Look who's all grown up! Finally able to pull the trigger and get an oscilloscope

[Chuck H]

... turn up the vertical far enough they eventually see some sort of noise waveform, or the local AM radio station, or SOMEthing. I made up the term "trace chasing" for the phenomenon. You chase after a trace long enough, you'll find one.

Still laughing. I did that too. Not much though because I always have other amps around for a reality check.

I did have an odd thing happen once where micro scanning the waveform helped. Sometimes with tone stacks the HP cap will create a distortion artifact when there is a lot of LF being "blocked" (if that's even the right way to think of it?). I had an amp that would crackle just audibly on some clean tones and I traced it to the treble cap. Replacing the cap didn't help. So I took a good close look at the wave form and saw the artifact. I struggled for hours trying to mitigate it in one way or another before I opened up another amp and probed it for a reality check. It turns out that this is normal, but worse on some amps and more or less audible in different overall designs. In this case the artifact was greater than usual and the design really allowed it to be heard. It was a silver face Champ. I fixed it by reducing gain to the tone stack a little, bumping the gain in the following stage and I can't remember what all else. But it was the scope that helped me find it and the scope that allowed me to see it was "normal"-ish and it was the scope that allowed me to monitor progress in the circuit modifications. I did a lot of mods to that one and the owner calls it his little *umble

[TimmyP1955]

Good idea, do you know that one from experience?

nosaj

No, but Murphy catches up with us all eventually ;-(

[SoulFetish]

Right now you just need to learn to use the scope, and for that matter when to stop scoping every darn thing in sight.

Too late. I'm a scopin' Mutha' F**ker now, Enzo.
If it beeps, bloops, blinks, or buzzes, it's getting scoped. I'm gonna scope like LT scopes. Hell, I'm gonna' Scope like The State of Tennessee vs. John Thomas for violating the Butler Act.... in 1925.

[SoulFetish]

do you guys use active differential probes to measure high p-p voltages (ie. plate)? Or, passive 100x probes?

[nickb]

do you guys use active differential probes to measure high p-p voltages (ie. plate)? Or, passive 100x probes?

Passive 100x for me. I doubt anyone would use a diff probe for tube tests unless it's a lab situation

[SoulFetish]

Passive 100x for me. I doubt anyone would use a diff probe for tube tests unless it's a lab situation

Are most 100x high voltage rated (one would think)? I should probably look for at least CAT II, yeah?

[Chuck H]

I only have a 10x/1x switch probe. My scope has a max 400 input voltage. I haven't had a problem. I don't scope the power tube plates much though. Usually if I'm already out that far on the signal chain I'm scoping the output jack.

What is the input voltage of your scope?

[SoulFetish]

I only have a 10x/1x switch probe. My scope has a max 400 input voltage. I haven't had a problem. I don't scope the power tube plates much though. Usually if I'm already out that far on the signal chain I'm scoping the output jack.

What is the input voltage of your scope?

Same. 400VDC or peak AC. but can withstand transients to 1000V for short a duration. Now I'm paranoid that I'm going to ground the probe out to a +V terminal by accident.

[Chuck H]

Now I'm paranoid that I'm going to ground the probe out to a +V terminal by accident.

Yeah, don't do that.

And you almost certainly won't. It's a hard mistake to make when the CHASSIS IS RIGHT THERE!!! Unless you're measuring a swing voltage across two points. But that's usually pretty specific.

10X means you can have peaks to 4000V on your scope using a 10X probe. I think you'll be OK if you're paying attention. Though I admit that I've had a couple of close calls with "my" scope by NOT paying attention.

[Justin Thomas]

Probes were cheap enough; I got a x1/x10 AND a x10. And I may be willing to make up whatever little safety gadgets I have to to spare my scope. When I get one! Eventually... I try not to look at these kinds of things as disposable, even if I <DO> get a great deal. That $10 multimeter is worth playing safe with; you'll be that much more conscientious later on with the good stuff. And who knows when the next great deal will come along? Besides, less spent on blown-up equipment = more spent on amp parts!

Justin

[nickb]

Are most 100x high voltage rated (one would think)? I should probably look for at least CAT II, yeah?

The one I have is rated at 1200V AC+DC.


My scope has a limit of 150V so it's really not the best solution for probing anywhere where you need AC coupling and where there is more than 150V DC. When the scope is switched to AC the DC input voltage appears across that cap. It's quite common to hear people use a capacitor on the end of the probe to look at AC signals in the presence of high DC and I do this often myself, but there is a short period of time when you first probe during which that scope input cap charges and nearly high DC voltage appears across the scope input cap. Keep the cap small say 1,000pF for a x10 10Mohm probe. The best solution is really an external attenuator with switchable AC coupling and a limiter to prevent scope damage - Juan has a posted a suitable design on these pages before.

[Enzo]

ANyone really concerned could make a tiny voltage divider with a couple resistors and clip the probe to it, then use an additional probe to the circuit.

[SoulFetish]

Fahey (or anyone), if your out there, do you have alink to the probe topic that Nick was refering to?

[Tom Phillips]

Fahey (or anyone), if your out there, do you have alink to the probe topic that Nick was refering to?

There is a good bit of information in the discussions at http://music-electronics-forum.com/t31492/ and http://music-electronics-forum.com/t37999/

Still looking for the discussion that includes Juan's attenuator that will substitute for a good HV probe.
Edit: Juan's post is in the discussion at Test bench amp setup advice - Music Electronics Forum

[nickb]

Fahey (or anyone), if your out there, do you have alink to the probe topic that Nick was refering to?

Here is my version. Ideas shamelessly stolen from Juan




Oh..., the 1pf is just a guess. It's there to maintain the HF response and can usually be left out. The actual value depends on the zeners and chosen to get a flat top on a square wave. Resistors do have a voltage limitation so a string of six is used to get to 3kV