Oscilloscope + Signal Generator

Discussion in 'Shock Brother's DIY Amps' started by jchabalk, Oct 17, 2020 at 2:28 PM.

  1. jchabalk

    jchabalk Tele-Meister

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    I'm thinking about getting a scope and signal generator to help with my learning and troubleshooting. I've been reading a bunch of threads here and on other forums, hitting up youtube etc.

    I'm thinking about getting a Siglent 1202x-e. I wouldn't be opposed to getting something with more channels but everything i'm seeing tells me that this scope will do more than i'll need. The next model up (1104x-e) can be software unlocked to be essentially a 4 channel version of the 1202x-e but that appears to be pretty big overkill for this work. I thought about picking up a used Tek, but my desk space is at a premium and i don't want to roll the dice on equipment i'm not familiar with already.

    I'd like to get a set of high voltage probes (100:1 is what i think i need) to be able to examine the b+ power supply in my amps. I'm don't have a ton of experience here so i'm wondering if anyone could suggest a good set of probes for this use?

    I'm also looking at this signal generator which seems like it will work well for what we do here. I understand i can use my phone, or an ipad or whatever for this, i'd like to get a separate device so i can keep my phone separate. I think i'll need to get an appropriate BNC cable for this and then wire up a simple 1/4" jack to connect to to interface with my amp - and i can use mini-grabbers to attach the signal generation to different gain stages in the amp directly as needed.

    Does this sound like a good setup for working on tube amps? I understand it's probably a bit more than is necessary in a lot of ways but also seems to be a relatively modest digital setup.
     
  2. Strebs

    Strebs Tele-Meister

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    That o-scope will meet your needs and then some. I can't offer any opinions on that signal generator though.
     
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  3. jchabalk

    jchabalk Tele-Meister

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    These 100:1 test leads should let me work with the power supply rail, and they're 200mHz which is big overkill here but they only cost $3 more each than the 100mHz ones (which are also overkill).

    And this simple BNC -> MiniGrabber lead should work well for the signal generator (both to a 1/4" plug and inside the amp when needed. There's a cable set which this lead and one with alligator clips too for a couple of dollars more that i might get instead.

    Any other signal generators that would be good to look at? There seem to be a lot of similar ones in the $30-$50USD range and then you get into the $100 knock off generic ones that are bit more full featured, and then the name brand ones for a lot more. Pretty much everything is way overkill for me for this purpose. The one mentioned above covers well below and above the frequency range needed and seems to be pretty well reviewed.
     
    Last edited: Oct 17, 2020 at 4:44 PM
  4. mrriggs

    mrriggs TDPRI Member

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    Higher-bandwidth probes aren't always better. Inside an attenuating probe there is an adjustable capacitor that compensates for the input capacitance of the oscilloscope.

    In the specs on that probe it says the "Compensation Range" is 10pF - 35pF. This means that it will only work properly on scopes with a input capacitance of 10pF - 35pF.

    A quick glance at the specs of that oscilloscope didn't reveal what the input capacitance is. You should find that out before ordering probes.

    It's probably fine. Higher-bandwidth probes are usually only a problem with older/lower-bandwidth scopes.
     
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  5. tubelectron

    tubelectron Tele-Afflicted

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    @jchabalk :

    I bought the SDG-810 SIGLENT function generator :

    [​IMG]

    And it's an excellent instrument ! I borrowed a RIGOL DG1022U to a friend and found it ergonomically inferior.

    And like you, I am interested by the SDS-1202-X-E oscilloscope and the SDM-3045-X multimeter... When I'll have the money to devote to !

    -tbln.
     
    Last edited: Oct 18, 2020 at 12:30 PM
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  6. jchabalk

    jchabalk Tele-Meister

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    I checked the specs and it says the inputs are 1Mohm / 18pf 400v peak so it looks like the one i found will work. Does 400v mean that a 10:1 probe is (much more than) sufficient?

    I'm a bit unclear on the voltage, when i was looking around the last few days there was a lot of mention of needing 100:1 probes to get into the 400-600 volt range but maybe that was with equipment that couldn't natively handle as high a voltage as these scopes do?
     
    Last edited: Oct 17, 2020 at 8:10 PM
  7. mrriggs

    mrriggs TDPRI Member

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    With a 400 Volt input, a 10:1 probe would be all you need. Some cheaper scopes have much lower input voltage which would require greater attenuation.

    Most probes are only good up to 600 Volts. That means that they are only meant to be used on circuits 600 Volts or less. It's a safety thing. I'll admit to exceeding the voltage of a probe a time or two, but it was clipped into the circuit before power up, I wasn't holding it.
     
  8. jchabalk

    jchabalk Tele-Meister

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    Cool, the probe piece has been confusing - mostly because in the manual it doesn't state what probes actually come with the scope, nor how the operating voltages work. This seemed to be consistent with the Rigol and Siglent scopes, and the 3rd party makers selling x10 and x100 probes on amazon and ebay are equally non-specific about it.

    If the scope can natively support <=400v And i don't think the capacitance at 1:1 will affect the valve amp circuit) an x10 probe should work up to 4000 volts, seems like in most cases the 1:1 probe should even work fine. I'm going to buy a set of x10 and x100 probes to see how this actually works and will likely be returning the x100 set.
     
  9. BobSmith

    BobSmith Tele-Meister

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    These should do the job. For the kind of analysis/troubleshooting that amps need, spec requirements are actually quite low.
     
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  10. mrriggs

    mrriggs TDPRI Member

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    Most of the time I just use jumper leads rather than a probe. However, you have to consider the input resistance and capacitance of the scope.

    Hooking the scope straight to the circuit is the same as jumpering in a 1 MOhm resistor and ~20pF capacitor (actually more when you add in the capacitance of the leads). It will affect the circuit. If the circuit is sufficiently low impedance then the effect is negligible. On high impedance circuits, say the grid of a tube, it can drastically alter the circuit.

    A 10:1 probe will load the circuit with 10 MOhm and much lower capacitance.
     
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  11. jchabalk

    jchabalk Tele-Meister

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    Thanks riggs, this helps me understand more. much appreciated.
     
  12. LightningPhil

    LightningPhil Tele-Meister

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    The Siglent looks pretty good. Far better than my old Tek.

    You may prefer an analog signal generator. Digital ones are great until the frequency is changed where they miss a beat and start the new waveform. In other words they click when the frequency is changed. Analog ones can be changed in a nice smooth analog way. No horrid clicks.

    Regarding probes, a set of 100x and 10x should have you covered. The problem with the switchable 1x to 10x models is the chance of fragging you scope by accidentally by having it on 1 when you think it should be 10. I’ve got some 1000x and never get to use them. They were free though. Woot!

    Regarding BNC cables, always ensure you avoid 75 ohm. Scopes use 50 ohm.
     
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