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Need help measuring high B+ signals using oscilloscope.
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Author:  Danny [ 16 Nov 2020, 17:28 ]
Post subject:  Need help measuring high B+ signals using oscilloscope.

Hi everyone.

I am planning to buy a digital oscilloscope whose max input voltage, including for the 1X/10X probes, is 300Vrms.
But, I need to measure B+ voltages that go up to 460 Vdc. Tech support is telling me that I need to buy some expensive probes.
Is there a way to do this without having to buy the expensive probes?



Author:  Suncalc [ 18 Nov 2020, 19:09 ]
Post subject:  Re: Need help measuring high B+ signals using oscilloscope.

Danny wrote:
Tech support is telling me that I need to buy some expensive probes. Is there a way to do this without having to buy the expensive probes?
There are a few things to consider here.

First, you need to do some studying up on safety procedures. The voltages in most tube amps can kill you deader than dead in a heartbeat (literally). Please be very careful working on this equipment.

Second, when it comes to equipment, multimeters are for high voltages, oscilloscopes are for examining waveforms. This is an important distinction. I would never even think about hooking up a 400V B+ line to my oscilloscope. And neither should you. High voltages are for meters rated to take it. I use my Fluke 179 to check circuit voltages and I use probe covers when working in the chassis to avoid inadvertent shorts. Note that the Fluke 179 meter is a 600V/1000V CAT III/IV rated meter.

Oscilloscopes are for examining waveforms not checking voltages. Usually, the only place I have my oscilloscopes connected in a tube amp are the input signals (a few volts) and the speaker outputs (also a handful of volts). Sometimes I will measure the AC output swing of a signal or driver stage, but only AFTER the DC blocking capacitor. Usually these ac voltages are far less than 80v rms (≈114v peak). There is almost never a reason to look at high voltage (<120v) waveforms unless you are working on some very specialized equipment. I understand the desire to maybe look at the various waveforms in the tube amp power supply as it goes from AC to DC, but there is really no reason to do this. And if you find you must, you should probably install a high impedance divide-by-10 voltage divider across what ever test point you want to measure. It usually best to leave the high voltage measurements to the multimeters.

Also remember that you NEVER probe with a oscilloscope. An oscilloscope is meant to be hooked up to the circuit before the circuit is powered on. That's why scope probes have little locking hooks on the ends instead of points like a multimeter probe.

Finally, when you talk to tech support, they are going to protect themselves from liability. Most typical probes like you find on amazon (like these) and the like, are typically 300v probes. The probes that come with the Rigol scopes are all 300v. They are "Cat II" with a max rating of 150vac (1X) or 300vac (10x). CAT ratings are a safety rating based on power system source impedance and are issued by the IEC. These ratings really apply to electrical power systems and not electronics. The minute you say 460v to tech support, they are going to point you at some very expensive equipment. Most of this is not particularly relevant to your situation.

To summarize, when you're looking at test equipment, the MOST important piece of equipment is a high quality multimeter. This is the real workhorse on your test bench. Personally, I would skimp almost anywhere else before skimping on my meter. The oscilloscope is all about waveform analysis. My multimeter is ALWAYS the first thing I grab when working with anything electrical.

I realize that most of this is not indirect response to your question, but I felt it important to clarify the differences in usage between meters and oscilloscopes. I was/am afraid that you are looking to do something that is well out of ordinary.

Author:  Danny [ 19 Nov 2020, 07:58 ]
Post subject:  Re: Need help measuring high B+ signals using oscilloscope.


Thanks for the insights. Yes, I was planning to look at the waveforms in the PS to check the AC to DC ripple at various stages. But, you make some good points.
So far I have exclusively been using my multimeter. But, it would be nice to go to the next step.



Author:  gofar99 [ 19 Nov 2020, 21:30 ]
Post subject:  Re: Need help measuring high B+ signals using oscilloscope.

Hi, Plus 1 on the not using a scope on high voltages. I have a rather complete array of test gear and worked with electronics for about 55 years. I can count on one hand the number of times I measured the ac component of a high voltage supply with a scope. Then it was with a 100X probe or a divider network and blocking capacitor. Issues with ripple on a power supply almost always show up in the output if they are significant. If the output is clean then by default the power supply is too. Additionally using a PC scope on such voltages is often a quick way to smoke one and possibly damage the PC via the connection to the scope. BTW: when using PC scopes beware of AC mains ground loops. The ground of the PC and scope may not be the same as the unit under test. If you are in a sensitive mode (like the 30 volt input) there can be enough difference in the grounds to fry the input of the scope. Many are not protected against such failures. I found that out the hard way a few years back. Now everything in my shop has a common ground system and I measure the voltage between something I want to test with an auto ranging meter before I connect the scope to it.

Good listening

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