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
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
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.
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.
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.
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.
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
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.
This control positions the neutral horizontal deflection of the
beam. This can make for more convenient viewing or measurement of a
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
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
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.
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.
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
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 oscilloscopes almost always have a 'single trace'. While
this isn't important to many, it does make it almost impossible to
compare phase angles between the input and output of a coupling
circuit, as an example. I say 'almost' because you may be able to find
an electronic switcher, which was devised for just such a purpose.
Still, a dual-trace oscilloscope is just infinitely more versatile.
- Older oscilloscopes usually do not calibrate their vertical
amplifier or their horizontal time base. Many real 'dinosaurs' do not
even feature a graticule on the CRT. This makes it hard to
measure voltage or time. While this isn't really important, it can come
- Typically, all 5MHz (and less) oscilloscopes will utilize vacuum
tubes. While this may seem 'cool', these oscilloscopes will always have
a set of quirks and headaches waiting for you. I haven't seen many
10MHz oscilloscopes that used vacuum tubes. One notable exception will
be the 'laboratory grade' Tektronix models. More on these
later. But for now, vacuum tube oscilloscopes will have leaky
capacitors (that affect the sawtooth waveform), as well as resistors
and tubes that drift. It can be fun to restore these classic pieces,
but the upstart technician is better off to take on these 'fun'
projects later on during his education. Until you learn how to
interpret the waveform presented to you, new oscilloscopes will often
have a waveform that is sharper, clearer, and more accurate because the
components inside aren't fifty-years old, and are much more stable.
utilizes 'banana jacks', which can be troublesome. At right, we see
adaptors are available.
- Older oscilloscopes routinely utilized 'banana jacks' for their
input connectors. The 'average' technician will not be able to use a
low capacitance shielded cable for his measurements, and this can add
some instability or noise to the signal being measured. Also, it is
most likely that the capacitance of your 'home brew' test leads will
seriously affect the waveform you see1.
The astute technician knows that there are adaptors available today
that allow a modern probe with a BNC connector to be used in these
instances. The banana jacks are almost always spaced 3/4" apart, and
the people who make the adaptors know this. The adaptors are available
to convert from banana jacks to a BNC connector, an RCA jack, or even a
1/4" female jack (perfect for signal generators). Those pesky banana
jacks are on a lot of older test equipment, so please have a few
adaptors handy. Using unshielded cable for the signal generator and
the oscilloscope is just asking for a lot of 'hash' on top of your
oscilloscope waveforms is an art in and of itself, especially when a
simple probe is not so simple.
- Along with the solid-state technology, the 'modern' oscilloscopes
will also often have a higher accelerating voltage for the CRT. This
yields a trace with superior intensity and sharpness. They will also be
much more sensitive, giving more accurate waveform readings. As a
digital analogy, think of this as comparing 8-bit to 16-bit or 32-bit.
A complaint common to users of older B&K oscilloscopes is
their rather 'ambiguous' trace at higher Television frequencies. My
manual for the B&K 1470 oscilloscope lists the accelerating
voltage as 1600VDC. I had one old-timer mention that he preferred to
use his B&K oscilloscope with the room lights off, so he
could really observe the trace as he was checking TV waveforms!
My advice is avoid B&K and (those rebranded) Kenwood
oscilloscopes as your #1, because of this quirk. Unless, of course, the
price is just too irresistible. They do make an adequate back-up unit,
and work well for using as a curve tracer. For comparison; cheaper Tenma
oscilloscopes list their accelerating voltage as 2KVDC, some cheaper Telequipment
oscilloscopes list their accelerating voltage as 3.5KVDC, the
'intermediate' Tenma oscilloscopes list their accelerating
voltage as 12KVDC, and some Tektronix oscilloscopes list their
accelerating voltage as 32KVDC! Which do you think has the
brighter, more accurate trace?
- Along with higher accelerating voltages, you can also expect a
faster edge speed or rise time. Lesser quality oscilloscopes could not
keep up to faster edge speeds, because these generally require higher
currents to produce them. Higher currents tend to cause 'ground
bounce', especially on wide busses in which many signals switch at
once. Moreover, higher current increases the amount of radiated
magnetic energy and with it, crosstalk2.
This means a re-design of the circuit boards, and not just slapping
more B+ into the circuit. It also lead to the much-improved digital
operating system for oscilloscopes (DSO). For analog oscilloscopes,
higher accelerating voltages means clearer, more accurate waveforms.
This did not go unnoticed until higher accelerating voltages were
'discovered'. Many magazine articles and books were written, advising
the technician how to interpret wave forms, including those that did
not appear exactly as expected. Other 'culprits' can include lack of
calibration, as well as the probe itself.
- Much like an automobile, newer oscilloscopes simply have less
mileage on them. This means they should last for quite some time yet.
Of course, there are 'lemons' out there, just as with automobiles.
There is nothing wrong with owning any oscilloscope, but be
aware that until you learn its quirks and idiosyncrasies, your readings
may not be accurate or meaningful. Much like the venerable tube tester,
the astute technician learns how to interpret what he believes the
oscilloscope seems to be telling him. You are essentially buying an
electronic Ouija board, until you shell out the serious dollars for
'laboratory-grade' test equipment. Until you get a good feel for using
test equipment, start small, and work your way up to the 'big-league'
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.
- B&K should be avoided unless relatively new or the
price is extremely attractive. As mentioned before, they usually have a
lower accelerating voltage, and therefore an 'ambiguous' trace at
higher frequencies. For simple audio sine-wave viewing, they will
suffice nicely. They do make an adequate backup unit, and seem to hold
up to constant usage. The older Model 1470 is very
popular, and is a good, basic dual-trace 10Meg oscilloscope. The Model
1564 is much newer, has a 60MHz bandwidth, and is more preferred.
Other models include the Model 1541 (40MHz), and the Model
1580 (100MHz). Avoid the Model 1450, the Model 1465,
and the Model 1466; unless you can get them for about $40. When
they do break down, they can make great door-stops, thanks to obscure
Japanese transistors that are difficult to replace. Lastly, older B&K
oscilloscopes (including the 1470) 'feature' a UHF connector
(properly called a PL259) for the probe. This makes it hard to use just
any old probe you may have, unless of course you have the handy-dandy
adaptor. These adaptors are available, and they work just swell, so
this isn't a major concern. It is still something to consider.
- Bell & Howell oscilloscopes (made by Heathkit)
are best describes with three words; (1) avoid, (2) avoid, and (3)
avoid. Vacuum tube units like the Model 34 have a trace that
makes a sine wave look almost like a sine wave. These were
mostly school-training units, and I wonder how many students got
discouraged about electronics after using one of these oscilloscopes.
For the nostalgia, or for $5 or $10, you can have one of these as a
conversation piece; to show how the 'good-old-days' weren't always so
- Eico is one of those 'oldies' names you'll see frequently
at the Ham Radio Swap Meets. They made everything from battery
eliminators to stereo receivers to tube testers, so finding an
oscilloscope really isn't out of the ordinary. Their very common Model
460 (4.5MHz) isn't anything to write home about, but does work
reasonably well. Of course, this is only after you've replaced every
capacitor and resistor inside. As a beginner's oscilloscope, it will
do, providing you do not pay more than about $30 for it.
- Heathkit is another one of the more popular 'oldies' to
find. Quite a few vacuum tube oscilloscopes like the OM-3, the 10-12,
or the 10-30 out there. You know my feelings about any
oscilloscope that uses vacuum tubes; if you can get it for cheap, go
ahead. But, be prepared to spend your days getting it into tip-top
shape. And, when all is said and done, the 900VDC accelerating voltage
can still make you squint until you get a headache. A 'step up' is the
solid-state single-channel IO-4530 (10Meg), but this is still
just a 'so-so' beginner's oscilloscope. You can do a lot better, and
for about the same amount of cash to lay out.
- Kenwood are usually rebranded B&K
oscilloscopes. The CS-5350 is a good oscilloscope (50MHz), but
for $1,300 (list price today) I would want something that could also
butter my toast and sign my kid's report card.
- RCA is another of the 'oldies'. The old vacuum tube based WO-33A
or WO-91B may look like a classic, but getting one to work
'properly' can be a chore. They look impressive on your vintage
workbench when friends drop by, but as an 'everyday' oscilloscope, you
can do much better. I own a WO-33A, and it isn't my 'go-to'
oscilloscope, by any stretch of the imagination. Aside from the 4.5MHz
bandwidth, the teeny 3" screen, and the non-calibrated graticule, the
built-in clip-lead probe simply doesn't inspire confidence.
- Tektronix would have to be the Hickok of the
oscilloscope world. They were very expensive in their day, and command
high prices on eBay today. There are very old units using
vacuum tubes, and more modern solid-state units. Good vacuum tube units
include the Model 310 (very useless for anything other
than audio work because of a 3.5MHz bandwidth, but it is still a solid
unit) or the Model 506 (an excellent 23MHz unit,
although it uses plug-in modules that may be hard to locate), while the
Model 453 (50MHz) is one of the best 'basic' solid-state
units out there. Also look for the Model 465 (100MHz), the Model
454 (150MHz), the Model 475 (200MHz), and the absolute
'Cadillac' in the Tektronix line of analog oscilloscopes; the Model
485 (350MHz). Most Tektronix oscilloscopes have a high
accelerating voltage, and a very nice trace that is clear and accurate.
However, even Tektronix made a lemon or two, so check out your
prospective purchase very carefully.
- Telequipment is simply the British Tektronix. What
is strange is the Telequipment oscilloscopes I have or have
seen seem like they were made from a kit. They are not made to a
heavy-duty standard like the Tektronix, but are still around,
and they usually still work fine. The Model D52 and Model
S51e are very good and very basic, but aren't my first choice;
least of all because of their banana-plug inputs. The Model D61a
is a better bet; still a basic 10MHz unit, but it seems to work well.
The trace appears fine, partly due to its 3.5KVDC accelerating voltage.
And better still is the Model 67a. This is a 25MHz
oscilloscope, with a 10KVDC accelerating voltage. The trace is very
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.
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.
A selection of
reliable signal generators isn't necessary, but it makes
experimenting a little more fun.
- Audio generators. This a whole 'Lesson' in and of itself, but for
now I chose to use either an RCA WA-504A or a B&K E310B.
The RCA unit is a very well-made unit using MOSFET's, with a
THD (Total Harmonic Distortion; a relative
indicator of how 'pure' the sine-wave is, or the ratio of distortion to
sine-wave present in the signal) figure of less than 0.25%. The B&K
unit is another excellent audio generator, with a very clean and
accurate waveform. You may have an older tube unit like the Heathkit
AG10 or an Eico 377. If the unit is well maintained,
whatever you have will suffice. While the square wave function is ideal
for testing the frequency response, distortion, and tone control
operation, I simply used the sine-wave function. I chose a 1kHz signal,
with a measured output of 250mVAC. This simulates the output of an
'average' single coil pickup, and at a frequency somewhere in the
middle of the range to the combination of the guitar/amplifier/speaker
you'd probably be using. The test parameters were not chosen to
try and obtain a predetermined outcome; I'm a little older than that,
some variety of 'dummy loads' to
test amplifiers. Shown above are just two examples.
- Dummy loads. You have to use one, so how do you choose? Do I have
to go with a non-inductive load? Since I wasn't sweeping the audio
range, this does not matter. Do I go with 7.5-ohms or exactly
8-ohms? Since the rated speaker impedance is a nominal figure, the
value I use is not as important as some may argue. I have tried various
setups; two 15-ohm resistors paralleled for 7.5-ohms, and two 4-ohm
resistors in series for a total load of 8-ohms. The results were
identical. Therefore, I won't mention exactly what I use,
simply because it isn't important. Use what you can get your hands on,
as long as the power rating is about four-times the rated power of your
amplifier. Often I used 150-watt resistors, available from people like Mouser.
They'll still get warm, but not as hot as using a 50-watt resistor to
test your Bassman. Make sure your resistors are mounted to a
solid 'base' and have adequate ventilation. If I plan to leave the
amplifier running into a dummy load for extended periods of time, I
will also have a small fan mounted near the resistors.
- Volt-Ohm-Meters. You need to measure the bias range, and the
output power of your amplifier. I use digital VOM's and VTVM's. Use
what you have, as long as you are cognizant of the fact that
your readings will only be as accurate as your meters. However, the
exact reading isn't crucial. We are more interested in knowing that the
output power we have, and our bias voltage/plate current, seem
correct, rather than the exact value down to the microvolt.
- Tube testers. I checked the NOS tubes, and the new tubes, for
'balance' in the 12AX7 halves. I also chose output tubes with a gm
reading that read within 5% of each other. The tube testers used were a
Hickok 752A, a Triplett 3423, and a B&K
747A. I chose these testers thinking that they were popular on eBay,
and very likely to be similar to what the aspiring technician is using.
- Tubes. I decided to bias up the amplifiers with at least two
different sets of output tubes, to compare if optimum bias settings
were much different with different tube brands. I could have chosen new
tubes from Sovtek, Svetlana, Groove Tubes, or JJ,
as well as NOS American tubes I had in my collection. Output tubes were
not 'burned in' before the testing, but the amplifier was left idling,
played for some time, and the bias was checked again. I understand that
after the amplifier has been 'out in the field' for a while, the bias
will possibly need to be tweaked, but this is not the issue at hand
today. We are interested in setting the bias as quickly as we can, in
an effort to keep the 'professional repair shop' moving along
efficiently. For those of you at home, with all the time in the world,
you can perform this procedure as you see fit. Burning in the tubes is
not a bad idea, and it will save time later.
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
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
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.
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
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
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.
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.
has a name that says it all. You will get a lot of dry theory, but
you will learn, damn it.
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.
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.
Hey! Get a glimpse into the
way things really went down over at the Fender laboratory! If
have any curiosity at all about what test equipment Fender
'good enough' for them, CLICK
1) Probes For Test Instruments, by Bruno Zucconi and Martin
2) The XYZ's of Oscilloscopes, published by Tektronix, 1985.