I just bought my 1st O-scope!

[dwinstonwood]

I just bought my 1st oscilloscope. I did a bunch of research, read many reviews and watched lots of videos.

I ended up buying one of the new(er) Rigol 12bit resolution scopes:
https://www.rigolna.com/products/digital-oscilloscopes/dho800/

I also used a discount code, so it was only $263.00. That's close to Hantek prices.

Now, I need to buy a signal generator. I'm not sure exactly what I need. I guess I need one that goes down close to 1Hz, and puts out at least a 1/2 volt or so? Any inexpensive recommendations are appreciated.

Edited to say I think I can just use a sine wave tone generator app on my laptop.

Happy Holidays! David

[shooter]

now that's a proper tool!


spend quality time reading the manual, get to know the knobs n features.  I'm a book guy, so I take the PDF's to the kids at Staples n tell 'em to print on quality paper, single-sided, 3 hole punch, with a proper binder.  The "back blank page" is a great place to make notes to your future self.


and ya, a simple Amplitude adjustable 1K signal from any source will work fine, you'll want a full audio range though to get your proper geek on.  If you really go all geek, then frequencies way beyond audio come in handy but they cost more

[dwinstonwood]

Thanks shooter. This scope has a 266 page PDF manual. I can get it printed and spiral bound for under $20 (b/w). I'm also a book person - I hate reading screens - so, this is a good, worthwhile investment that I can set down right in front of me while learning.

[dwinstonwood]

I went ahead and bought an inexpensive signal generator, too. It will be good enough for guitar amp work, and it has two channels so I guess I can do something like monitor two different frequencies at the same time on the scope.

It also has a nifty computer interface that's a lot easier to use than the confusing buttons on the physical unit.

[shooter]

by the looks of your scope, without downloading the manual it appears to be both scope and "spectrum analyzer", FFT on my 1st gen version.
If so; spend some quality time to "understand" the difference.
a scope is a "time based" system, the other a "frequency based" system.


both have their uses but moving between the two takes an understanding of the theory not just an ability to twist knobs to see the signal.
the "theory" gets pretty deep converting time based to frequency based, so unless you're a math masochist you don't need to go that deep, just the understanding of the differences between them

[dwinstonwood]

Yeah, apparently my scope does Bode Plots and FFT. Way down the road from where I am at this point.

But Google AI says, "A Bode plot is a graphical representation of a system's frequency response, showing both the magnitude and phase of the signal across a range of frequencies, typically using logarithmic scales, while an FFT (Fast Fourier Transform) is a mathematical algorithm that converts a time-domain signal into its constituent frequencies, allowing you to see the individual frequency components of a signal at a specific point in time; essentially, a Bode plot displays the overall frequency behavior of a system, while an FFT provides a snapshot of the frequency spectrum at a given moment."

Sounds like endless fun. That latest single-ended amp I built is both the sweetest, cleanest (beautiful jazz tone) amp I've built, and the most savage and stable screamer I've built (I'm convinced I used the right tubes in the right order). It's the perfect design to analyze in depth with this scope's abilities.

[dwinstonwood]

Well, as it turned out, the scope I ordered was out of stock. Which is actually a good thing because the Rigol has an external power supply that connects with a USB-C cable; the chassis is not earth grounded (it has a ground jumper that you have to connect instead).

So, I bought the Siglent equivalent: the SDS802X HD that's also 12bit. It's a better scope and also has an interal power supply with a regular IEC 3-prong cord.

Right now, I'm just trying to figure out how to use it! The cheap waveform generator is just that: cheap. I think I'll return it and get something a bit better.

But, anyway, I did manage to feed a 1kHz sine wave at 150Mv into the scope. Next, after studying the manual some more, I'll feed that signal into an amp...

[shooter]

Next, after studying the manual some more

I know the new scopes have "all the answers" displayed, but take the time to count the "squares", look at the time base, (sec/dev) do the math to "prove" it is 1khz.
put in FFT or bod, look at the 1khz, the reason, it's MUCH easier to "see" stray noise or harmonic interference in these modes than standard time base.


enjoy, a scope is one of the cool kids toys 

[dwinstonwood]

Great advice shooter! I'll think of it as an old Tektronix until I get the basics down.

There's a lot of little stuff about scopes that you have to dig to learn. Example, my signal generator has a 50 Ohm output that I think was throwing off the voltage readings on the scope. Plus, I had alligator clipped the two together, which is probably noisy. I bought a BNC-to-BNC with a 50 Ohm impedance just for feeding signals directly into the scope for learning.

[shooter]

BNC-to-BNC

I just pitched my "gender bag" of connectors, when you work in RF you get every connector, every mechanical engineer thinks is cool! somedays the stack of "adapters" n gender changers looks like cool steampunk art


that "added noise" you mention comes in handy to learning, make the connection noisy, try n sync it, measure it in time base, then switch to FFT and find it again.  You're learning "conversational scope"  you want to be able to flip from on mode to the other without having to "glitch out" translating in your mind.

[dwinstonwood]

I wanted to make sure it's is working as it should. Not that I know how to test it.

Same 1kHz, 150Mv sine wave. This time using a 50 Ohm BNC to BNC cable. I now have a very steady and clean image on the screen.

I'm still reading the manual, but the Siglent controls make logical sense and are pretty easy to navigate and adjust. I like the touch-screen menus.

[SEL49]

I bought a Siglent SDS1202X-E back in 2020. No touchscreen or HDMI but I find the controls and layout very easy to use. Made the transition from Tek 465B very easy. Rigol controls are not as intuitive as the Siglent.

FYI, when using a termination on a cable, the terminator does not connect to the source (sig gen). It connects to the other end of the cable (scope). The purpose of the terminator is to prevent ringing on the cable. Not useful at audio frequencies.

[dwinstonwood]

Thanks SEL49. I'm not even at the point where I know what a terminator is. I only bought the BNC-to-BNC cable so I could connect the signal generator to the scope and play around with the knobs. It's just for learning the scope's controls.

Tomorrow, when I start looking at my latest amp design, I intend to connect the signal generator to the amp's input. I intend to use the passive scope probes that came with the scope to look at points along the circuit ( after coupling caps).

[SEL49]

You're gonna want more than one BNC cable, some tees, and assorted adapters. Here's one adapter that comes in real handy for guitar amps. Wow! Kinda pricey...

     https://www.amazon.com/Pomona-1297-Adapter-Phono-Plug/dp/B06Y1HDSCS/

[shooter]

I intend to use the passive scope probes that came with the scope to look at points along the circuit ( after coupling caps).

USE AC coupling on the scope, DON'T probe the plate side of your PA tubes, Make that reading at the speaker



most what you need for guitar amps you can make outta 'ol guitar cables and gator clips

BNC cable, some tees, and assorted adapters.

somedays the stack of "adapters" n gender changers looks like cool steampunk art

[dwinstonwood]

most what you need for guitar amps you can make outta 'ol guitar cables and gator clips

I did just that!

I'm going to start learning on my AC4, so the 25Watt/8Ohm dummy load is fine. When I move on to Push-Pull stuff I'll make a heftier dummy load, maybe with switchable settings for 4, 8, and 16 Ohm loads.

[shooter]

these are scraped outta old equipment, a couple jumper wires, ohms law, good to go

[acheld]

BNC->1/4inch plug

Wow! Kinda pricey...

Well, yeah, it's a Fluke.   

Actually, it's a fair price considering the materials and low volume production.

But dwinstonwood's BNC to plug cable looks very well made!

[dwinstonwood]

Thanks everyone!

So, I measured my AC4's output power...

Before seeing any distortion at the speaker jack I measured about 11.5 volts p-p.

So, 5.75^2 = 33v times 0.707 = 25.46v divided by 8 (Ohm load) = about 3.2 watts of clean power.

So, it's only an AC"4" after it's started distorting!

[SEL49]

BNC->1/4inch plug

Wow! Kinda pricey...

Well, yeah, it's a Fluke.   

Not a Fluke. It's made by Pomona. Pomona it top quality adapters. Pomona is to adapters as Fluke is to digital meters.

BNC to 1/4" phone plug seems a better option. You can have two 6' RG-58 cables for less than that one adapter.

[shooter]

times 0.707

ya know..the scope will read in True RMS, save a math teacher, cheat 

[SEL49]

So, I measured my AC4's output power...

Before seeing any distortion at the speaker jack I measured about 11.5 volts p-p.

So, 5.75^2 = 33v times 0.707 = 25.46v divided by 8 (Ohm load) = about 3.2 watts of clean power.

Your math is out of order. First convert 11.5V_pp to V_rms The answer is 4.07V_rms

Now plug that into the power formula P = E²/R = 4.07²/8 = 2.07W

BTW, you can set your scope to measure V_rms voltage rather than V_pp.

[dwinstonwood]

Your math is out of order.

I was never good at math. I always forget that the order matters.

BTW, you can set your scope to measure V_rms voltage rather than V_pp.

I'll read the manual and figure out how to do that next!

[shooter]

2.07W

now run that 2W into a speaker cab filled with SPL > 100 and ask the audience what the power of the amp is 


I left a 23Wrms amp n a 2 X12 cab with 5 guitarists to evaluate, went back a week later and I could NOT convince ANY of them the amp was 23W  They all swore it was louder than any of their 40W amps.  a Tech and his gear are NOT the same as the audience or the guitarist.

[dwinstonwood]

OK, I turned on the RMS feature (there's also something called Cycle RMS that I didn't use).

With the scope's probe on the output jack I can get about 4.94V RMS before the wave starts to change shape.

So, I get 3.05 Watts of clean.

With volume and tone dimed I get 6.14V RMS and a "rounded square wave."

That's 4.7 Watts max output.

What would I do without this forum to correct my errors?

I did figure out how to save a screenshot to a thumb drive.

[dwinstonwood]

There's a weird little thing going on in the red circle. Is that a high frequency spike? I need to zoom in on it with the scope.

Is it a bit of oscillation/instability? I've read that those things can rear up on the upper-left corners of wave forms.

It's only there when the amp is maxed out, and I never really play it at that setting.

Expert diagnoses encouraged.

[shooter]

guessing just the scope's "limitations" converting an analog sig to digital.


A "transient" or overshoot would be more like 1st image
you've found why switching to FFT will help "diagnose"  The 2nd image are a couple stray signals that come n go

[acheld]

I agree with Shooter.

While there may be a bit of ringing present, it is trivial at most.  The wave form looks good for a dimed amp, IMO.

That said, I would have expected your scope to deliver a less jaggy image than that, in other words, better resolution.   

Perhaps by decreasing the volts/division setting -- making the image "larger" on screen both vertically and in the time domain (X-axis) would help clear up what you're seeing.

I assume your scope allows screenshots?  Yes?  Those often have more complete information on your settings . . .

Are we havin' fun yet?

[shooter]

I would have expected your scope to deliver a less jaggy image than that

look close at my sinewave image, it's about the best you can expect from 12bit A/D, even with "smart smoothing" from some wiz-kid software embedded.

[acheld]

OK, though my Hantek scope screen shots "look" less jaggy,  (it is most assuredly NOT a better instrument!),

my main point is that the scope screenshots have a lot of good information, just as yours does, and

that by increasing the size of the signal "image" on the scope screen you can decrease the visual effect of the A-D convertor on the overall image.

[shooter]

scope screenshots have a lot of good information

my 1012B has a USB stick and a print button, since it's an antique it can only handle a MAX USB of 128KB.  try finding them in the modern world 

[dwinstonwood]

Thanks for all the help so far y'all!

There's still a ton to learn with this scope. But, if I'm interested in recording Gain along this circuit, as well as output power, do these look like the best Test Points to measure (the red ovals)? I'm assuming that control grids are the best and safest places to connect the scope.

Thanks!

[shooter]

I prefer the "right side" of the coupling cap as my Vin, plate as my Vout.


probing directly on G1 may cause un-expected issues do to impedance matching, loading....other things that may skew results




EDIT:  ALWAYS know the VDC at the plate and change/adjust scope probe accordingly so magic smoke doesn't escape
I never normally probe PA plates, even if my probe is capable of the VDC's there

[dwinstonwood]

Thanks shooter!

So, like this?

[dwinstonwood]

I finally had a good block of time to slow down, take my time, learn the scope controls a bit better, and record some measurements of my 6V6 cathode biased amp that I built a while back.

Before I started, I swapped the 270 Ohm cathode resistor for a 250 Ohm. This pushed my dissipation up to 93.4% at 13.2 Watts, 39mA per tube and 20.61V across the cathode resistor. This is with a pair of ANOS GE 6V6GT's. I also added the usual voltage measurements to the schematic.

I'll upload scope screenshots taken at the red dots on the schematic in the next post. They are all at maximum clean levels.

Anyway, with a clean signal at the speaker jack I got 10.18V RMS, so that's 12.95 Watts clean power. I think that seems pretty good with only 339V plate to cathode.

[dwinstonwood]

Here are the scope screenshots. The image numbers are in order from first gain stage to speaker jack.

I'm sure I didn't have the scope set perfectly. I used 10X for measuring inside the amp and 1X for the speaker out. That seemed to work best.

Here are the first four...

[dwinstonwood]

And, the fifth image, the speaker jack at 1X settings...

[shooter]

Nice!  Those images tell me you ain't gonna get away with playing sloppy through that amp n hope nobody hears 


get used to labeling those images, so next week they make sense to you
MaxCleanSpkr.xxx, InpuT100mV.xxx...


I store my scope-shots in a folder with the schematic, build notes, etal, on a computer, in a dark room 

[dwinstonwood]

...so next week they make sense to you

By tomorrow afternoon I'll have no idea which image is what.

Thanks, the amp has adequate clean volume, but can descend into Metal World, too.

In comparison, my 6G3, biased around 60% with 374V a>k is putting out about 10.86 Watts of clean.

[shooter]

once you get to using the FFT part of the scope, you'll be able to look at "harmonics" and "see" where a SE amp tends to favor the odd's where PP the evens.  so many rabbit holes, so little time