UNDERSTANDING & USING
YOUR (analog) OSCILLOSCOPE

Many Internet tube 'gurus' will vicariously demonstrate their prowess servicing a vintage tube guitar amplifier utilizing nothing more than a Fluke DMM. They make no mention of using old-fashioned test equipment like a VTVM, a dummy load, or even a signal generator. Mention the word 'oscilloscope' to any of them, and you are answered with a sneer. Mention anything about biasing your output tubes utilizing an oscilloscope, and the rest of your day will be spent listening to complaints about how 'wrong' it is to do so. Ignoring all the canard, let's see what little I can 'teach' you about what an oscilloscope is, and how to use it to your benefit. Towards the end of this 'Lesson', I will demonstrate one method of biasing your output stage, using a number of analog oscilloscopes (and other test equipment) I purchased during the last year specifically for this purpose. While I do not have the deepest of pockets, I did spend considerable amounts of cash over the previous twelve months just for this 'Lesson'. I spent a lot of time getting acquainted with my 'new' test equipment, and I believe I now have the minimal required experience and insight few 'gurus' possess. I will now attempt to pass along some of this acquired knowledge. The oscilloscopes used will be of varying ages and qualities; the point will be to show that the quality and condition of your oscilloscope will definitely make a difference in the outcome of your 'work'. With that out of the way, let's get to it!

The oscilloscope is of course based on the cathode-ray tube, which displays the electrical signals in graphic form. It is probably the most widely-used test instrument because it can be used to observe waveforms as well as measure voltage, time, frequency, and phase angle. Below is a simple drawing demonstrating a basic oscilloscope cathode-ray tube.

Oscilloscope CRT is a simple device in and of itself.

There are opposing vertical and horizontal deflection plates inside the CRT, which is not evident in the drawing above. The electron beam is centered, and without any voltage is applied to the electron gun, we end up with a simple 'dot' on the viewing screen. The drawing below should make this a little clearer.

Oscilloscope CRT has opposing pairs of deflection plates.

One vertical deflection plate and one horizontal plate are typically 'grounded', and the deflection of the electron beam itself should obviously be dependant on the applied voltage to the electron gun and the opposing deflection plates. Any signal to be observed is typically applied to the vertical deflection plates, and a sawtooth waveform (to be explained later) is applied to the horizontal deflection plates, in order to accomplish 'sweep'. The drawing below will demonstrate that quite nicely.

Deflecting the beam from the electron gun isn't a difficult task.

If you are still following along, so far so good. We are almost done with the dry theory part. In normal oscilloscope operation, the beam is horizontally deflected from left to right across the screen at a certain rate. This sweeping action produces a horizontal line or trace across the screen. The actual sweeping of the beam is accomplished by the application of a sawtooth voltage across the horizontal plates. The rate at which the sawtooth goes from negative to positive is determined by its frequency. This in turn establishes the sweep rate of the beam. All oscilloscopes have provisions for selecting various sweep rates. A pictorial example of how a sawtooth voltage results in horizontal sweep is seen below.

Achieving horizontal sweep is not very difficult.

Now that we have dealt with the mundane (but necessary) theory, let's continue, and learn the controls on a typical oscilloscope. Keep in mind that while a wide variety of oscilloscopes are available, all have certain operational features in common. Whether it is a relatively simple instrument with limited capabilities, or a sophisticated model that provides a variety of optional functions and precise measurements, these 'most common' front panel controls operate identically. How more advanced features may help us will be discussed later.

INTENSITY

This control varies the brightness of the trace on the screen. Caution should be used so that the 'Intensity' is not left too high for an extended period of time. Damage to the screen can result from excessive 'Intensity'.

FOCUS

This control focuses the beam so that it converges to a fine point on the screen. An 'out-of-focus' condition results in a trace that is not sharp, but rather fuzzy.

HORIZONTAL POSITION

This control positions the neutral horizontal deflection of the beam. This can make for more convenient viewing or measurement of a voltage.

VERTICAL POSITION

This control positions the 'crossover' point (or 'reference' point, or 'zero' point) for easier measurement or observation of the waveform. Utilizing a dual-trace oscilloscope, for example, we could not clearly note each waveform if they both referenced to the same 'zero point'.

SECONDS/DIVISION

This sets the horizontal sweep rate. This is also called the time base control, and selects the time interval that is to be represented by each horizontal division in seconds, milliseconds, or microseconds.

VOLTS/DIVISION

This sets the number of volts to be represented by each division on the vertical scale. This allows most waveforms, regardless of their voltage, to be displayed conveniently within the entire screen.

AC-GROUND-DC

This switch allows the input signal to be AC coupled, DC coupled, or Grounded. The AC coupling eliminates any DC component of the input signal. The DC coupling permits DC values (or 'offsets') to be displayed. The Ground position allows for a zero-volt reference to be established on the screen.

TRIGGER

This allows the beam to be triggered from various selected sources. This triggering of the beam causes it to begin its sweep across the screen. It can be triggered from an internally generated signal derived from an input signal, or from an externally applied trigger signal. The modes of triggering are most often Auto, Normal, and TV. In the Auto mode, sweep will still occur in the absence of an adequate trigger signal. In the Normal mode, a trigger signal must be present for the sweep to occur. The TV mode provides triggering of a frequency that is a harmonic of a Television sync signal. There will be a Slope switch, which will allow the sweep to begin on either the positive or negative slope of the trigger waveform. The Level control sets the voltage level on the trigger signal at which the triggering will occur.

With all that out of the way, what makes one oscilloscope superior or inferior to any other? This will all depend on the application. The first 'spec' everyone knows to ask about is the bandwidth. In the good old days, the oscilloscopes were crude, and had a limited bandwidth. There was no need for anything fancy, until Television came along. Here we need to observe certain waveforms that have a frequency of approximately 4.5MHz. Before this, the highest typical frequency to observe was the 455KHz IF in the radio receivers of the day. Therefore, it is very common to find really old oscilloscopes with a bandwidth of 500KHz or 1MHz. With the advent of Television, oscilloscopes now had a typical bandwidth of 4.5MHz or 5MHz. Eventually, 10MHz oscilloscopes became the most prevalent. For audio work, almost any oscilloscope will suffice, but the higher bandwidth models usually also came with a few other 'upgrades' that makes them far more preferable. My advice is not to settle for anything less than a 20MHz bandwidth oscilloscope to use as your '#1'. The reasons are as follows.

Older oscilloscopes are chock full of drifting resistors and leaky capacitors.
Older test equipment utilizes 'banana jacks', which can be troublesome. At right, we see adaptors are available.
Interpreting oscilloscope waveforms is an art in and of itself, especially when a simple probe is not so simple.

There are many brands of oscilloscopes available, and it quickly becomes a jungle out there differentiating between any two. Here is a quick synopsis of a few of the brands I see at Ham Radio Swap Meets and on eBay.

I have left out many brands of oscilloscopes here. These would include Hameg, Hewlett-Packard, Hitatchi, Leader, and a few others. This does not mean they aren't worth considering. What this means is that I simply do not own more than one, nor do I have the resources to 'try a few out' before I pass any opinions along. Owning one or even two Hitatchi oscilloscopes (which I do) is not enough to form a valid opinion and pass it along to you. Once I have used the aforementioned oscilloscope enough to understand their quirks, only then will I gladly pass along any observations. The opinions listed above are 'first impressions', and are subject to change at the drop of a hat.

KICKING THE TIRES

What I hope to do here is show you a handful of oscilloscopes I recently purchased. I will demonstrate these oscilloscopes 'in action', as I attempt to bias a few different vintage tube guitar amplifiers. Let's meet the oscilloscopes, and the amplifiers. Also, while it should seem obvious, I'll also have to reveal the rest of the test setup. These items include the following.

THE TEST EQUIPMENT
A selection of good, reliable signal generators isn't necessary, but it makes experimenting a little more fun.
You'll need some variety of 'dummy loads' to effectively test amplifiers. Shown above are just two examples.

Lastly, while it should seem obvious, to some it may not be as plain to 'see' the proper setup arrangement for the whole enchilada. That would be shown below, in my best artwork. This is a simplified setup, and geared towards your particular amplifier. I'll discuss an 'advanced' version in a second or two.

Test setup is not elaborate, nor complicated. This is a good thing, because I wouldn't have been able to explain it.

The 'key' points are to keep the leads as short as possible, and to have coupling 'losses' at a minimum. This is achieved by using the proper cables, and the necessary adaptors where needed. If you plan to do a lot of bench work, a semi-permanent setup is advised. I use a shelf above the bench itself, with a panel utilizing all the jacks and switches I need. From there, it is one cable from the signal generator to the amplifier, and one cable from the amplifier to the panel. Everything else is 'hard-wired' behind the panel, to keep any losses as insignificant as possible. This is far neater than having a bench top that resembles a plate of spaghetti. I have multiple dummy loads, that can be switched in or out of the circuit, and an AC VTVM hardwired to the dummy load switch! This lets me 'see' at a glance the output power of the amplifier, or the gain factor of any modification/extra stage/FX Loop/etc. I may be experimenting with. However, I really don't think many of you are going to go that far.

THE AMPLIFIER(S)

The amplifiers I chose are listed below, and the dedicated 'page' can be accessed by clicking on the appropriate 'link'. This was done to keep loading times as brief as possible. Select the amplifier most similar to what you are interested in learning how I dealt with it. There is nothing more to add, so buckle up, and pay attention.

AA1164 Princeton Reverb. This combo was chosen for numerous reasons. There is also an anticipation that this will be a very interesting exercise. For starters, there is not a lot of choices when shopping for a 6V6 today. Secondly, there is no real 'adjustment' possible for the bias voltage setting! There is only a very narrow range of resistors to choose from, and hopefully have them act as a voltage divider and supply the 'proper' bias voltage. In other words; expect a lot of surprises on this trip.

AB165 Bassman Head. This head was chosen for a few reasons. It is a popular/common amplifier, and I had a chance to try a few mod ideas seen elsewhere on this website. However, for the purpose of biasing and demonstrating the results, the amplifier was left stock except for replacing the bias 'balance' circuit with a conventional bias circuit. *WARNING* I demonstrate a lot of different oscilloscopes, so it will take a few minutes for all of the images to load. Please be patient, and you will be rewarded!

50-Watt Marshall JCM800 Head. This head was chosen to demonstrate two things. First, biasing any Marshall is a matter of personal taste. Secondly, biasing many EL34 amplifiers is a lot more complicated than biasing 6L6 amplifiers.

 INTERESTING WEBSITES; WHERE TO LEARN MORE ABOUT OSCILLOSCOPES

Here are a few websites you can visit, and learn a little more about oscilloscopes. There are also of course vintage electronics text books, but I'll concentrate specifically on the websites (for now). There are actually quite a few oscilloscope websites out there, but many caused my computer to crash(!), or were erratic in loading. The websites listed below all passed my 'no hassle' test.

CATHODE-RAY OSCILLOSCOPE TRAINER is absolutely recommended for the fact that you can play with an analog oscilloscope 'on-line'! Adjust controls, and see how they affect the waveform. Hook up different signals, use the 'Chop' function, etc. This is for beginners that feel intimidated in asking a live person how to use an oscilloscope.

OSCILLOSCOPE-TUTORIALS.COM has a name that says it all. You will get a lot of dry theory, but you will learn, damn it.

USING AN OSCILLOSCOPE is an interesting British website. You even get a few little experiments thrown in for good measure. The Hameg oscilloscope used for demonstrative purposes won't be too common on this side of the big pond, and there is no 'virtual' training, but it is still a pretty nifty site. You should understand all of the oscilloscope controls a little better after a good visit or two.

VIRTUAL OSCILLOSCOPE is one of the cooler virtual oscilloscope 'trainers'. You have to download the program, and have 'Flash Player', but it is definitely worth it. After you 'click-and-drag' various signal sources to be hooked up to the oscilloscope trainer, you set controls, and watch the display. Another 'beginner' website, but still one of the most explicit learning experiences available.

*NEW!*

Hey! Get a glimpse into the way things really went down over at the Fender laboratory! If you have any curiosity at all about what test equipment Fender felt was 'good enough' for them, CLICK HERE!

 SOURCES

1) Probes For Test Instruments, by Bruno Zucconi and Martin Clifford, 1957.

2) The XYZ's of Oscilloscopes, published by Tektronix, 1985.

 

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