scope waveforms?

[daz]

I've acquired a scope and i need a source for waveform pics that are applicable to my needs, which most of you know by now....just wanna be able to know exactly what i'm looking at to see where the signal is getting clipped. I think i know...rounded is clean, sharp edges show it's starting to clip and at full clipping it becomes more or less square. I've observed this so far. But while i have found a lot of waveform pics googling it, I'd waste less time sorting thru the stuff i don't need and getting some pics of the exact forms showing clean to clipping and parasitic oscillation in guitar amps If youse guys can point me to a good page.

[nashvillebill]

There's many good books on this site Technical books online Although the scopes they describe are dated, there's three or four great references under the "Test and Measurement" category. Download and read for free!

Dozens of other great books there also, including RDH, all free.

[Jazz P Bass]

As an alternerative thought, how about posting waveforms that you find interesting.

[Chuck H]

I'm backing Jazz (again). Put a 250mV/400Hz to 1000Hz signal into the front end of the amp and a load matched resistance of the appropriate wattage and load on the output. Then you can see what your amp is doing at each stage. Post pics if you can. There are so many scenarios and I've never found a site that's very comprehensive for clipped wave forms in tube amps. I have a lot of questions myself that I would like answered in that area but I can't find a site that covers it. The only "wrong" wave form for a distorting guitar amp would be one that indicates something bad is happening or one that doesn't sound good to you.

I've seen oscillations look like a waveform on top of, and fuzzing the line of the input wave form. Sometimes you need to learn your scope to get images that will show them. It's really not that hard. There will be knobs and switches on the scope. You can read the manual and twiddle and it'll all become clear very quickly (pun intended).

One more piece of advice... Do use a 10X or 100X probe. Division is easy enough so there's no sense risking your new purchase. Most scopes can't handle tube amp voltages at all points in the circuit. You can easily damage the scope. I use a 10X because that's what I have and my scope is good to 200V on the input. Not all are so a 100X probe is usually recommended.

[daz]

It has a 10x switch on the probe so i'll use that. I will post some pics once i get more familiar with things. One thing i don't get is that it has nothing on the probe for a ground wire, but it does have a ground connection on the scope. However, i noticed it works without even using that. I assume that's because both scope and amp are grounded thru the wall?

Funny thing is, at this point the amp sounds and feels better than ever and i'm not sure i even wanna mess with it, but i know if i start with this damn thing it will have me screwing with it again.

[JoeM]

If it's sounding good, then you will have a reference for the waveforms at various points in the amp.

What kind of scope did you get?

[daz]

Just a old beat up gold star. It was dirt cheap.

[Jazz P Bass]

I agree with Chuck, but I would use a 100mv input signal.

Simply place the scope leads on the output.

A dummy load is best at first.

[Enzo]

Take a picture of your probe. I assume the probe is a used one that came with the scope? I get there is a small hole or maybe a circle around it where a ground clip snaps on. Not always, but usually the ground clip is removable.


yes, the fact your amp and scope are both plugged into grounded outlets is providing the necessary ground. That won;t always work. But I get you have a ground post on the front of the scope somewhere, and a clip wire from that to the amp will usually do.


There is no standard waveform library that I know, but most discussions of circuit phenomena include some waveforms. I can show you examples of parasitic oscillations, but you cannot infer they all will look the same. This is like noises in a car. A car can make many noises, and examples of some will not show them all.

[daz]

I agree with Chuck, but I would use a 100mv input signal.

Simply place the scope leads on the output.

A dummy load is best at first.

On the output? I'm new to this, but wouldn't that tell me nothing about where things are happening? I thought the way would be to test each stage I'm using a signal i general on a pc by the way. I have a audio app that can generate 1k or several other frequencies.

Enzo...when i get to using this thing i will upload a pic of the probe with any waveforms i've got questions about. And yes, it came with it, tho the scope is a old used one and i don't know if it's the original one.

My first question is going to be in regards to the 2k cathode i recently put in place of the 470R on the PI and why it lowered the apparent preamp OD at low volume levels where neither the PI or outputs should be remotely near distorting. I know it cause a huge increase in plate voltage so maybe thats it. In fact, i wanted to ask about that now.....what is the max a 12AX7 can stand on the plates? It went from around 230VDC to almost 100V more, about 315 ! The tone and mostly feel is much much better and the amount of OD at any given gain pot setting is lower and it cleans up a lot more on the guitar volume. I want to find out why it worked so well, but for now, anyone wanna tell me if 315VDC on the PI plates is going to cause any issues?

[Enzo]

Don't worry about the 12AX7 plates, you worry about the voltage on the cathode. There is a limit to the amount of voltage the cathode can be above the heater without it breaks down.


Just watching the output as you turn the controls will help educate you on what to expect. As you gain experience, it will tel you more and more. Yes we alswo will look stage by stage as well.

I suggest at first, just continue using your meter like always, but every time you use the meter, try then using the scope for the same reading. Obviously not for resistance, but when you measure he B+ voltage with a meter, then scope the B+. Set the scope input coupling to AC. Now you can see for example the ripple on that supply.

[cyclone]

Oscilloscope Testing of the Audio Amplifier.
This document has some drawings of various audio waveforms including some with parasitic oscillation.
If your scope is not two channel you should get another Its very interesting to look at the input, phase inverter output and at the speaker output, across the dummy load. I have four scopes, I hook at least two of them up with both channels scoping different points, you learn so much about the amp under test watching four points as you crank it into distortion

you definitely want to ground the scope probes, you'll pick up random stuff that may confuse you or send you down the wrong path.

[daz]

Thanks Cyclone. That page looks quite informative and i'll check it out further tomorrow.

Enzo, theres about 20VDC on the cathode. Is that too much? I have a tung sol RI there by the way which is supposed to be sensitive to high cathode voltages like in a CF. But 20v isn't close to what a CD has on the cathode.

[Enzo]

No, I would be concerned if it got over 100 volts at the very least. My point was not that you needed to be concerned here, I was merely pointing out that the plate voltage is rarely a concern. The cathode would be the limiting condition. That usually only comes up in cathode followers and sometimes phase inverters.

[daz]

No, I would be concerned if it got over 100 volts at the very least. My point was not that you needed to be concerned here, I was merely pointing out that the plate voltage is rarely a concern. The cathode would be the limiting condition. That usually only comes up in cathode followers and sometimes phase inverters.

Thanks Enzo. And of course thats a typo in my above post....CD was suppose to be CF, but you knew that. Any idea why the result was less OD for a given setting on the gain pot in the preamp and a more dynamic tone? I'm just real curious about that because it worked so well yet no one else does that. So i begin to think it's bandaid on something else. Here again is the schematic.