Harmony Central Forums
Announcement
Collapse
No announcement yet.

Vacuum Tubes

Collapse



X
  • Time
  • Show
Clear All
new posts

  • Vacuum Tubes

    What causes the actual wear and tear on vacuum tubes.



    2 scenarios -



    a) Amp turned on, volume cranked no guitar plugged in



    b) Guitar signal passing through the amp.



    c) a bit of each

  • #2
    Cathode boiling off electrons the minute you turn on the tube. As soon as they're born, they start dying...kind of like people
    Simplicity, my new album project, is now streamable from my YouTube channel.

    Subscribe, like, and share the links!

    Comment


    • #3
      Generally tube filaments fail before low emission from the cathode material boiling off becomes a problem. Back in the day of black-and-white TV sets, you could buy a picture tube booster which was a transformer that you'd plug between the CRT and its socket. This raised the filament voltage so the cathode got hotter and more electrons boiled off when needed. It was a short term solution.



      What causes tubes to fail, assuming that they're operated within their ratings and the glass hasn't cracked, is the mechanical stress on the filament when current is first applied. The resistance of the wire is lower when cold than hot, so the filament draws more current for the first few seconds after power is switched on. This puts a little extra stress on the wire.



      Some tubes last longer if the filaments are left on all the time so they aren't subject to the stress of the higher "cold" current. Others don't. There are devices which will limit the current so the filaments have a "soft" turn-on, but they're not commonly used in instrument amplifiers. Whether the tube is passing a signal or not makes little difference in how soon they fail.



      Basically, as Craig said, they're like people. They die when it's time and there's only so much you can do to delay that.
      --
      "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
      Drop by http://mikeriversaudio.wordpress.com now and then

      Comment


      • #4
        My son has worn through tubes in 6 hours of full-tilt recording volume. I OTOH can go years it seems. There's something to be said for lower volumes and letting pedals do the work.

        Comment


        • #5
          Back in the days of vacuum tube computers, NORAD used to have two identical computers to handle incoming data, one always in service and the other being maintained or on standby, alternating daily.



          To avoid tube failures during operation, during the daily maintenance cycle, they raised the (plate?) voltage and ran for say an hour. Any tubes that were at the edge would blow and they'd replace them. This avoided losing tubes during normal activity. I don't remember how much they raised the voltage, but it was a number that made the tubes about a thousand times more likely to blow. Probably not what you'd want to do with your vintage gear. ;-)



          I don't know whether they also switched all the tubes on, so this doesn't really address the question.









          Quote Originally Posted by blue2blue


          What causes tubes to fail, assuming that they're operated within their ratings and the glass hasn't cracked, is the mechanical stress on the filament when current is first applied. The resistance of the wire is lower when cold than hot, so the filament draws more current for the first few seconds after power is switched on. This puts a little extra stress on the wire.




          An EE I respected (co-worker at Ford Scientific Research Lab, decades ago) did the math for this for a light bulb and dismissed it as a factor, because the filament heats up so fast that the extra current draw is insignificant. However, the question there was whether it wasted electricity (answer: no) and it was for lightbulbs, not tubes. Tubes definitely take longer to heat up. Still, I doubt the extra current draw is the culprit. The heat stress is a bigger issue than current draw: most problems with drawing too much current either attack other components, or are caused by heat buildup. Still, I do believe it's quite likely that some tubes last longer if left on all the time. I just don't think the extra current draw is the reason. My guess (admittedly just a guess) is it's more likely to be physical stress due to thermal expansion.
          learjeff.net

          Comment


          • #6
            In 2005, I replaced the 1961 tubes in my Leslie. The 1961 tubes still worked.

            Comment


            • #7






              Quote Originally Posted by cereal
              View Post

              My son has worn through tubes in 6 hours of full-tilt recording volume. I OTOH can go years it seems. There's something to be said for lower volumes and letting pedals do the work.




              I suppose it's possible to drive them too hard and the plates melt. The amplifier just needs more cooling. In my ham radio days, people were making 100 watt transmitters with a single 6L6 (usually good for about 35 watts Class C) as the final amplifier by mounting it upside down and putting it in a bucket of oil for cooling. The plate ran a dark red but the tube didn't burn out as quickly as if it was in air. I wouldn't recommend that for a stage amplifier, though.



              When I was in college, the engineering school inherited FLAC-II, a computer that used a couple of thousand 12AU7s. Every year a couple of grad students would work on it, replacing enough tubes to get it to run a program, then at the end of the semester, turn it off, assuring a fresh crop of blown tubes for the next crop of grad students.
              --
              "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
              Drop by http://mikeriversaudio.wordpress.com now and then

              Comment


              • #8






                Quote Originally Posted by learjeff
                View Post

                Back in the days of vacuum tube computers, NORAD used to have two identical computers to handle incoming data, one always in service and the other being maintained or on standby, alternating daily.



                To avoid tube failures during operation, during the daily maintenance cycle, they raised the (plate?) voltage and ran for say an hour. Any tubes that were at the edge would blow and they'd replace them. This avoided losing tubes during normal activity.




                That's usually called a margin test. I've heard of doing it by lowering the filament voltage to decrease the cathode emission, but not by raising the plate voltage. That would probably cause all sorts of havoc since the bias would be off its design value.



                The DEC PDP-9 computers that we had on the Naval Oceanographic Office research ships I worked with back in the mid 1960s had a Margin Check switch on the power supply to both raise and lower the logic supply rail voltage by a set amount so as to aggravate marginal components when running diagnostics. That test always passed, but the computers went down fairly regularly anyway.
                --
                "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
                Drop by http://mikeriversaudio.wordpress.com now and then

                Comment


                • #9
                  The original tubes in my '63 Vibrolux Reverb amp lasted about 32 years. At that time (around 1995) they were still working, but I had torn it apart to make some mods (increased the B+ by about 75 volts by changing to a solid state rectifier, put in a heavier output transformer to handle the increased power, and changed out all the electrolytic caps) so I thought it was time for new tubes as well. I put in Russian 12AX7 & 12AT7's, and bought a matched pair of 6L6 (GC? GLC? I don't recall... but they were the old style coke-bottle shaped ones.) power tubes from Ruby tubes, and set the bias with an oscilloscope for Class B to get the maximum power & efficiency.



                  Anyway, to the point: I burned it in for about a month into 100W 8 Ohm power resistor running a 400 Hz sine tone, with the output power at 50W output (it was formerly a 35W amp). The tubes held up fine, and are still in the amp today. I don't believe tubes are all that unreliable, with a few caveats:

                  - If the power tubes are not biased properly, especially if they are biased on too much, the lifetime will definitely shorten. (I've seen amps where the bias was set so high the plates glowed dull orange from the electron bombardment - - not good!)

                  - Do NOT pump signal through while the tubes are warming up; this is known to damage the cathode surface.

                  - Use care when moving them. Banging them around is just as bad for filaments in tubes as it is in lightbulbs.

                  Comment


                  • #10






                    Quote Originally Posted by MikeRivers
                    View Post

                    That's usually called a margin test. I've heard of doing it by lowering the filament voltage to decrease the cathode emission, but not by raising the plate voltage. That would probably cause all sorts of havoc since the bias would be off its design value.



                    The DEC PDP-9 computers that we had on the Naval Oceanographic Office research ships I worked with back in the mid 1960s had a Margin Check switch on the power supply to both raise and lower the logic supply rail voltage by a set amount so as to aggravate marginal components when running diagnostics. That test always passed, but the computers went down fairly regularly anyway.




                    PDP-9 had tubes? I find that surprising. I spent many hours on a LINC-8, even had it open, and helped to find a design flaw that clobbered one memory word each time it switched from LINC mode to PDP8 mode. I bet that drove some poor experimenters nuts. Even PDP-5 had solid state logic cards; I remember seeing them. Discrete logic (!)



                    In any case, I'm probably misremembering about plate voltage. I do know they ran them overvoltaged, but not necessarily plate voltage.



                    Oddly enough, though, tubes can work remarkably well with plate voltages too high. Years ago I got my son a used Music Man HD130, to use as a bass amp. It worked fine until one day it blew a tube. I replaced with a matched set of Ruby Tubes. It worked, but it would sputter, and IIRC there were other occasional issues -- enough so that I took it to the local amp guru (I figured it needed biasing for the new tubes anyway). He said it had power tubes intended for 450V, but the plate voltage was much higher. It surprised me they worked as well and as long as they did! It had 4 EL34's in it; not sure what they got replaced with but 6L6 or 6CA7 comes to mind. Tubes are confusing.
                    learjeff.net

                    Comment


                    • #11






                      Quote Originally Posted by learjeff
                      View Post

                      PDP-9 had tubes? I find that surprising. I spent many hours on a LINC-8, even had it open, and helped to find a design flaw that clobbered one memory word each time it switched from LINC mode to PDP8 mode. I bet that drove some poor experimenters nuts. Even PDP-5 had solid state logic cards; I remember seeing them. Discrete logic (!)



                      In any case, I'm probably misremembering about plate voltage. I do know they ran them overvoltaged, but not necessarily plate voltage.



                      Oddly enough, though, tubes can work remarkably well with plate voltages too high. Years ago I got my son a used Music Man HD130, to use as a bass amp. It worked fine until one day it blew a tube. I replaced with a matched set of Ruby Tubes. It worked, but it would sputter, and IIRC there were other occasional issues -- enough so that I took it to the local amp guru (I figured it needed biasing for the new tubes anyway). He said it had power tubes intended for 450V, but the plate voltage was much higher. It surprised me they worked as well and as long as they did! It had 4 EL34's in it; not sure what they got replaced with but 6L6 or 6CA7 comes to mind. Tubes are confusing.




                      The plate voltage itself is not that big of factor (unless it's high enough to cause arcing, or the plate current is enough to cause secondary emission).



                      The output level and power dissipated by the tube depends on the plate current and the value (impedance) of the plate load. The plate current is controlled by how much bias voltage is on the grid.



                      Anyway... enough technologizing for now...

                      Comment


                      • #12






                        Quote Originally Posted by learjeff
                        View Post

                        PDP-9 had tubes? I find that surprising.




                        No, the PDP-9 was solid state, with real transistors. But it still had the margin test.








                        Oddly enough, though, tubes can work remarkably well with plate voltages too high. Years ago I got my son a used Music Man HD130, to use as a bass amp. . . . .I took it to the local amp guru (I figured it needed biasing for the new tubes anyway). He said it had power tubes intended for 450V, but the plate voltage was much higher.



                        Odd that the plate voltage was much higher than 450 V. I wonder if he measured it with half of the tubes out. These things don't have regulated power supplies and depend on everything drawing about the right amount of current to keep the voltage where they want it.



                        Odder, though, is that tubes work with much lower voltages than where they're normally run. There was a rash of tube mic preamps that used a starved plate design, running a 12AU7 with 40V or so on the plate for that phat toob distortion, with the actual gain provided by a solid state amplifier. And I had a 1957 Ford with a radio that had tubes which ran with 12V on the plates - but they were designed that way. I wonder if any of those are still around.
                        --
                        "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
                        Drop by http://mikeriversaudio.wordpress.com now and then

                        Comment


                        • #13






                          Quote Originally Posted by philbo
                          View Post

                          (I've seen amps where the bias was set so high the plates glowed dull orange from the electron bombardment - - not good!)




                          I thought that was normal. When I was a kid I remember looking though the ventilation holes on the old TV and you could see it light up inside after it warmed up.



                          Dan
                          http://musicinit.com/fastfingers.php An Experiment in 80's Technology

                          http://youtube.com/techristian My YOUTUBE channel
                          Music videos at http://musicinit.com/video.php

                          Comment


                          • #14
                            Filaments glow red when they warm up. Plates shouldn't.



                            For a better understanding of the parts of a vacuum tube, make your own tubes on your kitchen table using only a Swiss Army knife, spot welder, glassblowing torch, and vacuum pump.
                            --
                            "Today's production equipment is IT-based and cannot be operated without a passing knowledge of computing, although it seems that it can be operated without a passing knowledge of audio." - John Watkinson, Resolution Magazine, October 2006
                            Drop by http://mikeriversaudio.wordpress.com now and then

                            Comment


                            • #15
                              Back to amps for a minute. What is the difference as far as driving the tubes between boosting volume in these 2 scenarios:



                              1) Crank up the amp itself (in my case old fender champ no master volume)

                              2) Run a boost into the amp, which by all accounts gives me even more push

                              Comment













                              Working...
                              X