Oscilloscopes and oscillation

Keefy

I watched the video below and it made me think two things:

1. It's probably about time I got an oscilloscope

What is worth getting? I don't mind spending on quality, but I don't want to sink a lot of cash into features I may never use. What's a good budget buy, preferably something compact? Any pitfalls to avoid if buying used?

I have an old XP laptop sat doing nothing, so a gadget that would work with simple PC software would also be an option.

2. I wonder whether my Princeton Reverb rebuild could be subject to oscillation?

I followed the original circuit (no grid resistors on the 6V6 valves), whereas the '68 Custom had 1.5k grid resistors. It would be an easy job to add them so I may as well do it anyway. Obviously if I have a problem, an oscilloscope will tell me!



Full disclosure: I have one of those little £18 pocket oscilloscopes off eBay but unsurprisingly, it's shit!

urban_winter

I got a picoscope for christmas (the cheapest one, can't remember the model). I'm very happy with it. Made in the UK, cheap, does more than I need. Two channels plus a signal generator. The software is good. I recommend it. I'd love a big old traditional scope but they take up so much space!

SlopeSoarer

Would one of these do? Lots of reviews and not a lot of outlay can be had on UK eBay for a tenner more.

springhead

urban_winter said:

I got a picoscope for christmas (the cheapest one, can't remember the model). I'm very happy with it. Made in the UK, cheap, does more than I need. Two channels plus a signal generator. The software is good. I recommend it. I'd love a big old traditional scope but they take up so much space!

I have both.  Love the Picoscope, spectrum analyser is nice for playing with filters and wah pedal sweep tuning.  However you have to be aware of the voltage limit if you're going to use one on a valve amp.  +/-20V max measurement with +/- 100V overvoltage protection, so 10:1 probe is a must, 100:1 even for the higher voltage spots in an amp.  I use my old CRT scope there for safety, you can pick them up cheaply now.  You don't need huge bandwidth, 2 channels 25MHz is more than enough.

Keefy

@urban_winter I’ve been looking at the Picoscope range but as @springhead  says, it will only take 20V max.

@SlopeSoarer I think that’s pretty much what I’ve got but in a nicer box. I perhaps ought to give it another go.

I’ve also had the offer of a free ‘scope from a kind member here, once it is legal and practicable to pick it up.

urban_winter

Picoscope - true, re voltage, but 100:1 probes are cheap. I haven't stuck mine inside a valve amp yet, mind. 

U

springhead

urban_winter said:

Picoscope - true, re voltage, but 100:1 probes are cheap. I haven't stuck mine inside a valve amp yet, mind. 

U

If you've got a source for cheap (but not crap!) 100:1 probes let me know.  I've only seen them around £50 and up.

Keefy

Resurrecting my own thread here...

I am looking again at this, having saved up some money from recent gigs (yay!). I had an online chat with Pico Tech, explaining that I'll be working on valve amps where there could be 600V DC lurking, and they confirmed that the 2204A will only withstand a combined potential of 100V. The cathode in a cathode follower typical sits at a higher DC potential than this, and I wouldn't want to blow up either the Pico device or my computer by connecting a probe to it! Do 10:1 and 100:1 probes act as potential dividers, so that the 'scope only 'sees' 1/10 or 1/100 of the actual voltage? And can you get DC-blocking probes with built-in capacitors rated at, say, 1kV?

@urban_winter have you used yours (guessing its the 2204A) in anger yet, and if so how did you get on?

urban_winter

I still haven't used mine on a valve amp. And I'm really not qualified to advise on safety-related stuff.

...but...

Googling for circuit diagrams of probes seems to show that they are a voltage divider - which means it should be OK. You could check by putting a multi-meter in place of the oscilloscope input and seeing what voltage you get before you risk plugging it into the 'scope.

I would guess that a DC-blocking capacitor would render the probe unusable for observing waveforms.

U.

m_c

1/10 and 1/100 probes are just potential dividers.
I have a couple 1/100 probes, for when I was doing some experimenting with VFDs, 3 phase, and sine wave filters, and they showed a nice 5.4V sine wave.

Keefy

Thanks chaps, I’ve made some progress in my own understanding of oscilloscopes and probes and those comments line up with how I now understand things. I have just taken delivery of a Yeapook ADS1014D (model has also been branded FNIRSI). It comes with 1x/10x probes and I have ordered a 100x probe. I spent a good chunk of yesterday getting to know its features. So far, so good!