Oscilloscope coming soon - please help!

[rogb]

Guys I have ordered a PC-USB o/scope. I have got a bunch of 50w resistors on their way from good ole China for the dummy load.
I have looked at XYZ of O/sopes.
I have read all the threads here.
I have looked at You Tube.


Assuming I have learned to set the scope up from the info available, please explain in a simple way how to go about testing one of our style amps for oscillation and general fitness tests. Andy F was going on about 2nd order harmonics in the PI, how for example would you test for this?

I am really trying to advance my knowledge, I would really appreciate your help.

I am drowning in the snowstorm of internet information.

Thanks
Rog

[xtian]

Do you have a sine generator?

I learned to use my scope for the basics. Put a 1KHz sine at 1v p-p at the input, and observe its shape throughout the amp. It's very illuminating to see the various shapes of distortion, but it's a hieroglyphic language that takes a while to learn. Crossover distortion is easy to see, and some other shapes are recognizable, too.

You can also measure approximate output power by adjusting volume levels until the sine begins to distort at the output, and then observing the output voltage swing.

I'm looking forward to learning more, so bring it!

[markusw]

2nd order harmonics in the PI, how for example would you test for this?

Probably you USB scope will have a spectrum analyzer anyway.
If not you might use visual analyzer freeware.
http://www.sillanumsoft.org/prod01.htm
It has a scope but also includes a spectrum analyzer and also a sine generator you could use for checking 2nd order harmonics.
You just need to talk care not to overload the audio input of your PC.

I use the input of an USB Audio interface for the scope and spectrum analyzer function and the "normal" audio output of my PC for the sine generator.

[rogb]

Thanks guys - I have a 1khZ sine wave samples and generator on my phone also on my PCs.
A simple way to get started, easy to understand - phew thanks
Any more info is welcomed by me and many others I am sure who are moving up rung by rung.

[mcinku]

I have one doc where you can find explanation how to measure Output Power of the amp using a scope.
It's not much, but it's a start.

[rogb]

Thanks for that, so I would need a dedicated function generator which supplies a voltage and a test signal rather than just audio files of test tones?

[mcinku]

Thanks for that, so I would need a dedicated function generator which supplies a voltage and a test signal rather than just audio files of test tones?

I strongly suggest you invest in some function generator... it doesn't have to be something fancy. Ebay is full of them... or you can get something like this for example.
http://www.conrad.com/ce/en/product/123 ... r?ref=list

...but I guess on ebay you can score something cheaper than that.

[rob@tele]

I have an earlier model of one of these USB scopes with signal generator:
http://www.picotech.com/entry-level-oscilloscopes.html
Works perfectly!

[rogb]

I have an earlier model of one of these USB scopes with signal generator:
http://www.picotech.com/entry-level-oscilloscopes.html
Works perfectly!

Thanks! Think I will get that one and return this.
The software is very buggy and will not run.
Time I get a signal generator I will have spent the same.

[Luthierwnc]

If you do this for fun, try whatever rig you get on a known-good amp. Record what it sounds like at different settings with a guitar, remembering your settings and then listen to it as you look at the waveform through the same progressions on the scope. Remember to unplug the speaker(s) and use a dummy load. An amplified sine wave should be seen and not heard. sh

[rogb]

I have an earlier model of one of these USB scopes with signal generator:
http://www.picotech.com/entry-level-oscilloscopes.html
Works perfectly!

Thanks! Think I will get that one and return this.
The software is very buggy and will not run.
Time I get a signal generator I will have spent the same.

I got it going, it was the DPI settings I use to get text bigger on the computer screwing things up. The software doesn't like this kind of magnification, it seems.

Thanks Luthier, good advice there. I know my 2204 JCM800 is perfect, nice simple circuit so I will go through that one and get an idea of the waveforms at the different test points.

[Luthierwnc]

It may be on here somewhere but I'd like to see a dialed Skyliner with the appropriate (meaning I'm not sure) input signal at all the important signal points in the circuit (ie first coupler, V1b grid ...) showing how the waveform is magnified and altered down the line. Maybe as a sticky or a YouTube vid.

That way, when you get a scratchy, scooped, furry, wimpy or otherwise uncool noise, you can check and maybe find out that the cap (which looks fine on its own) is too big for the stage it is feeding next. That's one of the issues when a hot non-HRM signal from V1b slams the OD entrance too hard. Since I don't have a climate-controlled room full of legendary amps to compare, I fiddle around until it sounds good but might now know what I'm missing.

I'd be happy to post one with this 102 variant and a 2nd gen hybrid. They aren't reference rigs but sound pretty good. I don't have a recording capability so it would just be the pics but it is a start. Perhaps someone who has made the same amp more than once will contribute.

Something to think about, sh

[rogb]

OK I set up to measure the output power of my 50w 102. 16R dummy load attached.

Do I measure with x1 or x10?. Can't get it to clip with my simple audio 1kHz sine wave except at full phone volume.

Not too sure if the 10.2v p-p is the right measurement?
It also reads out at RMS.

If I have the vRMS across a 16R load, what would be the output?
I'm not very hot at equations.

[xtian]

Before you put the sine wave into your amp, measure it with your probe, and adjust the amplitude of the sine to be 1v p-p.

Your screenshot shows a 10.2v p-p signal. What makes you think this is an RMS reading?

You should get around 75v p-p before clipping. (75 x 0.3535)^2 /16 = 43.9watts.

[rogb]

Before you put the sine wave into your amp, measure it with your probe, and adjust the amplitude of the sine to be 1v p-p.

Your screenshot shows a 10.2v p-p signal. What makes you think this is an RMS reading?

You should get around 75v p-p before clipping. (75 x 0.3535)^2 /16 = 43.9watts.

Ah OK thanks, great to know that, no I didn't think the p-p was RMS, it has an RMS display too (left hand measurements column), which reads out at 70.7% of the peak (IIRC).
i will try that. I didn't adjust the amplitude.

In the equation you show, can you please explain the working out in words, I am having trouble with getting it clear in my mind, much appreciated