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  • Noob 10x scope probe question

    Please tell me if I have the correct 10x probe I ought to use with my Tenma 72-320 oscilloscope for use on tube guitar amps.

    The one I have is a Test Products Int'l P100B 10x rated to 300V Category II. That doesn't seem high enough!

    Shouldn't I have it rated as high as my coupling caps?

    In the directions it says its "working voltage" is 600V Category I (a graph shows a line @ 600V to about 25 KHz, then it slopes down to 300V @ 100KHz). It also says that it is rated to 300V Category II, (DC + peak AC). I don't know what that means yet.

    I remember somewhere here that I only am dealing with frequencies up to 20KHz on tube amps.

    If that's true, then I think I can use the probe I already have.

    Will someone please confirm this or recommend a different 10x probe I should use?

  • #2
    If you really need to know about the probe, why not ask the manufacturer?

    Technical Support | TPI USA

    Cat I & Cat II: What are Measurement Categories (CAT I, CAT II, etc...)? - National Instruments

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    • #3
      I'm glaze-eyed from the Tektronix Probe Fundamentals pdf already, so that's not going to happen any time soon, thanks.

      All they care about is which of their scopes a probe is compatible with. Pooh!

      CAT II: used on single phase mains final sub-circuits. So that means amps… I better find a higher rated 10x probe for less than the $500 Tektronix P5102 somewhere.

      Yay! Pomona's 72940-8 passive oscilloscope probe is CAT II 600V!! It's $175!!

      Enzo:
      I have always gotten just fine performance in my audio work with the basic cheap $30 probes you can get anywhere.
      Then again, km6xz prefers ones that cost at least $150… I think I'm going to get the Pomona one.
      Last edited by deci belle; 03-28-2016, 03:49 AM. Reason: update my research

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      • #4
        Tektronix is high end laboratory gear, so don't read the Tek probe documents, try the simpler one from TPI that Jazz linked.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Well I don't need that!! OK, Enzo— and thank you, Jazz!!

          I read the link's short document with an example and a table showing categories I through IV (at least), and am no where near understanding what my applications' requirements are (other than something to do with CAT I or II.

          This is the place to find out. Here. On this forum. In theory anyway…

          It took me two years to be sure I was bleeding caps correctly. I laugh at the way I was faking it the first few times I "bled the filter caps" when I swapped in a choke on a Crate V115 six years ago. The only reason I'm not dead is because of modern electrical design codes!! haha!!

          I assure you that I did not learn how to bleed caps on this forum. I don't learn the same as other people.

          So why is a $30 10x scope probe good enough, Enzo? Examples?
          Last edited by deci belle; 03-28-2016, 02:58 PM.

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          • #6
            Cat II is what we work with.

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            • #7
              Originally posted by Jazz P Bass View Post
              Cat II is what we work with.
              Now you've got my attention, Jazz. Excellent!! And no one's dead yet…

              Shouldn't I have it [the probe] rated as high as my coupling caps?
              in Cat II (whereas my probe says 300V)?

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              • #8
                The rating is obviously too low for tube amp work.

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                • #9
                  Here's the amazing answer I received from the brilliant probe tech/rep~ hahahhaa!!
                  In my research it appears to be a very old scope. I can only provide information on our P5102. The attached manual covers all the specifications.
                  His research!!

                  And nice of you to grudgingly address the OP, Jazz!! I know that sounds sarcastic, but its true! What exactly IS the old-guard hanging onto so tightly? You guys can wax poetic on things that are exquisitely nebulous (which I enjoy knowing is comprehendible by the experienced wizards— and that's the beauty of this site).

                  But Jazz must use a scope probe for guitar-amp work~ so, what might be the specs on that piece of equipment, Jazz?

                  I bet its more expensive and higher-rated than Enzo's $30 piece of junk (how embarrassing)!!

                  —just kidding …I know it's so fun to tease me!

                  So I'll go spend $175 now~ cuz I has NO dignity to lose, but I still have to deal with the fact that I'm not dead yet. Pooh on you, Jazz!!
                  Last edited by deci belle; 03-28-2016, 10:24 PM.

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                  • #10
                    deci belle,

                    A couple of past discussions that covered high voltage concerns regarding scope probes are listed below.
                    http://music-electronics-forum.com/t31492/
                    http://music-electronics-forum.com/t37999/

                    Cheers,
                    Tom

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                    • #11
                      Originally posted by deci belle View Post
                      Please tell me if I have the correct 10x probe I ought to use with my Tenma 72-320 oscilloscope for use on tube guitar amps.

                      The one I have is a Test Products Int'l P100B 10x rated to 300V Category II. That doesn't seem high enough!

                      Shouldn't I have it rated as high as my coupling caps?
                      I mistakenly assumed you were referring to the coupling caps of the scope input, but I realize you probably meant the coupling caps in the amps you are working on.
                      As far as I can tell, the specs for your scope say the max input voltage is 300V (DC + AC peak).

                      No matter what your probe is rated for, you should not exceed this. So the probe you have is probably adequate for the capabilities of your scope. A 10x probe still passes the full DC value on to the scope.

                      The "cat" is a measurement class, and refers to the total amount of energy possible in the class. It is not about voltage. As previously mentioned, units that plug into household AC are CAT II.
                      Pricing is more indicative of the degree of accuracy, an expensive probe may not necessarily be safer or better quality, just more accurate than may be needed outside of the laboratory. And bandwidth is also a factor, we don't need to look at anything over 20khz, so no need for a 100Mhz probe. So a cheaper probe can sometimes be fine for our purposes, although they can also be cheap junk. On the other end are $1000 probes with the same voltage ratings and quality as a $50 probe, just way more accurate.
                      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                      • #12
                        Awesome , G1!!

                        The Pomona probes are back-ordered until JUNE (how can that happen?), so The tech at Tequipment helped me find a Fluke VPS210-G (gray) comparably rated probe that was a little more money (but he knocked the price down to that of the back-ordered Pomona probe). It's a 10x 200MHz CAT II 1kv.

                        Ya, I told him I didn't need the bandwidth, but there aren't any at that rating with the modest requirements I felt I should have. Maybe in 20 years I can be as cavalier as Enzo!!

                        I got some cool little pliers too with my birthday money!!

                        Thanks to everyone who provided the valuable information for beginners like me!!

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                        • #13
                          Happy Birthday.
                          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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