Ok, so i got my new O-scope. Now I have a very Noob question. is this right...

[jimmybjj]

...is it normal for the the square wave in a analog scope not to have any vertical lines in the trace? The first picture is with the probe at the calibration notch on the scope. The second is with the BK 3001 signal generator at 1k square wave. The third is the bk 3001 1k sine wave. thanks for any advice and most of all everybody's tolerance to my questions.



[img width= height=]http://lh5.ggpht.com/_yFex0loOVcw/S8ztG9LJURI/AAAAAAAAAcI/3o5ruiWfWzc/s800/IMG_0496.JPG[/img]

[jjasilli]

Yes, and I feel your pain.  Checkout my thread:  http://www.el34world.com/Forum/index.php?topic=8533.0

[tubesornothing]

That square wave is totally normal.  There are vertical lines, but they are very very faint.  This is normal.


OK, now the fun begins.  First a couple of rules:

(1)  when you connect your scope to a circuit, NEVER connect it directly to a tube plate or B+

(2)  before you connect your scope, always turn the X Scale (voltage) to the highest number (like 5V/Div or 10V/div)




OK, now for the first exercise.  In this exercise we will be setting up your signal generator to approximate a guitar signal (500Hz at 200mV Peak)

You will need a tube amp, your scope and a signal generator.

(1)  Turn your signal generator to 500Hz.  Plug this into the input of the amp.  Turn the volume of the amp right down.

(2)  Set your scope to 5V/Div, connect the scope to the GRID of the first preamp tube - we are going to measure the signal going into the first gain stage.

(3)  Turn the amp on. Let it warm up.

(4)  Adjust the scope timebase and vertical to get a nice sine wave on the scope.

(5)  Adjust your signal generator output so that the height of the top of the wave is 200mV - you may need to adjust your vertical to you still have a nice size wave on the screen.

Post  your results, and then we'll go on to the next exercise - measuring the output signal.

[sluckey]

is it normal for the the square wave in a analog scope not to have any vertical lines in the trace? The first picture is with the probe at the calibration notch on the scope.

Yes. Well, there really are vertical lines but your time/div is so slow you can't see them. Your time/div (AKA horizontal sweep speed) is set to 2 ms which is too slow to measure anything on that particular square wave. Turn the time/div clockwise (faster) until you are displaying only one or two cycles of the square wave. Then the vertical trace will be more visible. You'll also be able to measure the time of one cycle (and freq = 1/time). In fact, you can speed it up enough to actually see that the vertical line has a slope to it. This slope is called risetime on the positive slope and falltime on the negative slope. Risetime doesn't mean much in the audio world but it's very important in the radar world.

Take it slow and remember, a scope has two main functions... measure amplitude (vertical) and measure time (horizontal). In fact, your display is a graph that plots voltage and time.

[PRR]

> square wave in a analog scope not to have any vertical lines in the trace?

If the square-wave were PERFECT, the rise happens in ZERO time, so there is zero mark on the screen.

Even the imperfect square from a $1 calibrator will rise in very-small time and leave very-little trace. Absolutely normal.

What Sluckey said: Turn your sweep to get less than 20 cycles on the screen. One or two full cycles is often good. If you still have "invisible rise", then get one at the center of the screen. Hit your "Horiz X10 Mag" button, twist the H.Pos knob until the fractional trace jumps from top to bottom (a rise section). Now dial-up the brightness "INTEN". You'll probably see a faint rise.

If that still won't show the rise: put a 0.1uFd cap across the calibrator output. That will give slanty-waves with full trace. Now try 0.01u, 0.001u, 270pFd.... it gets less slanty and more square. With smaller loading-caps the rise goes up faster and fainter.

Now put your X10 back to normal (it's not much use in audio! this isn't radar) and dial the INTEN down till the trace fades-out, then up just until it shows well (excess brightness is not good).

[jimmybjj]

Awesome, thanks for all the great and educational responses! That's what makes this board so great. You guys put my mind at ease, i thought something was wrong.

tubesornothing, thanks for the great exercise! Thanks for giving me something to apply to my new scope. I won't get back to the house until the weekend so Its going to be a couple of days until i update this thread again, but I will update with plenty of pictures :)

One question, when i am doing the exercise how do i know when the top of the wave is at 200mv? Thanks again.

[sluckey]

how do i know when the top of the wave is at 200mv?

Use your scope to set the sig gen output. Connect the scope directly to the sig gen output. Set the scope volts/div (AKA vertical gain) to 50mv and set the time/div to give about 2 to 4 cycles. Adjust the sig gen attenuator so the scope displays four vertical divisions. This will be 200mV_pp or 100mV_p.

While you are at it, set the sig gen to approx 500Hz. 500Hz = 2ms. So, set your scope time/div to .5ms. Use the scope vertical position to place the waveform such that the positive peaks are 2 divisions above centerline and the negative peaks are 2 divisions below centerline. Centerline will now represent the zero crossing (baseline) for the sine wave. Now fine tune your sig gen until the displayed waveform takes exactly 4 horizontal divisions to repeat (one cycle). 4 divisions x .5ms/div = 2ms = 500Hz. This is the basic procedure for measuring time (frequency).

These steps will become routine for you anytime you use a sig gen. We basically just set your sig gen output to 200mV_pp @ 500Hz using the scope as your reference test equipment.

[jimmybjj]

how do i know when the top of the wave is at 200mv?

Use your scope to set the sig gen output. Connect the scope directly to the sig gen output. Set the scope volts/div (AKA vertical gain) to 50mv and set the time/div to give about 2 to 4 cycles. Adjust the sig gen attenuator so the scope displays four vertical divisions. This will be 200mV_pp or 100mV_p.

While you are at it, set the sig gen to approx 500Hz. 500Hz = 2ms. So, set your scope time/div to .5ms. Use the scope vertical position to place the waveform such that the positive peaks are 2 divisions above centerline and the negative peaks are 2 divisions below centerline. Centerline will now represent the zero crossing (baseline) for the sine wave. Now fine tune your sig gen until the displayed waveform takes exactly 4 horizontal divisions to repeat (one cycle). 4 divisions x .5ms/div = 2ms = 500Hz. This is the basic procedure for measuring time

Ok, I am confused. Please educate me  I followed your instructions the best that i could, I am still learning so I confuse things alot. I set everything as you said (i think, the picture should clarify if i wrong) but my meter says the sig gen is putting out 0.7v. According to the procedure you just typed out it should be at 200mv, am i miss understanding something? BTW what does V_pp and V_p mean?

[tubesornothing]

Vp - means "voltage peak" - measure from zero to the top of the waveform
Vpp - means "voltage peak to peak" - measure from the lowest part of the wave to the highest part.
Your Fluke meter on AC will measure RMS. which is about 0.7 the size of Vp.

Looking at your scope I think it say the vertical is at  50mV/div,  so the Vp is 100mV and the Vpp is 200mV

What is confusing me is your meter.  If your fluke meter was reading the true RMS of the wave, it should read about 0.070V.  So why is it reading 0.700V?  I think it is because the frequency is at 500Hz and that is goofing with the meter.  ANyways, I will now let PRR or sluckey correct my thinking.

[sluckey]

You set it up perfectly and everything is in agreement. The only confusion factor is that the Fluke 87 measures true RMS and the scope displays peak to peak. One other confusion factor is that you're using a 10:1 probe. Soooo....

The Fluke reads .7VAC. (I trust the Fluke over that scope any day)
The scope reads 200mV_pp which is 100mV_pk. But.... you're using a 10:1 probe, so the scope is really measuring 2V_pp or 1V_pk.

The voltage conversion factor for a sine wave is V_rms = V_pk x .707.

This means that your scope and your Fluke agree with each other. You're good to go.

When using the scope you need to train yourself to think in terms of peak to peak voltage. When using the Fluke, you need to think in terms of RMS (root/mean/squared) voltage. And you need to always be aware of the relationship between VAC_rms and VAC_pp when dealing with sine wave signals.

[tubesornothing]

ahhh 10:1 probe - that explains it!

[jimmybjj]

ok, i'm still trying to figure things out. I understand that x-axis is voltage and we set the scope for 200mv peak to peak. I don't understand how the number .7 comes about. If the y-axis is time, then how do we get 100vp from 0 to the top of the wave form. I though it was time not voltage. Another thing how does 100vp = 0.7v I am very confused. Can someone explain this or point me to a link that might explain this to me. Thanks for your patients guys.

[sluckey]

http://wiki.answers.com/Q/What_is_the_conversion_for_rms_voltage_to_peak_to_peak_voltage

[tubesornothing]

X axis is TIME.  Y axis is VOLTAGE.  Measuring from the horizontal 0 volt line to the top of the wave form is along the vertical Y axis - voltage.

First thing is you have your probe switched to 10x so what you are seeing on the screen is actually 1/10th of the voltage.  So if you have your probe turned to 1X (make sure to readjust your vertical to 0.5V/div) you will see the wave form will be 1V peak (or 2V peak-peak)

#2 your DMM does not measure peak AC voltage, it measures RMS AC voltage.  So its not going to measure the 1V peak.  RMS value of 1V peak is about 0.7V.  (read the link sluckey posted for a proper explanation of RMS).

hope that helps.

[tubesornothing]

Hope the picture here helps.

[HotBluePlates]

#2 your DMM does not measure peak AC voltage, it measures RMS AC voltage.

Well... it can measure peak volts, but the default mode is RMS and it has to be manually placed in the mode to read peak voltage. I have a Fluke 87III, so eveything looks identical, your meter might have some extra features or better accuracy.

Eveything else they're telling you is on the mark.

[jimmybjj]

Thanks guys, I know have a better understanding  tubesornothing, your pics helped alot i am visual learner. Whats funny is I am fairly new to this hobby, I have an obsessive personality and dive in head first, i probably have 8 or 10 books on tubes amps, and a couple on the way about oscilloscopes, and i didn't even realize that the multimeter i bought was "RMS" or for that matter even know there was a difference between RMS and a "standard" meter. Ok, know i understand what RMS is and how it is calculated, now how does standard meters calculate AC?

Ok, I was finally able to get around to my homework assignment, thanks tubesornothing  The first pictures show the sig gen setup how sluckly guided me to except the signal is going though the amp and being measured on the grid(pin 2) of v1. The  result, it looks exactly like it did before. I guess this makes since because its the same signal except traveling thought the guitar cable. Was this what i was suppose to do?



Well my first assignment wasn't very exciting so i decided i wanted to try something else. I attached the probe to the control grid of the 6v6 on my aa764 champ. The result is depicted below. It doesn't look as i was expecting, I was expecting a little more uniformity in the clipping (right or wrong?) Is this what its suppose to look like? Volume was on 10, all the way up.

[tubesornothing]

That looks just great to me!

Tube gain stages distort asymmetrically.  Saturation will be a soft rounded distortion, whereas cut-off will chop off the wave form.  Each gain stage flips the wave (except for CF).  This is the beauty of tubes.

Here is an article on saturation and cut-off distortion: http://www.guitarnuts.com/amps/g40v/index.php



CAUTION:  dont hook your scope directly to a plate or any high voltage source.  You will fry the scope.  If you want to see the signal after the plate, make sure it is after the coupling cap.

[PRR]

> now how does standard meters calculate AC?

ANY way they want to.

The most basic passive multi-meter (now obsolete) is a mechanical DC current meter. To read AC it uses a rectifier and a resistor. This actually reads "Average". But the scale is calibrated 1.111 times higher, so that the RMS of a sine-wave reads correctly. This actually gives a pretty-good approximation of "RMS" for most "reasonable" waveforms.

My VTVM uses a peak-to-peak rectifier, then scales it 1/2.828 to read "RMS" of sine-waves. On non-sine waves it gives numbers very different than the RMS.

A thermal-meter reads true RMS over some range. The RMS chips read true RMS if the waveform is not too spiky.

If you know the wave is a sine, any of these is fine.

If the wave is not a sine, you gotta ask: what do you really want to know?

If you want to run heaters, toasters, or incandescent lamps from a non-sine source, you want the RMS. This is the "heating power".

If you want to know if a musical signal will "fit" through an amplifier (without clipping), the RMS is not really important, you want the Peak-to-peak.

All this confusion is one reason why we usually use sine-waves for testing.

Your 'scope on 6V6 grid with BIG signal is correct. The 5-watt amplifier is attempting to put out 10W or 20W, and can't.

If you wanted a clean sound, you would play softer (try 20mV to input) and turn-down (try "5" on the volume control). Now the output should be very much like the input, only bigger, and (at the OT secondary) able to pull a heavy load (8 ohms).

[jimmybjj]

ok new questions :) what do you guys typically use the scope for? I don't need a detailed instructions on what you do (unless you feel like it) when you are, testing, debugging, experimenting, what do you typically do? how you guys use the scope. I have one, and have a basic understanding on how to use it, but I don't really know what to do with it now.

[tubesornothing]

I use it for a few things:

(1)  when analyzing gain stages of an amp I will input a 200mV 500Hz sine wave, then measure the signal as it goes through each gain stage.  I build amps, so this helps me ensure each amp within a model line is roughly the same as the next.

(2)  diagnosing oscillations. It is invaluable, often because oscillations are super sonic.

(3)  adjusting for blocking distortion

(4)  analyzing distortion when designing a new circuit.  Once you get use to how different types of distortion look, it helps with predicting what it will sound like.

(5)  impressing friends.

I am sure there is other stuff, I cannot recall it all right now.