Hi all! I was looking at USB oscilloscopes and I realized I will be working with 450VAC. Some of the USB oscilloscopes advertise ranges from 10V to 40V. Is there a line I could be made aware of? Should I stick with a benchtop for tubes? I was going to use my Xonar STX card to output arbitrary waves, so a complimentary USB oscilloscope would be better IMO. If I'm stuck with benchtop what's good for tubes?
thx Beka D:
thx Beka D:
I'm also currently looking around for a new oscilloscope and my current thinking is this. You need a bench scope as a general purpose oscilloscope. Then an USB scope (or simply a sound card) is good for measuring audio, e.g. THD.
There is really no need to probe 300+ voltages with a scope, mostly you only need to measure input and output of the amplifier. There you will also be fine with an USB scope. But keep away those probes from the B+ line, OR get a 100x probe that is safe for high voltages.
To be absolutely on the safe side, get an expensive high voltage active DIFFERENTIAL probe and you are good to go. The disadvantage is that they cost almost as much as an oscilloscope.
If you MUST poke into the high voltage rail for measuring e.g. ripple, you won't see much of it with a 100x probe (simply due to scale). Then it is better to attach a capacitor (e.g. 100nF) to the high voltage and from the capacitor a resistor (e.g. 220kohm) to ground. Then apply power to the device under test. AFTER applying power, you can attach oscilloscope probe in between capacitor and its grounded resistor. Use DC mode and 1x or 10x probe. Don't keep probe attached when powering on, because the capacitor will momentarily be charged with full DC voltage before draining off through the resistor attached to ground.
Now for USB scopes, my own opinion is they are a hassle. Unless you have a big lab where you can keep them attached and ready. I have an old parallel port scope and rarely use it, because it is such a pain to dig out all devices, boot an old computer (and where was the power supply again...).
And don't ever try to probe anything higher than the scope spec. You will destroy both scope and computer. They are still grounded devices, you would need an expensive differential high voltage probe to probe higher voltages.
Where a computer has its advantage is when measuring THD or frequency response using a good USB sound card (or USB scope). Almost any 16-bit soundcard will get you better accuracy than a regular 8-bit bench scope. As long as you stay away from the high voltage. And that is no problem when you only measure input and output from the amplifier.
Opinions and experiences are welcome. I don't have much practical experience with oscilloscopes.
There is really no need to probe 300+ voltages with a scope, mostly you only need to measure input and output of the amplifier. There you will also be fine with an USB scope. But keep away those probes from the B+ line, OR get a 100x probe that is safe for high voltages.
To be absolutely on the safe side, get an expensive high voltage active DIFFERENTIAL probe and you are good to go. The disadvantage is that they cost almost as much as an oscilloscope.
If you MUST poke into the high voltage rail for measuring e.g. ripple, you won't see much of it with a 100x probe (simply due to scale). Then it is better to attach a capacitor (e.g. 100nF) to the high voltage and from the capacitor a resistor (e.g. 220kohm) to ground. Then apply power to the device under test. AFTER applying power, you can attach oscilloscope probe in between capacitor and its grounded resistor. Use DC mode and 1x or 10x probe. Don't keep probe attached when powering on, because the capacitor will momentarily be charged with full DC voltage before draining off through the resistor attached to ground.
Now for USB scopes, my own opinion is they are a hassle. Unless you have a big lab where you can keep them attached and ready. I have an old parallel port scope and rarely use it, because it is such a pain to dig out all devices, boot an old computer (and where was the power supply again...).
And don't ever try to probe anything higher than the scope spec. You will destroy both scope and computer. They are still grounded devices, you would need an expensive differential high voltage probe to probe higher voltages.
Where a computer has its advantage is when measuring THD or frequency response using a good USB sound card (or USB scope). Almost any 16-bit soundcard will get you better accuracy than a regular 8-bit bench scope. As long as you stay away from the high voltage. And that is no problem when you only measure input and output from the amplifier.
Opinions and experiences are welcome. I don't have much practical experience with oscilloscopes.
You can use 1:100 probes for high voltage in tube amps. That should give you enough capability with either a bench top or a USB scope.
I am using a USB scope with a USB isolator so it totally floats free, so I can measure for instance across an output transformer primary. But you need to be careful not to touch it while measuring!
I also have a WiFi scope that does not need any galvanic connection to a computer and runs on an internal battery, there is no practical limit to hooking it up inside a high voltage unit.
But they are relatively expensive.
Jan
I am using a USB scope with a USB isolator so it totally floats free, so I can measure for instance across an output transformer primary. But you need to be careful not to touch it while measuring!
I also have a WiFi scope that does not need any galvanic connection to a computer and runs on an internal battery, there is no practical limit to hooking it up inside a high voltage unit.
But they are relatively expensive.
Jan
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Awesome! Exactly what I needed to know. I haven't used an oscilloscope in 25yrs and that was for debugging a custom RS232 pc card I made for a robot. I never would have thought to look for the 100x probes.
thx guys!
thx guys!
Working with tubeamps and oscilloscope without probes allowing at least 500Volt
is a risk for destroying the scope.
Examining phase balance is a not-uncommon task, examining plate's also.
Most 1:100 probes allows 500Volt, but access to a 4k probe is sometimes an asset.
is a risk for destroying the scope.
Examining phase balance is a not-uncommon task, examining plate's also.
Most 1:100 probes allows 500Volt, but access to a 4k probe is sometimes an asset.
Make sure its a non-switchable 100:1 probe with adequate rating class for safe use. (And long shrouded tip, 6 inches or so
is good to keep fingers away from circuit!)
I wouldn't trust a cheap copy for such a probe.
is good to keep fingers away from circuit!)
I wouldn't trust a cheap copy for such a probe.
I've been wondering about probes too lately for use with a distortion analyzer HP8903B or another audio analyzer I have that uses a computer and has a max input voltage of 50v (audio asylum device). I feel safe hooking it to the OPT. But lately I've wanted to measure the THD of a single voltage stage at different operating points. Would I measure THD of a voltage stage by useing a 100x probe at the output of a voltage stage? Leaving the coupling cap and subsequent stage grid leak in place? I've been hesitant to measure THD at plate voltage points, but the answers here seem applicable to what I want to do as well. Sorry to the OP but I thought this was pretty much the same situation... How to probe higher voltage waveforms without frying the gear.
I use -at least 40 year old- CRT oscilloscope for rapid viewing with 10:1 probes (up to - DC+AC - 500V).
For the correct measuring I use Kenwood VT-176 2 channel AC voltmeter (at HV measuring with 10:1 probes), which have 1V output for soundcard measuring (FFT).
Over 500V I only use 1kV 100:1 probes.
For the correct measuring I use Kenwood VT-176 2 channel AC voltmeter (at HV measuring with 10:1 probes), which have 1V output for soundcard measuring (FFT).
Over 500V I only use 1kV 100:1 probes.
Yes. As a former millwright, machinist, welder, I will be wearing HV gloves, a wrist strap, my welding leathers and a face shield with my hair in a net. My amp will be fixed on a plexi glass plate no cladding and all connections made using terminal bars. The amp will slow start, and switch stages. There will be plenty of breakers or fuses. I won't touch it unless it's up to my standards of safety. I've taken 240V across the chest 2x and it's not pleasant.
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Some fast A2D's are only 1 volt range !
So it needs a good front end to cope with high voltages and also x10 x100 scope probes.
So it needs a good front end to cope with high voltages and also x10 x100 scope probes.
A proper simplified example of a high voltage probe, that will work with inputs that can be switched to AC coupled is:
Tip to 9 Meg series resistor to a, 110k parallel resistor to ground, in parallel with the scope 1 Meg input. That makes a 100:1 divider.
You need the AC coupling cap to be able to take 10V, because a 500V DC tube stage in come cases will go to 1000V.
More than a lot of digitizers will take (the transient voltage when the cap charges, and when it discharges is 10V).
Blow-out for so many digitizer inputs.
Most scope AC coupling caps are rated for up to 300V, so a true 100X probe works there (the 110k keeps the voltage down, even with a DC blocking cap after that.
But a real High Voltage scope probe is so much more than that:
Of course, the 9 Meg resistor has to be paralleled with a high voltage trim cap,
and there are other components in the compensation box that is at the scope input.
The coax from the front of the probe that connects to the compensation box is very special coax, resistive center lead, and other details too.
That is why a really good High Voltage scope probe is so expensive, lots of design, lots of parts, lots of factory adjustment, and legal cal certificate.
I have one of those 2500V probes, it is certified to 275MHz, and it is worth its price.
Not simple.
Makes real square waves look like real square waves.
Protects the scope input.
Protects the user when the comp box is off the front of the scope (Maximum voltage is 25V at the BNC comp box tip, with 2500V in; do NOT float the scope, do not float the probe ground clip).
Makes accurate measurements possible.
"You should make things as simple as possible, but no simpler" - Albert Einstein
Tip to 9 Meg series resistor to a, 110k parallel resistor to ground, in parallel with the scope 1 Meg input. That makes a 100:1 divider.
You need the AC coupling cap to be able to take 10V, because a 500V DC tube stage in come cases will go to 1000V.
More than a lot of digitizers will take (the transient voltage when the cap charges, and when it discharges is 10V).
Blow-out for so many digitizer inputs.
Most scope AC coupling caps are rated for up to 300V, so a true 100X probe works there (the 110k keeps the voltage down, even with a DC blocking cap after that.
But a real High Voltage scope probe is so much more than that:
Of course, the 9 Meg resistor has to be paralleled with a high voltage trim cap,
and there are other components in the compensation box that is at the scope input.
The coax from the front of the probe that connects to the compensation box is very special coax, resistive center lead, and other details too.
That is why a really good High Voltage scope probe is so expensive, lots of design, lots of parts, lots of factory adjustment, and legal cal certificate.
I have one of those 2500V probes, it is certified to 275MHz, and it is worth its price.
Not simple.
Makes real square waves look like real square waves.
Protects the scope input.
Protects the user when the comp box is off the front of the scope (Maximum voltage is 25V at the BNC comp box tip, with 2500V in; do NOT float the scope, do not float the probe ground clip).
Makes accurate measurements possible.
"You should make things as simple as possible, but no simpler" - Albert Einstein
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