Scope debugging Qs...

[surfsup]

So I am going to start up the scope this weekend now that I'm back in town. Couple questions:

1-What peak to peak 1kHz signal amplitude should I use to accurately represent input from a humbucker and single coil?
2-How does one scope out the output or power tube plates? (I touch the probe here and I get a loud pop and a decent spark, is this normal?)

Are free probes with the scope good? Mine seem to have a mind of their own. Sometimes they work great, other times they seem to pick up and display only 385MHz signals.

[Cliff Schecht]

First and foremost, make sure that everything that you are using to probe your HIGH VOLTAGE tube amp is actually able to handle the PEAK-to-PEAK voltage swing that your amp can provide. Probes/scopes that can handle 400V peak to peak means that anything over ~150V RMS (RMS for a sine wave is peak amplitude, not peak to peak, divided by square root of 2) can potentially blow the front end of your scope.

For the high frequency signal, your amp might be oscillating (probably not as there is no gain in most audio tubes at 350+ MHz). You should see a few MHz max unless you are using some weird RF tubes and not snubbing them properly. Maybe you are getting a bad connection on the ground connection? Or adding too much parasitic capacitance to the amp with the 20pF probe?

I usually never probe the plates directly, unless I know for a fact that they aren't going to hurt my equipment. You can probe the secondary side and see what the speaker is seeing, this is usually good enough. If you want to avoid the effects that the speaker adds to the plate outputs (they throw lots of back EMF into the signals), use a resistive dummy load on the secondary and probe there. The resistor will give you a relatively faithful (and scaled down) version of the plate signal.

As far as the popping, what you are doing is adding a new path to ground through the scope probe. If there is any charge stored anywhere or any potential difference between the amp and scope grounds (not uncommon) then you will get a pop when you add the extra capacitance from the scope probe into the signal.

[surfsup]

The weird thing regarding the large MHz signal is i can solve it simply by turning off my scope and powering it back on. Strange.

Thanks for the warning about the plates. I have 1x and 10x probes. If i have the probe set to 10x, wont that divide my signal by 10 so i can read the plates directly? I did scope the plates of the PI already...but i dont wanna blow the scope up for sure.

[tubeswell]

I have 1x and 10x probes. If i have the probe set to 10x, wont that divide my signal by 10 so I can read the plates directly?

Yes, as long as the p-p swing of the plate doesn't exceed 10 times the scope's maximum rated p-p input voltage

[Bob S]

I know it takes more time, but if you get pops when you are scoping the
pre-amp path - especially the first stages, switch off standby before you probe the test point(s).
Hope this helps - I use 10x probes on my old(ish) Hitachi scope.
I had the same problem when I was poking around.

[Cliff Schecht]

I have 1x and 10x probes. If i have the probe set to 10x, wont that divide my signal by 10 so I can read the plates directly?

Yes, as long as the p-p swing of the plate doesn't exceed 10 times the scope's maximum rated p-p input voltage

The probe itself will have a voltage rating too, usually anywhere from 400V-1000V P-P. It doesn't care whether it's on 1x or 10x, if you go past this rating you are going to kill the probe. Just an FYI for those who don't know..

Also, FWIW, I always use 10x probes. For what we do they are usually adequate. With a 1x probe the loading you'll be seeing is about 1meg//100pf whereas a 10x probe the loading is about 10meg//10pf. Probes of the 1x nature therefore tend to give you the worst bandwidth and most loading, neither of which are good when you are sniffing out HF oscillations.

[surfsup]

The scope's max voltage appears to be 400 volts. This scope is a modern digital scope, btw. Not sure about the probe ratings, but a 10x setting that would mean the plates would need to swing 4000 volts yes? So i think i'm okay. At idle the voltage on the plates is 330 for the el84s.

I found the american branded scope name, its a b&k 2542 series. They seem nearly identical in lok and features. I downloaded the BK scope manual. So now i finally have an english manual. Some of the buttons make more sense now haha. Actually i didn't like the "fuzzy" traces i was getting but now with two filters turned on, they are nice and crisp/smooth.

[martin manning]

5V/division and +/- 8 divisions = +/- 40V @ 1x, +/- 400V @10x. Based on that I wouldn't probe anything that might be over 400V WITH the 10x probe!

[jjman]

I was not able to properly scope PP output plates. I tried 10x and 100x. I think it scoped as a triangle wave. I assume this is either because of the transformer (OPT) or the fact that it's PP. So I scope the 1ohm resistors on the cathodes instead and this works nicely. The screen current is also included using this approach. I have no problems scoping preamp/PI plates. I don't remember if scoping the plate of the 6v6 on my Champ type SE outputs works properly.

[surfsup]

My cathodes are connected and share the same resistor so i dont thing i can do that. Maybe i should get a couple 100x probes? The scope supports up to 500x for sure, possibly 1000x though not sure about the 1000x.