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Recommend a minimalist oscilloscope specifically for guitar amp testing?

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  • Recommend a minimalist oscilloscope specifically for guitar amp testing?

    Well, it looks like it's time for me to buy an oscilloscope. But I'm working under a few constraints. In order of most important to less:

    1) size. Pocket or USB oscilloscopes highly preferred for portability, so that I can go over to friends' homes to fix their tube amps. Those huge old CRT scopes are definitely out of the question, no matter how well built they are.
    2) budget. Would prefer under $100, but up to $200 is okay. Used is fine.
    3) high reliability and parts availability. "Buy right, buy once."
    4) easy to use

    I've seen the pocket oscilloscopes, but they are questionable in terms of reliability. I'm totally willing to give up all but the most essential features/specs in order to keep the size and cost down. Basically, the bare minimum to do amp repairs in the field, but built well enough to last a lifetime.

    Worst case scenario, I'll just get one of the pocket scopes, just cause the size is right. Or the DIY pocket oscilloscope, because the price is right.

    Oh, BTW, I have no idea of what specs/features are considered minimally functional for repairing tube amps. I understand that many have limits on the input voltage that rule them out. But as I don't know much about oscilloscopes beyond that, I have no idea what I'd need. Please school me on what to get.

    Thanks, guys!
    Last edited by dchang0; 12-12-2010, 06:34 AM.

  • #2
    When I last looked into this Pico oscilloscope range - compare oscilloscope specifications and prices , the problem was that the max voltage they'd accept was about 50v, which means messing around with attenuating probes in order to deal with tube amp signal voltages. As far as the rest of it goes, anything will do, even 1 channel at 1MHz would cover 99% of troubleshooting.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      A couple of X100 probes will fix that. Multi-Contact make some affordable ones, I have two of them. (You'll want a couple of X10s too, though.)

      Using probes with a scope is the norm. You don't just stick a wire from the circuit into the scope's input socket. That puts a heavy capacitive load on the circuit that can completely change the very signal you're trying to scope, especially in the case of tube amps, with their fragile high-impedance signals.

      Now, the bad news, the OP's dream oscilloscope doesn't exist. I can think of two alternatives:

      1- One of the low-end CRT scopes, the 20MHz kind used by old-school TV repair guys and college electronics labs. They run off wall power, but are fairly small and portable, and you should be able to get one on Ebay, Craigslist etc. for under $100. I've seen (and owned) a couple of really small, battery-powered ones, but those can be more expensive because of the cute factor.

      2- Seeed Studio are a Chinese company who specialize in remarkably cheap pocket DSOs. I've never tried their products, though. Latest happening of DSO Nano and upcoming QUAD! | Seeed Studio

      3- Rigol make decent digital scopes that can be got from Dealextreme etc. pretty cheaply. I know a few hobbyists who have Rigols and are happy with them.

      I don't much like PC-based scopes or low-end DSOs. If I had a couple of hundred bucks to spend on a scope for tube amp troubleshooting, I'd buy an old Tek 465 or 2465. Except I already have three scopes: a Tek R7603, a Tek 222A, and a Pico ADC212/100 PC-based one. OK, four if you count the Gameboy with GBDSO cartridge.
      "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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      • #4
        LOL--"dream oscilloscope?" Are those four criteria that pie-in-the-sky? I had no idea! Figured there'd be something like a low-end Fluke multimeter: clunky but still "handheld," moderately priced, sturdy, and just enough to get the job done. An "everyman's oscilloscope" that most people would toss in their toolbox and use maybe once a year.

        Thanks, guys--I'll check these options out. Thanks for being so specific too, about model numbers, etc. Love the GBDSO approach--it's certainly the right size, and the Gameboy itself is sturdy.

        Wow, I remember using those old Tektronix scopes back in college. Waaaaay too big--the Rigol ones look to be more reasonably sized.

        This is about the right size and price. Would this cut it as a tube amp repair scope?

        Velleman Inc.

        Looks sturdier than the DSO nano ones...

        If the Velleman will cut it, this looks like a lot more fun to build:

        http://www.sparkfun.com/products/9484

        I might do this one just because I love building stuff. Too bad they don't sell a molded plastic body for it to protect the exposed components.
        Last edited by dchang0; 12-12-2010, 03:50 PM.

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        • #5
          Whoa, turns out there are a couple iPhone oscilloscope apps. Since I have an iPhone sitting around doing nothing, and some credit at the iTunes Store, this is perfect!

          oscilloscopeapp.com

          And a German company just released a probe specifically designed for the iPhone for around 30 euros...

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          • #6
            Well, the GBDSO is a cool geek toy, but I'm not sure of its value as a diagnostic tool.

            Fluke actually do make handheld oscilloscopes that you can toss in your toolbox. But you really don't want to know the price, they're about a factor of 10 outside your budget. One of the older ones, you might be able to pick up used, and they're probably plenty good enough for audio work. They're called "Scopemeters".

            The Velleman is probably as good as you have a right to expect for a pocket scope that costs under 200 bucks. Velleman are a long-standing European maker of DIY kits, and that thing is an updated version of a handheld scope that they've made for years. It's probably a lot better than the GBDSO.

            I'd avoid the Sparkfun kit, the reviews on that site you linked include words like "bricked" and "piece of crap".

            Edit: No, if you're at all serious about scoping stuff, don't use the iPhone app. It'll use the phone's audio ADC as the input device, and I shouldn't have to tell you that that is useless for serious work.
            "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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            • #7
              Good deal. Thanks, Steve, for catching those pitfalls. The Velleman looks good, then. I wonder if they offer it in kit form--would be cool to put together...

              Edit:

              Wow, check this little bugger. Just released by Velleman. PERFECT SIZE and looks pretty rugged. I have no idea about the price, though. Probably $400 to $500.

              http://www.vellemanusa.com/us/enu/pr...iew/?id=525377
              Last edited by dchang0; 12-12-2010, 05:08 PM.

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              • #8
                For Europe: Suggested retail price HPS140: € 139,- / HPS140i: € 149,-

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                • #9
                  Now I'm no amp repair expert, but I've piddled in around in them using a scope and sig gen.

                  I'm questioning the intent to "go over to friend's houses to fix them". If it's non-functional to the point where a scope is necessary, IMHO it needs to be on your own bench at home, not on your friend's kitchen table. There's enough current and high voltage in there to KILL somebody who is careless, not to mention the hassles of fixing stuff in the field. Toting a soldering iron, sig gen, scope, and tools around is a major PITA. And I can foresee the situation: you're digging around in an unfamiliar amp, with your friend looking over your shoulder, feeling a little bit of pressure, which is the perfect scenario for inadvertently screwing something up.

                  I'm not intending to be a know-it-all and I hate to pee in your Cheerios, so to speak, but I'm trying to bring an element of practicality into this.

                  So my advice, for what it's worth: don't worry about getting a super-small portable unit if money is a factor--just keep your eyes out for a common 2 channel scope in the 10 to 20 Mhz range and set it up at home. Then if you want to try your hand at fixing your friend's amps, you can do so on your own turf, where the workplace is familiar (and without your friend watching you).

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                  • #10
                    Thanks, Albert, for finding the price. It's certainly within my budget.

                    Bill: I totally get your reasoning. Makes sense to me, and I would certainly prefer to work on these amps at my own home and at my own leisure.

                    Actually, it's funny that you mention these things now, because I am considering this purchase for a very specific amp repair. There is a friend who has a giant Plush amp that is too large to move. It is certainly too large to fit in my home--I wouldn't want them to bring it over. And at 400 watts, it is far too loud--my neighbors already complain about my little 15 watter. So it has to stay over there, at his storage facility.

                    The amp is running but has a weird oscillation near full volume. Seems to me that an oscilloscope would be a requirement for diagnosing that thing. So here we are, looking for a portable oscilloscope. I figure I can buy one using the proceeds from this repair and then keep it for future repairs. Considering that most of my amp "repairs" involves identifying and swapping out bad tubes and quickie biasing, I don't really foresee using the scope more than maybe once a year, so why take up precious workbench space with a huge clunker scope? And of course, why spend a lot of money?

                    Thanks for your concern for my safety. I'm not entirely crazy--when I do repairs, I actually wear Nomex fire retardant gloves to protect me from shocks and fires. Usually, they allow me to hold wires steady with my fingers without getting burnt by the soldering iron. A bit more convenient than holding wires with forceps or pliers. I've been shocked by 400VDC before--not fun, so safety is a habit with me.
                    Last edited by dchang0; 12-12-2010, 10:56 PM.

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                    • #11
                      A friend let me borrow his O-scope for a while, because I have a Sunn Beta lead that needs repair, so rather than just changing parts, I can maybe do a bit of proper diagnosing. This prompted me to look into oscilloscopes, and Fluke does make a battery powered hand held one model STL-120, that sells for just over $100. Looked it up on McMaster-Carr. Might have to look into that one for myself.

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                      • #12
                        Fluke STL-120 is just for the test leads, not the scope itself. $114 for a pair of leads!

                        But hey, Fluke has proven that they are actually "cheaper" than the competition in the long run, in that you only need to buy a Fluke once and can use it for a lifetime.

                        Thanks to everybody for their help. I ended up buying a Velleman PPS10 used offa ebay for just under $60. Good enough until the need arises to get a real desktop scope, in which case I'll probably get a Tek 465 just cause I have training on them.

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                        • #13
                          Originally posted by dchang0 View Post
                          Fluke STL-120 is just for the test leads, not the scope itself. $114 for a pair of leads!

                          But hey, Fluke has proven that they are actually "cheaper" than the competition in the long run, in that you only need to buy a Fluke once and can use it for a lifetime.

                          Thanks to everybody for their help. I ended up buying a Velleman PPS10 used offa ebay for just under $60. Good enough until the need arises to get a real desktop scope, in which case I'll probably get a Tek 465 just cause I have training on them.
                          I jsut went back and read that... hah! I shouldn't read so fast. And I though 38 bucks for an O-scope test lead was a bit much. That meter is probably ten times that, at least.

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                          • #14
                            Yeah, the tough part of Fluke stuff is that they focus on professional product lines and only occasionally extend products down into the "prosumer" price range. Unfortunately, their oscilloscope is never likely to be released in prosumer form.

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                            • #15
                              Hi all...
                              I just got an e-mail from Makershed.com for a hand held O scope.

                              DSO Nano V2.0 - Pocket Sized Digital Storage Oscilloscope

                              If I am understanding some of the previous discussion, this one may not handle the voltage of a tube amp either. Seems cool though.

                              Can you guys give your thoughts?

                              Thanks in advance.
                              Mandopicker

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