How Class A1 (normally called Class A), AB1, B, and C Operation Work and Differences.
Class A2 means grid current flows over a portion of the cycle
I thought it would be informative to discuss the differences between
Class A, AB1, B, and C operation. By doing such, one will more fully
understand how each component in your system operates.
Knowledge is power and the more you understand, the less chance
of being misinformed. I am going to keep this discussion as simple as
possible for our newbie friends. I will not cover every detail nor every proof.
However, there will be information presented that has never before been
mentioned, let alone discussed, as far as I have seen.
Caveat: Lets leave out transformers from our discussion.
Note: It might be good idea to print out all the figures at the bottom
of this post to examine while reading.
So let's get started.
What is a sine wave? A sine wave is a constantly varying voltage. Figure 1 is a
pictorial of one 360 degree sine wave. 120 vac at the wall outlet is
basically a sine wave.
So is music made up of sine waves? The answer is yes.
Although looking at a musical signal with an oscilloscope might look
haphazard, with sharp peaks, those sharp peaks are simply varying
high frequencies. Even a solo instrument's signal might look haphazard
due to natural harmonics from the instrument.
It will be easier to understand the different classes of operation if we use
a single frequency sine wave as pictured in Figure 1. As mentioned earlier,
the entire signal is one complete sine wave, 360 degrees. Half of a sine
wave is 180 degrees. One fourth of a sine wave is 90 degrees, one eighth
of a sine wave is 45 degrees etc.
Class A operation.
Suppose we have a single vacuum tube and we have it drawing
current (idle current, Point Q of Fig. A1) with no signal present. Now we apply
the input signal to the tube's grid and the output appears as X and Y
output in fig. A1. Notice X and Y has the same shape, the whole wave
as the input sine wave signal.
Very important. The only requirement of Class A is 360 degree conduction.
There is no center of the Load line requirement or any other requirement or consideration.
The input voltage applied to the tube grid controls the current flowing
through the tube to the plate. It is similar to the foot peddle (grid) controlling
the output of an engine (plate current).
Virtually all phono stages, pre-amplifiers, input and phase
splitters in amplifiers are operated Class A. The next/following
tube stage presents a fairly constant load. That is good news.
Let us continue for tubes operated in Class AB1, B, and C.
Will all classifications work in linear audio applications? Class B is
generally for PA systems, low quality audio and Radio Frequency (RF).
Class C is usually for RF, and industrial applications.
Class AB1, B, and C are defined as operating a single tube when the
current through the tube can be stopped, cut off, meaning 0 ma.
(ma is milliamps) at different portions of the waveform. So what is the
difference between AB1, B, and C operations?
First, we need to see something significant in figure Fig. A1 below, Class A operation.
It has to do with the tube's idling current at the Q point, which, in this
case is set to 65 ma, half way between 0 ma and maximum 130 ma. in our
example. Notice we can go 65 ma. to 0 ma. and 65 ma to 130 ma.
For an output stage, we generally want maximum audio power output,
although any idle current could be used for Class A as long as the tube
conducts 360 degrees. (In fact, less distortion is present if we up the idle
current above 65ma, but output power will be less.) Back to our discussion.
Above and below current differences are equal. So X and Y are equal output
and mimic the input signal, except larger amplitude. We need to understand
Class A operation, 360 degree conduction, as it allows us to understand
Class AB1, B, and C operation correctly. Please re-read if necessary.
Let's bypass fig. AB1, for now.
Let's jump to fig B, Class B operation/mode. Notice the Q point is different.
It is not 65 ma idle current but now 0 ma idling. We still have the same exact value
input signal, but only X appears at the output, Y being absent.
Only half the input signal is at the output. What happened to the other half?
Q point is set at 0 ma. As the signal goes positive,
more current flows through the tube, so X output appears due to more current.
However, how can we go less than 0 ma. current as the input signal
voltage goes negative and the tube stays shut off? We cannot. Thus no Y output
signal voltage. Thus only ½ of the input signal appears at the output (180 degrees).
Again, this is a classic definition of Class B operation, higher efficiency,
higher power output, higher distortion.
Class B presents severe distortion to the input signal, and is generally
used in RF and industry. It can be used in audio if we go Push Pull, but it
will produce crossover distortion, higher distortion in general, so is mostly
used in PA systems etc, where fidelity is not important.
A couple of points.
1. The crossover distortion one sees in Class B, Class C operation
is caused by the output transformer. The tubes are easily capable of millions of hz,
so the output tubes are not the cause of switching problems. It is the mismatch of
the output devices and steep slope of the output waveform that causes the problem.
2. The gradual slope of the output signal, and slower increase of the plate current in the
off tube in Class A and AB1 eliminates any crossover distortion. If present, extremely low
value.
Fig. AB1 operation is between Class A and Class B.
Let us check out fig. AB1 operation. Once again we have our input
signal sine wave, and X and Y output voltage. However, we have
some Y output sine wave signal present. Notice, however, the tube's idle
current, Q is between our Class A and Class B Q points, 65 ma and 0 ma
respectively.
In our AB1 example, the idle current is set to 45 ma (could be 55ma, 35ma). Ok,
as the input signal is increased from no signal, X and Y output rise equally,
Class A operation, until the negative input signal causes the tube current to reach 0 ma.
or cutoff. At that point the tube cannot go less current, so Y signal cannot continue to follow
the negative input signal.
So what good is it if X output signal becomes larger than Y? How about adding
a second output tube which mirrors the first tube, except it
handles the negative portion of the input signal. Then X and Y output sine wave
mirrors the input sine wave signal. They naturally blend together when properly designed.
That is called Push Pull AB1.
So is there any advantage in designing Push Pull? If designed properly, efficiency is much
higher than class A, much more power output with much less harmonic and intermodulation distortion
is produced. There is no notch distortion that Class B produces.
Very important. Up until each tube at cutoff, both output tubes operate in Class A mode.
Each tube conducts during the whole 360 degree cycle.
A quote from RCA Radiotron Designers Handbook, 1960, by 26 electrical engineers.
(The engineers checked each other's work.)
"A Class A amplifier is an amplifier in which the grid bias and alternating grid voltages are such
that the plate current of the output valve or valves flows at all times. The suffix 1 indicates that
grid current does not flow during any part of the input cycle."
One can also eliminate the inherent negatives of a class A output stage.
See below *.
For example, a 6L6GC, beam power tube in AB1 mode can produce
55 watts rms output in Class AB1 operation. In Triode mode, we can figure
about half the power output of beam power mode. That is approximately
30 watts output. The tube operates in Class A mode for at least 7.5 watts
output before sliding into AB1 operation.
Even at 1 watt Class A output, a typical speaker can at least peak
into the mid 80s+ spl, depending upon the efficiency of one's speakers.
And the harmonic distortion is extremely low. My entire KT88 amp
produces only 0,05% at 1 watt output, with no global or stage to stage
negative feedback.
Ok, we have discussed Class A, AB1, and B operation. Let us check out
fig. C1, Class C operation.
The first thing one notices is that Q idle is below 0 ma. How can that be?
Notice the perforated line to Q. What is actually pictured is the grid bias
is so negative that less than half, in fact, a very small portion of the input
signal is even large enough to cause plate current flow through the tube. Thus X
appears to be small and Y does not exist at all. A larger, huge input signal
must be presented to obtain lots of power output in Class C mode. The tube
conducts less than 180 degrees. Usually much less.
The plus is that the efficiency can reach 80%, but the minus is that
the distortion is huge. Class C operation is usually for radio
frequencies (RF) and Industrial applications.
So what have we learned?
A. Class A is used in virtually all small signal applications since the load is relatively constant.
B. Class AB1 Push Pull and A are used in most output applications.
C. In Class AB1, both output tubes X and Y run Class A until each
tube reaches 0 ma. cutoff on positive and negative peaks of the
input sine wave cycle.
D. The damping factor (DA) is virtually constant over the entire waveform. Not so with SETs,
which varies from low damping to no damping factor at plate cutoff, maximum power output.
See * below.
E. There is no crossover distortion, no notch in AB1 operation.
F. There is a smooth blending in properly designed Class AB Push Pull stages.
G. Class B is used as Push Pull, almost exclusively for PA systems, RF, and in industrial.
It is virtually never used in high fidelity audio components.
H. Class C is never used in linear analog audio designs.
I. 120 hz power supply hum is virtually eliminated in PP operation.
* For a single output tube amplifier, different considerations apply.
For instance, we want the amplifier's output impedance (Z) to be
constant with varying power output and over the entire signal cycle,
360 degrees. To accomplish this, the tube's plate resistance (Ra) must
remain constant.
However, Fig. D shows the Ra line of a typical single
ended triode tube varies/curves drastically as the current changes.
Of course as the current changes, the output power also changes.
At peak power output, the damping factor varies from
maximum damping of the SET amplifier design to virtually no damping
near cutoff. Push Pull remains virtually constant under the same conditions,
so the PP amplifier controls the driver.
There are other pros and cons that we might discuss later.
I hope this has helped in understanding how Class A, AB1,
B, and C amplifiers work.
One can check out:
RCA Tube Manual
RCA Radiotron Designers Handbook, 1960, written by 26 electrical engineers.
College textbook "Semiconductor and Tube Electronics" by James G Brazee
And other electrical engineering textbooks.
steve
