All About Audio Amplifiers.

Push-Pull Outputs.

Now we have reached the part that can make or break an amplifier
design. A poorly designed output can make an otherwise good amplifier sound
terrible. A unit which has been properly designed from phono input to power
output will give that wonderful tube sound that makes listeners mouths drop
open in amazement.

Why Push-Pull?
Why not. You can get more power out of two tubes
than one. You may be surprised to learn that you can get more than twice
the power out of two tubes in push-pull then you can get out of just one.
In simple terms it's like two people cranking a human powered winch. There
are two cranks, one on each side of the winch. The two cranks are 180
degrees apart and just when one man gets to that part of the circle where he
is lifting and pushing away on the crank (his weakest point) the other man
is pulling down and toward himself (his strongest point). It works much the
same way with two tubes in push-pull. They are working 180 degrees apart
and when one tube is in a condition where it is making the weakest
contribution to the power the other tube is at its strongest.

The other good reason is distortion. When distortion occurs harmonics
(integer multiples of the frequency) are created that weren't there before.
This gives music a highly unnatural sound. The new frequency which is twice
the original frequency is called the second harmonic, the one which is 3
times, the third and so on. What happened to the first harmonic? That's
the original frequency. Tubes in push-pull cancel even numbered harmonics
which were not part of the original input signal. The strongest distortion
product is almost always the second harmonic so if you cancel it out you
have greatly reduced the amount of distortion. Don't worry, I'm not going
into the mathematical proof of this cancellation.

Classes of Amplifiers.
You knew it had to happen sooner or later. I
have to tell you what the difference is between class A, AB1, AB2, B and C
amplifiers. So hang on tight because here we go.

Class A Amplifier.
The resistance coupled amplifiers we have already
studied are class A. In such an amplifier plate current flows over the
entire input wave cycle. A resistance coupled amplifier won't work right in
any other class. Single ended power amplifiers, such as those used in All
American 5 radios are always class A. The theoretical maximum efficiency is
50% but in practice, consider yourself lucky if you can get 20.
Theoretically the average, DC, plate current will NOT change from zero
signal to maximum power output. In a push-pull output stage practice often
comes very close to theory.

A single ended class A amplifier is a different story. Such an amplifier
can meet the conditions of class A but still show a considerable increase in
plate current from zero signal to maximum signal. This is because the tube
is not a perfectly linear device. When the plate current is high it will
increase by a larger amount for a given change in grid voltage as compared
to when the plate current is small.

In a push-pull amplifier this effect is almost un noticeable because the
nonlinearities of the two tubes cancel each other and produce a nearly
linear transfer function. Many audiophiles favor class A outputs because of
the lower distortion which results. However a well designed AB1 output can
sound just as good as a class A and run a lot cooler.

Remember the definition. Class A, the plate current of both tubes is
flowing uninterrupted over the entire cycle.

Class B Amplifier.
Hold it, what happened to AB1 and AB2? As you
may already know or have guessed class AB is a hybrid of pure class A and B.
After we learn what class B is then we will go back to see what those AB
classes are all about.

In ideal class B the plate current is zero when there is no input signal.
When the signal appears each tube conducts over half of the cycle. The two
tubes work alternately so the whole cycle is amplified. This assumes that
the plate current versus grid voltage is a perfect straight line going right
down to zero. Unfortunately, it is not. In order to avoid large amounts of
distortion as the task of amplifying is being handed off from one tube to
the other we have to allow some plate current to flow in the absence of
input signal. The average, DC, plate current for maximum power output can
be from 5 to 10 times the zero signal current. The class B amplifier that
was the modulator for my AM kilowatt used a pair of 810 tubes which are
power triodes. The idling current (plate current in the absence of audio
signal) was about 60 milliamps and the peak current for 100% modulation was
about 360 milliamps. (The plate voltage was 2000 volts.) The theoretical
maximum efficiency for a class B amplifier is 78.5 % but in the real world
expect about 60. In class B amplifiers the grid must always be driven
positive for part of the cycle. That means that the grid draws current over
that part of the cycle and a transformer must be used between the driver
tubes and the power output tubes instead of resistance-capacitance coupling.
A power amplifier with a transformer coupled driver is shown in Figure F
below.

Just as a side note, a very popular transmitter used by small,
250 watt, AM broadcast stations and made by Collins Radio company was a
pair of 810s for the RF power amplifier and another pair for the
modulator. Stations running this rig were easy to spot, they had a
terrific sound.

Remember the definition. Class B, the plate current of
each tube flows for approximately 1/2 of the cycle and the two tubes work
alternately.

Class AB1 Amplifier.
Class AB is intermediate between A and B. The
1 in AB1 means that grid current does not flow, the tube grids are not
driven positive. A class AB1 amplifier will produce much less distortion
than a class B amplifier using the same tubes. The distortion figures are
often comparable to class A. The rise in plate current from zero signal to
maximum power may be as little as 10 % to as high as 70 %.

Remember the definition. Class AB1, somewhere between A and B but the
grids are NEVER driven positive.

Class AB2 amplifier.

Class AB2 is closer to B than to A. The 2 means that the grids ARE driven
positive and grid current will flow. An output stage in which there is grid
current such as AB2 or B must be transformer coupled, as shown in Figure F,
instead of R-C coupled. If R-C coupling IS used the grid behaves like the
plate of a diode and rectifies some of the audio signal which makes the
grids more negative. Instead of class AB2 you would have a kind of moving
class AB1 and a lot more distortion than you had counted on. If you were to
attempt class B with R-C coupling, the extra grid bias could actually bias
the tubes beyond cutoff and lead to heavy distortion. The rise in average,
DC, plate current may be anywhere from 50 % to 200 %.

Remember the definition. Class AB2, somewhere between A and B but the
grids ARE driven positive and there will be grid current.

Class A2 amplifier.
Some amplifier designs use a type of operation
known as class A2. The tubes are running in class A but control grid
current flows over some part of the input cycle. Tubes such as the 811A, a
zero bias power triode, have to be operated in this mode because of their
nature. Some other tubes may be operated this way depending on the tube and
the load impedance.

Class C amplifier.
Class C amplifiers are never used in audio. This
section is given in the name of completeness. They are used as radio
frequency power amplifiers because of their high efficiency, approximately
80 %. They conduct plate current over 125 degrees, or less, of the input
cycle. A resonate L-C circuit restores the wave to the familiar sine wave.
The efficiency is so high because the tube conducts current only when the
instantaneous plate voltage is low and is cut off when the plate voltage is
high. That means the tube dissipates little power and most of it goes to
the antenna.

Cathode Bias or Fixed Bias.
In cathode bias the cathode current of
both tubes is allowed to flow through a resistor to ground which develops a
positive voltage on the two cathodes. As the grids are held to ground
potential by the resistors they are actually negative with respect to the
cathode. For a more detailed discussion of cathode bias see Resistance
Coupled Amplifier, a basic building block. Cathode bias is used with
class A amplifiers ONLY. A cathode biased amplifier is shown in Figure A
below. Engineers disagree about using a capacitor to bypass the cathode
resistor. Some feel that leaving it off improves balance of the circuit and
reduces distortion. Others will say that the amplifier will deliver more
power at less distortion when being pushed to the limit. You pays your
money and you takes your choice.

For a verbal description click
here.

In Figure A the outputs from the
driver, phase inverter or phase splitter are capacitively coupled to the
grids of the two output tubes. As the signal on the grid of one tube goes
positive the signal on the grid of the other tube goes negative. The plate
current of the first tube increases while the plate current of the other
tube decreases. B+ is applied to the center tap of the transformer primary
so the steady state plate currents are flowing in opposite directions in the
primary. As long as the currents are equal there is no net magnetic field
from the primary coil. When the currents begin to change a changing
magnetic field is produced. Because the currents are flowing in opposite
directions an increasing current on one side has the same effect as a
decreasing current on the other side. The changing magnetic field induces a
voltage in the secondary winding which is connected to the voice coil of a
speaker.

In relatively low power tubes such as 6V6s or 6L6s the screen voltage can
be the same as the plate supply voltage and the points labeled "plate
supply" and "screen supply" may be connected together. In many amplifiers
the screen grid voltage passes through an additional filter section so the
two are not quite the same. In higher power tubes such as 6550s or 6146s
the plate may run at 600 volts while the screens may be at 200 or 300 volts.
For results to please someone with golden ears the voltage to the screen
grids of any tube type should be regulated in some way.

In a fixed bias circuit, shown in Figure B, the cathodes are grounded and
the power supply has been designed to develop a negative voltage which is
applied to the grids through the resistors which formerly went to ground.
Because the amount of bias voltage is not effected by the plate current, as
it is with cathode bias, fixed bias should be used when the tubes are to be
run in class AB1. Except for that the circuit operates the same as
described above.

Triode Connected Amplifier
Because many people feel that pentodes
produce higher levels of distortion than triodes, the triode connection is
sometimes used. The people who designed and sold tubes made a ton of beam
power pentodes but almost no power triodes. There doesn't seem to be much
between the 6AS7G/6080WA (10 watts) and the 810 (600 watts. There is the
811A but that's a zero bias triode so even a class A amplifier using this
tube would have to draw grid current.) To make a pentode act like a triode
you connect the screen grid to the plate as in Figure C..

For a verbal description click
here.

The major drawback seems to be that
the maximum screen voltage now becomes the maximum plate voltage. Depending
on what tube type you have that may cause a considerable reduction of power.
Another disadvantage is that the triode connected tubes will require more
driving voltage. If the driver is not well designed the improved distortion
from the triode connection may be canceled out by an increase in distortion
from the driver stage.

The Ultra Linear Connection
Figure D is a compromise between triode
and pentode connection. It's called the "ultra linear connection". Like
any compromise it has all of the advantages of both along with all of the
disadvantages of both. Its major disadvantage is that it requires a special
output transformer. Not only that but the transformer has to be designed
for optimum performance with a particular tube type.

The paragraph which used to occupy this
space put a knock on the ultra linear configuration. The reason was that I
was performing the tests with tubes of the 6L6 family. This tube does not
perform very well in ultralinear connection. The EL34 family was designed
for this configuration. I have designed and built such an amplifier which
is described elsewhere
on this site. I listen to it every day.

The Cathode Loaded Output Circuit.
The cathode loaded circuit was
described in an article titled "Amplifiers and Superlatives by D. T. N.
Williamson and P. J. Walker Wireless World, Sept. 1952, pp. 357-361. At
this time in history there seems to have been some transatlantic sniping
over ultra linear versus cathode loading. I used to side with the British
on this one. Williamson and Walker found that the so called ultra linear
connection produced more distortion and delivered less power than a similar
conventional pentode connection. Evidently they were using 6L6 tubes for
their tests. The cathode loaded circuit they described looks like this.

For a verbal description click
here.

The circuit above uses two
identical output transformers. That could get expensive but the alternative
is to get a custom made transformer which is likely to be even more
expensive. So you pays your money and you takes your choice. If you want
to take the time and have the design skill I would recommend the custom made
route. A custom made transformer would have another center tapped winding
similar in appearance to the plate winding.

Some mistakenly believe that the only purpose for cathode loading is to
apply negative feedback to the cathodes of the output tubes. I myself once
was a member of that misinformed group. It is called cathode loading
because part of the load is placed in the cathode circuit of the tubes. In
an amplifier where the impedance of the cathode winding is equal to the
impedance of the plate winding half of the power to the load comes from the
plates and the other half from the cathodes. Such an amplifier was sold in
the 60s.

There is an element of negative feedback just as there is in a cathode
follower. This is no doubt responsible for the reduction in distortion
claimed for these amplifiers. I have to say "claimed" because I have not
tested this configuration for myself.

You might think that the feedback effect on distortion could be obtained
by using separate cathode resistors and leaving them unbypassed.
Transformer coupling the signal back to the cathodes is NOT the same thing.
Unbypassed cathode resistors give current feedback as the fed back voltage
is proportional to the cathode current, which is almost the same as the
plate current, of the tube. With transformer coupled feedback the fed back
voltage is proportional to the output voltage of the transformer. Current
feedback increases the output impedance while voltage feedback reduces it.
And as pointed out above feedback is only a part of the reason for using
cathode loading. Power is being delivered from the cathodes to the load.
In the case of equal impedance windings, half of it.

I have never heard a push-pull cathode loaded amplifier but I have a 1956
Silvertone tape recorder, still working, that uses a single ended cathode
loaded 6V6 output circuit. It sounds better than the average single ended
6V6 amplifier.

Local feedback.
As I will discuss in detail in the next section I
believe that it is possible to have too much overall feedback around an
amplifier. Local feedback (around just one stage) should be used as much as
possible. In fact the ultra linear and cathode loaded circuits discussed
above are a form of local feedback. Figure E below shows another form of
local feedback around the output stage. The resistors go from the plates of
the output tubes to the plates of the driver tubes. This eliminates the use
of an additional DC blocking capacitor. The two sets of plates are not at
exactly the same DC voltage but they are close enough.

For a verbal description click
here.

Transformer coupling.
If you want to run a large amount of power you
will need to operate the output tubes in class AB2 or B. These classes of
operation require the use of an input transformer as shown in Figure F.
Audiophile equipment would never be designed this way. Even the output
transformer is almost too much for golden eared listeners. Another
transformer is over the line. You would find this only in ultra high power
PA amplifiers and modulators for AM transmitters.

Stop the presses, this just in.
People who view this site are
constantly thinking. I received an email message suggesting that direct
coupled cathode followers could be used as drivers and bias supply for a
class AB2 or B output stage. I built and tested it. To view the circuit click
here then use your back button to return. You will recognize the phase
splitter. It's output goes to a push pull stage which I found in a 1950
tube manual for driving 6AS7G triodes. They need a lot of drive voltage and
that's what this circuit gives. The original circuit used a 6SN7. I used a
6CG7 which is a 9 pin mini version of the 6SN7. When you look up a 6CG7 in
a tube manual you are referred to the data for a 6SN7. The signal is
capacitively coupled to another 6CG7 where each section is a cathode
follower which is directly coupled to each grid of a pair of 6146s. A pot
in the grids of the cathode followers sets the bias and therefore the idling
current of the 6146s. This circuit is to be the modulator of a 200 watt AM
ham transmitter I am building.

Understanding Tube Manual Information.
Tube manuals give you lots of
information on how to operate power tubes. The problem is that there is so
much extraneous data and someone who is not an electrical engineer may have
difficulty sorting out what is important and what is not.

The best way to explain what is what is by example. The problem is that
I can't just scan a page from a tube manual, post it and then tell you what
it means. Copyright law you know. What I will do is to give data on a
fictitious tube. Let's call it an XII124. I'm not going to give everything
you are likely to find, jus what is important. Unless you are the type of
person who likes to solve puzzles I don't recommend you spend a lot of time
looking over the fake tube manual data below. Give it a quick look and then
scroll on down to the explanation below.

Tube Type XII127

Beam Power Amplifier

Heater Characteristics

Heater Voltage
........................................................................

12.6

volts.

Heater Current
........................................................................

1.25

Amperes.

Maximum Heater-Cathode Voltage

Heater Negative with Respect to
Cathode .....................

200

volts.

Heater Positive with Respect to
Cathode ......................

100

volts.

Maximum Ratings

Triode

Pentode

Connection

Connection

--------------

--------------

Plate Voltage
......................................................................

350

750

volts.

Grid No. 2 Voltage
.............................................................

--

350

volts.

Plate Dissipation
.................................................................

75

75

watts.

Grid No. 2 Dissipation
........................................................

--

8

watts.

Grid No. 1 Resistance

Fixed Bias
..................................................................

100 k

100 k

ohms.

Cathode Bias
..............................................................

470 k

470 k

ohms.

Characteristics and Typical Operation.

Class A Amplifier

Triode

Pentode

(Single Tube)

Connection

Connection

-----------

---------

-----------

---------

Plate Voltage ......................
..................

350

400

600

750

volts.

Grid No. 2 Voltage ...............................

--

350

250

350

volts.

Grid No. 1 Voltage ...............................

-30

-20

-16

-25

volts

Peak A F Signal Voltage .......................

30

20

16

25

volts

Plate Current (Zero Signal) ....................

105

125

100

95

mA

Plate Current (Max. Signal) ...................

115

135

110

105

mA

Grid No. 2 Current (Zero Signal) ...........

--

5.0

2.5

2.5

mA

Grid No. 2 Current (Max. Signal) ..........

--

7.3

4.7

7.0

mA

Transconductance ................................

4700

6000

5300

5200

umhos.

Plate Resistance ....................................

1700

22500

35000

33000

ohms.

Load Resistance ...................................

2000

2500

4500

6000

ohms.

Power Output ......................................

5.0

11

20

32

watts

Total Harmonic Distortion .....................

5

10

12

16

%

Push-Pull Amplifier

(Two Tubes)

Class A1

Class AB1

Class AB2

--------

--------

--------

--------

--------

--------

Plate Voltage ..............................

250

400

400

600

600

750

volts.

Grid No. 2 voltage ......................

250

350

350

350

300

350

volts.

Grid No. 1 Voltage .....................

-15

-20

-25

-30

-30

-30

volts.

Peak A F Grid to Grid Voltage ...

30

40

50

60

75

100

volts.

Plate Current (Zero Signal) .........

200

150

100

50

50

55

mA

Plate Current (Max. Signal) ........

220

165

250

125

250

350

mA

Grid No. 2 Current (Zero Signal)

30

12

8

5

5

5

mA

Grid No. 2 Current (Max. Signal)

40

18

12

9

12

17

mA

Transconductance (Each Tube) ..

5500

5700

5000

5300

4500

4000

umhos.

Plate Resistance (Each Tube) .....

24500

23500

26000

25500

28000

29000

ohms.

Load Resistance (Plat to Plate) ...

3600

7200

4800

16000

8000

7400

ohms.

Power Output ............................

21

24.5

50

75

100

175

watts.

Total Harmonic Distortion ..... ....

2

2

2

2

2

2

%

What It All Means.
The name "Beam Power Amplifier" refers to the
geometry inside the tube. Instead of an actual grid used as the suppresser
it is a gate that forms the electrons into a beam as they head for the
plate. This concentration of electrons forms a negatively charged region
between the screen grid and the plate which suppresses secondary emission.

There is usually some mechanical data, size and shape. If you have the
tube in your hand and you know it will fit into the space you have planned
for it, you don't need to bother with this data. You will also find data on
interelectrode capacitances. This data is of vital interest to someone
designing a radio transmitter but it is mostly irrelevant to audio
designers.

Heater Characteristics.
Then there is the data on the heater. You
can see that for our XII124 it is 12.6 volts at 1.25 amps. I'll go ahead
and state the obvious. You need a transformer that has among its other
windings one that delivers 12.6 volts and will supply enough current to run
as many of these tubes as you intend to use, 2.5 amps for two, 5 amps for
four and so on. The data on maximum heater to cathode voltage is useful if
the tubes are going to be used in some application where the cathodes will
be at some high potential from ground, such as a voltage regulator. In an
audio amplifier using fixed bias the cathodes will be grounded so just skip
it.

Maximum Ratings.
The maximum ratings data tells us how far we can
push the tube before it is in danger of being damaged. The only thing you
might find puzzling are the plate dissipation and grid No. 2, screen grid,
dissipation. The plate has a voltage from it to the cathode and there is a
current. This means power and power means heat. The plate is the place
where the power is dissipated. That is where the electrons impact. their
kinetic energy has to go some where so it is turned into heat. The only way
the plate can get rid of this heat is by radiation.

Time to debunk another popular myth. Some people think you
can run a tube at higher power if you invert it and submerge all but the
base in a pan of water. The water cools the glass envelope but does that
do the plate any good? After all, the plate is where most of the power
has to be dissipated. Remember the three ways heat is moved from one
place to another? They are conduction, convection and radiation. For
conduction to take place between two objects the two must be in physical
contact. Look at any handy tube. The plate is inside the glass envelope
not touching it. For convection to take place there must be a gas or
liquid between the two objects. They're not called vacuum tubes for
nothing. More correctly, nothing is why they are called vacuum tubes. A
vacuum is nothing so there is no gas or liquid just nothing between the
plate and the glass envelope. That leaves only one, radiation. The
slight cooling of the envelope, in terms of absolute temperature, makes no
significant difference in how much heat the plate can radiate. So, if you
want a 200 watt amplifier, use 200 watt tubes.

Actually, the plate of a tube CAN get rid of energy in
another way; it can deliver it to a load. Look at the first entry under
"Characteristics and Typical Operation" "Class A1 Amplifier (Single Tube)".
The plate voltage is 350 volts and the zero signal plate current is .105
amps. That makes 36.75 watts and the plate of the tube has to radiate all
of it into space. Because the power is coming from the power supply and
going to the plate of the tube it is called plate input power. Now let's
look at the maximum signal condition. The voltage is still 350 volts but
the current is .115 amps. The input power is now 350 x .112 = 39.2 watts.
But that's not the end of it. the tube is delivering 5 watts to a load.
That is power that the plate does not have to radiate. Of that 39.2 watts,
5 watts goes to the load and 34.2 watts is converted into heat and radiated.
The efficiency is the load power divided by the input power times 100 %. 5
/ 39.2 x 100 % = 12.8 %. I told you class A amplifiers were inefficient.

Except for triode connected tubes the screen grid (grid No. 2) does not
deliver any power to a load. It has to dissipate all of the DC power it
absorbs from the power supply. The wires of the screen grid are fine but
they are positively charge so any electrons that are on a course that will
take them near one of the wires can have their paths bent enough to impact
on the wire. That is why there is screen grid current. Exceeding the
maximum grid No. 2 dissipation rating can destroy a tube.

There is a maximum amount of resistance you can have between the control
grid (grid No. 1) and ground or the negative bias supply. This maximum
resistance depends on whether fixed bias or cathode bias is being used.
There is no such thing as a perfect vacuum. The vacuum of space in low
earth orbit is not nearly as good as that at the distance of the moon's
orbit. Even so this low earth orbit vacuum is still considerably better
than that within a vacuum tube. Some of the gas molecules become positive
ions and are accelerated towards the cathode by the electric field. They
are neutralized when they meet up with the space charge surrounding the
cathode. Before that happens a few can impact the grid imparting a positive
charge to it. A more positive grid will increase the plate current.
Increased plate current can increase the number of ions which imparts more
positive charge to the grid. If the resistance in the grid circuit is too
high this can turn into a runaway condition which can ultimately destroy the
tube. The manufacturer specifies a maximum grid resistance to ensure that
this runaway condition never gets started.

When cathode bias is used the tube has a built-in regulator. If the grid
starts to build up a positive charge the increase in plate current will
cause the voltage at the cathode to become more positive. This diminishes
the effect of the positive going grid. Because this happens relatively
slowly a bypass capacitor will not interfere with this self protective
effect. This self protection means that a large grid resistor can be used
without the risk of damaging the tube.

Some experimenters are using zener diodes and even light emitting diodes
in the cathode circuit to produce bias. In these circuits the bias voltage
changes by a very small amount as plate current is changed. When this
circuit configuration is used the bias should be treated as fixed bias and
the lower value of grid resistance used.

You occasionally may find other entries such as maximum plate current.
If it is there, stay within it.

Characteristics and Typical Operation.
This is the really useful
part of the data. Everything is pretty much self explanatory except for one
thing, the plate resistance. I can remember being confused by this myself.
The plate resistance is NOT a resistor you have to put in the plate circuit,
it is a parameter of the tube. It is defined as the change in plate voltage
divided by the change in plate current while the voltage on all grids
remains constant. This number is useful to electrical engineers and others
who may know how to use it. Just to fill in the rest of the picture the
transconductance is also a tube parameter and is defined as the change in
plate current divided by the change in grid voltage while holding the plate
voltage constant. There are calculations that can be made using these
parameters but they are not very accurate in output amplifiers which are
known to engineers as large signal amplifiers.

The one that is important to you is the load resistance. You must order
an output transformer that will match the stated load resistance to a
speaker voice coil. Using the wrong output transformer can give you more
distortion and less power than you expect. The transformer must also be
able to handle the power you want. You can't push 50 watts of power through
a 25 watt transformer.

You may be disturbed by the large amounts of distortion you find stated
for all tube types. Who wants to be listening to an amplifier that has 2
percent distortion. Here's a sneak preview of the next section. It's all
done with negative feedback. If you put enough feedback on an amplifier to
lower its gain by a factor of ten, a distortion figure of 2 % becomes .2 %.
Lowering the gain by a factor of 30 will reduce the 2 % distortion to .066
%. Since gain is easy to come by an overabundance of it is always designed
into any amplifier so it can be traded off for lower distortion.

Next;
Overall Feedback, Pros and Cons.

Previous;
Phase Inverters, Phase Splitters and Drivers.

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This site begun March 14, 2001

This page last updated October 17, 2010.