (eg 30 seconds or more), by which time collateral damage may have been caused, eg to choke or output transformer windings.
                Even with B+ fusing, the screen grid resistors may burn out in the face of a shorted tube.
                So it may not be an advisable power tube test method for someone that doesn't have the competence / supplies to monitor the amp and replace anything that blows.
                Pete
                My band:- http://www.youtube.com/user/RedwingBand

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                • #9
                  Emission decline was the most common reason for degraded tubes but next came intra-element shorts. With newer power tubes, melted screen grids and terrible microphonics are common reasons to pull tubes, they are dying well before they can see the natural decline in emission. The old tubes seemed to hold together mechanically much better than new production tubes, with the possible exception of Wing C, or in the rest of the world "Svetlana Electron Device" which are made like the old days.

                  Power tubes wear out, they consume themselves in the process of working. Preamp tubes, if made well, can last decades but the microphonic problem with power tubes is common in preamp tubes as well. Power tubes always had a service life expectancy. For higher power tubes the commercial standard warranty was pro rated to 5,000 hours of operation. That is 5,000 hours of continuous maximum rated output CCS(Continuous Commercial Service) so might get 10,000 hours with a 50% duty cycle.

                  Transmitter tubes are still rated that way, so in light duty cycle ICAS (Intermittent Commercial and Amateur Service) they can last decades also. Guitar amps normally operating a low duty cycle way but are not in ICAS, they are used in modes far exceeding their design ratings, but for fairly short periods of minutes between cooling periods. We have tubes now which probably could last a long time if circuits were more conservative but they just are not built to last with abuse.

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                  • #10
                    Pete is right, "stick them in the amp and see what happens" is not for everyone.

                    I lied about the HT fuse, I use my Ninja Toaster amp as the tube testing guinea pig, since it has a regulated B+ supply with a current limit, and a meter to read cathode current.
                    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                    • #11
                      The mechanical issues as described above, as per heavily used tubes as well as new shoddy construction tubes, sounds like the greatest threat. Could not these mechanical issues be detected with some sort of low voltage capacitance measurement while agitating? I also like the idea of setting up an amp as a stress test bed. Thanks

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                      • #12
                        The changes in capacitance you're looking for would be so small that it would be a real challenge to separate them from the other changes in capacitance caused by holding the tube in your hand and shaking it around, while connected by wires to a capacitance meter that isn't moving. Even if you used screened cable, that has triboelectric effect.

                        You could build a tiny capacitance meter that attached to the tube, but I think it would be easier to use the "element short" function on a regular tube tester while flicking the tube with a finger. There is a schematic for a simple short tester using 6 neon lamps in the RDH4.

                        Or just shake it near your ear and listen for really bad rattles.
                        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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                        • #13
                          Sure you can "measure" the capacitance change. Condenser microphones do it very nicely everyday in every studio.
                          A microphonic test jig I am familiar with due to repairs and modifications to it is used by Ruby Tubes to sort 12AX's by mechanical noise. It uses a small geared motor to turn a cam that trips a row of hanging miniature bronze hammers that swing over a arc of about 2 inches with each trip, one every few seconds. A high gain amp is used to sequentially monitor each tube's output and a digital meter to record the peak. Ringing is more important than the initial hit so low freq is filtered out. It a tube meets gain and has low ringing in that test, it is going to be a great tube for V1. Chinese tubes do best.
                          Something similar could be used on power tubes.
                          Initial tests however, will only pick up the tubes with loose elements from new. Using modern tubes in combos is like a continuous shaker table abuse. There are ways to reduce combo tube destruction by isolating sockets with rubber bushings, using suspension systems for sockets and reducing direct sound wave contact with tubes.
                          Head tubes last longer if a 1/3 inch Insolite pad is used to cushion it from the cab.

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                          • #14
                            Originally posted by Steve Conner View Post
                            Tubes with low emission might well sound "muddy and muffled" when overdriven. The clipping due to running out of emission is softer and more rounded than the usual clipping due to grid current.
                            Or perhaps there's a difference between steady-state emission tests in a tube tester and the tube's ability to deliver transient peaks in the music, creating a compression effect. I'm mainly talking about Hi-Fi here. One red flag in my tester when testing output tubes is when the meter hits an initial peak and then falls off.

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                            • #15
                              Enlightening discussion. Thanks. I will make a note re the Winged C tubes as I have been hesitant to buy many new tubes. Have been offering pulled USA tubes to the few tube amp repair customers I have.

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