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  • #16
    Originally posted by Nate42
    Well, looks like I'm going to be heading out for the rest of the evening. Part 5: Stereo Power Amps will happen eventually though, promise.

    Glad you guys appreciate this, and hopefully I won't screw up any more.
    I'll be watching!
    "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
    -- Bob Parks

    "A thing is not necessarily true because a man dies for it."
    -- Oscar Wilde

    "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
    -- Oscar Wilde

    "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
    -- Theodore White

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    • #17
      Originally posted by Nate42
      Part 4: Tube Amps

      cab, their impedance combines in parallel as described in part 3.

      A note on running a tube amp with no cab: Don't do it. Edit: My original explanation for this was incorrect (thanks Isaac42). In fact the idea that you can't run a tube amp with no cab may just be folklore. But to be safe I still recommend you don't do it.


      Safer to say NEVER do it. Ever heard of flyback? Arced tube sockets? I'm confused by the edit, but this is a pretty simple rule, backed up by Ohm's law. When you have an infinite impedence, at the secondary you get a voltage spike at the primary. Sorry if I'm missing something here, but I thought this was Tube Amps 101.

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      • #18
        Originally posted by bassmantele


        Safer to say NEVER do it. Ever heard of flyback? Arced tube sockets? I'm confused by the edit, but this is a pretty simple rule, backed up by Ohm's law. When you have an infinite impedence, at the secondary you get a voltage spike at the primary. Sorry if I'm missing something here, but I thought this was Tube Amps 101.
        Why would you get a voltage spike? I can understand if you were playing and got unplugged (you'd get inductive kick), but that's not the question. Some say it's dangerous to power up a tube amp with no load. Is it really, and, if so, why?
        "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
        -- Bob Parks

        "A thing is not necessarily true because a man dies for it."
        -- Oscar Wilde

        "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
        -- Oscar Wilde

        "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
        -- Theodore White

        Comment


        • #19
          Originally posted by isaac42
          Why would you get a voltage spike? I can understand if you were playing and got unplugged (you'd get inductive kick), but that's not the question. Some say it's dangerous to power up a tube amp with no load. Is it really, and, if so, why?
          output transformers are the beefiest of the three(usually there are three transformers in a tube amp) transformers in a tube amp. they generate plenty of power and when there's nowhere for it to go, it heats up the transformer quite a bit. eventually, which is not THAT far off, this leads to failure of the transformer. if it's not too badly damaged, it can be rewound. if the core is shot too, the whole transformer is a write-off. Output transformers are in general an expensive part to replace. a replacement for an SVT can be $300 or more.

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          • #20
            Because if you don't do it, regardless of the cause, your Xfmr won't turn into a Ronco Toast-o-Matic.
            gp



            "There's only two things in life, but I forget what they are..."
            - - John Hiatt, (1952 - )

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            • #21
              Originally posted by isaac42
              Why would you get a voltage spike? I can understand if you were playing and got unplugged (you'd get inductive kick), but that's not the question. Some say it's dangerous to power up a tube amp with no load. Is it really, and, if so, why?



              Because E = IR


              If you have no speaker connecting the OT secondaries then R goes to infinity. Which means that E - voltage - tries to go there as well. The spike often causes arcing between tube socket pins or within the tube itself. At worst, the insulation in the OT can burn through and short out.


              Do a search for the alt.guitar.amps FAQ, or just ask your local tech.

              Comment


              • #22
                bump. There really is another part coming, promise. I've been busy with Christmas stuff and band stuff.

                I see there's been some discussion on the tube amp thing since I last posted. I still haven't heard an explanation for why not to run a tube amp with no load that I like, and I'm not comfortable changing my original post until I do. If any of you tube experts can give me a good explanation feel free to PM me and I'll change it and credit you in the original post. I'd rather keep that sort of discussion out of this thread if possible, I'm trying to keep this thread newby friendly.
                Girl, I wanna take you to a gay bar

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                • #23
                  I think this is the clearest understanding of this stuff I've ever gotten. Thanks Nate!

                  This definately belongs in the FAQ forum if it hasn't been added already.

                  One question...

                  You said on a tube amp, often they will have two outputs. One for 4 ohms and one for 8 ohms. Could you have a cab plugged into each outlet and be OK or do you need to stick with 4 or 8?
                  LOW TONE

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                  • #24
                    Originally posted by Low Tone

                    You said on a tube amp, often they will have two outputs. One for 4 ohms and one for 8 ohms. Could you have a cab plugged into each outlet and be OK or do you need to stick with 4 or 8?


                    I wouldn't go doing that unless the manufacturer says its ok.

                    If you plug one into a 4 ohm output and the other into an 8 ohm cabinet, those cabs are still (sort of) in parallel. They're not strictly in parallel because of the presence of the output transformer, but the effective impedance seen by the output tubes is definately going to go down. I wouldn't feel comfortable doing that without an ok from the manufacturer.

                    If the amp happened to have a 2 ohm output, I would hook them up to that. 2.66ohms is close enough to 2 that you would likely be okay.
                    Girl, I wanna take you to a gay bar

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                    • #25
                      Originally posted by Nate42


                      I wouldn't go doing that unless the manufacturer says its ok.

                      If you plug one into a 4 ohm output and the other into an 8 ohm cabinet, those cabs are still (sort of) in parallel. They're not strictly in parallel because of the presence of the output transformer, but the effective impedance seen by the output tubes is definately going to go down. I wouldn't feel comfortable doing that without an ok from the manufacturer.

                      If the amp happened to have a 2 ohm output, I would hook them up to that. 2.66ohms is close enough to 2 that you would likely be okay.


                      Thanks Nate.
                      That was more of a curiosity thing, but good to know just in case.
                      LOW TONE

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                      • #26
                        For whatever it's worth, if you have no load and no input on a tube amp, no damage will be done. Having no load and a given amount of input will lead to the transformer heating, possibly to the point of insulation damage and eventual failure. But this is something that you quite literally have to intentionally do with a bass amp, by playing into it with no speakers. It's not a case of "ZAP" instantaneously. It was a lot more common to have a transformer failure with an amp used for something like a stereo, where you could more easily have it on and driven with no load, and not be aware of this.
                        "If you don't know where you are going, you might wind up someplace else" - Yogi Berra, 1925-2015

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                        • #27
                          Part 5: Cabs and their power ratings

                          As mentioned in the newly edited Part 1, cabs will be rated for the amount of power they can handle. The basic rule here is pretty simple: if you don't want to blow you speakers, don't give them more power than they can handle for an extended period. This doesn't mean you need to match your amps power with the cabs rating exactly though. A cab that can handle more power than you've got is never a problem, and having one that can handle a little less than what your amp puts out is no big deal either.

                          In fact, with a solid state amp, I actually contend that with an ideal setup your amp would actually have more power than your speakers can handle. This is because you presumably want to be able to get the most out of your speakers, and also speakers usually sound their best at somewhere in the vicinity of their rated power. As you play, there will be occasional peaks in your signal that are higher than your usual operating levels. These peaks won't last long enough to harm your speakers, but your amp needs to have enough power to be able to reproduce them properly. This concept is often referred to as 'head room', basically you want to give your amp a little breathing room between what it can do and where you typically run it. This of course requires you be careful with the volume knob. You can run 100W speakers with a 1000W amp all day with no difficulty, but if you crank the volume all the way up, don't be surprised when the speakers go pop.

                          Please understand I'm not saying you HAVE to have more power than you speakers can handle. You won't hurt anything by having cabs with a very high power rating.

                          With a tube amp, you typically want the cab to be able to handle more power than the amp is rated for. This is because people like to overdrive their tube amps (that's pretty much the point) and an overdriven amp by definition puts out more power than its clean rating. Also, you don't really need to concern yourself with headroom so much with tube amps, because tube amps clip in a pleasant manner, unlike solid state amps which sound like **************** when pushed beyond their limits.

                          A note on running more than one cab: If you have more than one cab, they don't each have to be able to handle your amps full power, they split the available power. If they are of equal impedance, they split it equally. So a 500W amp with two 8 ohm cabs gives each cab 250W. If the cabs are different impedance its a little more complicated (the lower impedance cab gets more power) but still no one cab has to handle the full power of the amp.

                          Regardless of what kind of amp you have and your power ratings, always follow the golden rule: If your speakers are distorting, turn your **************** down.
                          Girl, I wanna take you to a gay bar

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                          • #28
                            Originally posted by bikehorn
                            output transformers are the beefiest of the three(usually there are three transformers in a tube amp) transformers in a tube amp. they generate plenty of power and when there's nowhere for it to go, it heats up the transformer quite a bit. eventually, which is not THAT far off, this leads to failure of the transformer. if it's not too badly damaged, it can be rewound. if the core is shot too, the whole transformer is a write-off. Output transformers are in general an expensive part to replace. a replacement for an SVT can be $300 or more.
                            First, none of my tube amps has three transformers. Next, infinite resistance is not possible. The primary windings have a DC resistance, and eddy currents in the transformer will prevent the primary seeing an infinite reflectdd impedance. The power suply cannot magically develop a voltage higher than the supply is designed for. The Power transformer produces a specific output based on the input and the turns ratio. That is rectified and filtered by the power supply capacitors. Volage cannot, will not exceed this maximum supply voltage.
                            "The Web puts all of the world's knowledge at our fingertips; unfortunately it's mixed with all of the world's bull****************."
                            -- Bob Parks

                            "A thing is not necessarily true because a man dies for it."
                            -- Oscar Wilde

                            "No man dies for what he knows to be true. Men die for what they want to be true, for what some terror in their hearts tells them is not true."
                            -- Oscar Wilde

                            "It is a trap of history to believe that eyewitnesses remember accurately what they have lived through."
                            -- Theodore White

                            Comment


                            • #29
                              Originally posted by Nate42
                              Part 5: Cabs and their power ratings



                              I thought part 5 was going to be Stereo Power Amps

                              -b
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                              • #30
                                It was. Now there's gonna be a part 6.
                                Girl, I wanna take you to a gay bar

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