The 12AX7 is a nine pin miniature, twin triode tube.
Twin triode means it has two separate tubes inside
one glass envelope. Each triode has three
electrodes: plate, grid and cathode.
At pin 1 is the Plate or Anode.
At pin 2 is the Grid.
At pin 3 is the Cathode.
The other triode is pins, 6-Plate, 7-Grid, and 8-Cathode.
There is a heater filament between pins 9 and 4,
and another between pins 9 and 5. The heater circuit is often omitted from circuit
diagrams, it is not considered to be a working electrode as it plays no part
in the audio circuit. To wire the heaters for 6.3 volts (heaters in parallel) you tie pins
4 & 5 together. |
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The Plate is connected to the high voltage B+
through the Plate load resistor.
The Cathode is connected through a Cathode resistor to ground.
The Grid is where the AC signal comes in, a Grid stopper resistor is usually connected
here.
The first electrode is called the cathode and it
gives off electrons easily when heated. Near the cathode is the heater
filament, pass current through the heater so that it gets hot and the cathode also
get hot.
A
second electrode
called the anode is added, if the anode voltage is made positive relative to the cathode,
it will attract electrons from the cathode and current will flow. If the anode voltage is
made negative relative to the cathode, electrons will not be attracted to it and no
current will flow. Being a one-way device with two electrodes, the tube is called a diode.
The current flowing between the anode
and cathode is called the plate or anode current (Ia). The voltage measured between
anode and cathode is called the plate or anode voltage (Va). The supply voltage is
called the B+ or HT.
When anode voltage is low it has
a low influence over anode current, as anode voltage is increased the anode becomes more
effective at drawing current from the cathode, until drawing all the current the cathode
is capable of is called saturation. As the B+ is increased the current in the diode
will also increase, so for every value of anode voltage there is a corresponding value of
anode current. These points are plotted on the static anode characteristic
graph. |
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A third electrode, called
the control grid, is added between the anode and cathode. With three
electrodes the tube is called a triode.
If the grid is made positive
relative to the cathode, electrons from the cathode will be drawn
towards its electric field, but because the grid is very fine most
of them will fly straight through the gaps and be captured by the
electric field of the anode.
If the grid is made negative relative
to the cathode, it will repel electrons, they stay at the cathode,
restricting anode current. If it is made very negative then the
anode current will be cut-off altogether.
Because the grid is much
closer to the cathode than the anode, a small change in grid voltage
has a much more powerful effect on anode current than a similar
change in anode voltage
For every value of anode voltage
there is now a whole range of anode currents.
These are plotted as grid curves on the static anode characteristic graph. |
If we apply a varying audio signal voltage to the grid then current
in the valve will vary too. By putting a resistance (plate
load resistor) in series with the tube as the current changes, the
voltage drop across this resistor changes, generating a
corresponding audio voltage at the anode.
To shows us any possible voltages and currents in the circuit, we will draw a
load line on the static anode characteristics chart. Anticipating the B+ voltage supply to the preamp tubes, at node C,
to be around 270 volts and using a plate load resistor of 91K. |
If the tube is cut-off so that no
anode current flows across the plate load resistor there will be no voltage drop, so the
full B+ (270volts) will reach the anode.
This point can be plotted on the graph: Va = 270v, Ia = 0mA.
With the full B+ dropped across the plate load resistor and none across the valve, using
Ohm's Law, Voltage / Resistance = Current, the anode current would be 270v/91k = 2.96mA.
This point can also be plotted at: Va = 0v, Ia = 2.96mA.
Draw a straight (purple) line between the two points, completing the load line. For any grid voltage value along
the load line, there is a corresponding value of anode voltage and anode current.
These are the only combinations that are possible in this circuit.
A small change in grid voltage causes
a change in anode current, which causes a large change in anode voltage. Where the
-1v grid line intersects with the load line there is 154 volts at the anode. Where
the -2v grid line intersects with the load line there is 211 volts at the anode.
The grid
voltage has changed by 1 volt but the anode voltage has changed by 57 volts. The gain of
this circuit is 57/1= 57
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Cathode biasing is when a resistor
is placed in series with the cathode and a voltage develops across it, raising the cathode voltage.
Instead of making the grid negative with respect to the cathode, the cathode is made positive with respect to the grid.
To choose a resistor that will set the cathode voltage to 1.5 volts, it will set the grid voltage to –1.5v.
On the –1.5v grid line the Current is .95mA. (1.5 volts / .95mA = 1.57K). The Cathode resistor needed is 1.57k.
For a -2v grid voltage bias point the graph indicates that Current is .65mA. (2 volts / .65mA = 3K).
The Cathode resistor needed is 3k.
Suppose that I have a 2.4k resistor I want to try.
Voltage/Resistance = Current. 2 volts / 2.4k = .83mA
. Mark this point on the -2 volt grid line.
1.5 volts / 2.4k = .62mA. Mark
this point on the -1.5 volt grid line.
Draw a (green) line between these two points where this line intersects the load line will be the bias point.
Adding a cathode resistor to the circuit will alter the load line, the cathode and plate load resistors are now both in series with the valve.
Because the value of the cathode resistor is so small it will not make much difference. 270v/(91k+2k) = 2.90mA.
At -1.8 volts the bias point is set close to centre biased with the anode voltage being around
201 volts. The AC input signal voltage swings symmetrically around this bias point, from the bias point the up going and down going
voltages are equal. It swings up 1.8 volts to 0 volts and down 1.8
volts to -3.6 volts.
But the anode voltage swings asymmetrically around this bias
point. The down going voltage is (111volts) 201volts down to 90volts on
the 0 volt grid line, and the up going voltage is (69volts) 201volts up to 270
on the -3.6 volt grid line, the full B+.
The output waveform is bent out of
shape (distorted) and no longer a perfect sine wave. This bending of the waveform
introduces new frequencies into the audio signal. These new frequencies are called
harmonics and are integer multiples of the original audio frequency. Triodes,
therefore, can have a pleasant warming effect on tone because they produce a decaying
series of all harmonics, dominated by the second. Because the waveform is compressed on
one side and stretched on the other, it is no longer equal above and below the bias
point. The average value of the output waveform is not at the bias point, but has an
average DC offset that can shift the average operating point of the tube dynamically with
signal level, having a compressor like effect.
Calculating Second Order Harmonic Distortion =
(AB-BC/2x(AB+BC)) x100.
AB is the difference between A & B(111volts), BC is the difference between B & C.
A=Plate Voltage where grid Voltage = 0, B=Plate Voltage at Bias Point,
C= Maximum Plate Voltage.
An AC sine wave swings both positive and negative
relative to an average grid voltage or bias point. If a large enough input signal is
placed on the grid, the triode will be overdriven into either anode current cut-off
Clipping or grid current Clipping.
Drive the grid negative enough and the tube will reach cut-off and stop conducting.
( no Current will flow) The grid voltage may
continue to swing more negative, the tube will remain in cut-off and
the top of the output wave will be clipped. Biasing a tube closer to
cut-off results in less power dissipation and is called cool
biasing.
When the grid voltage approaches the cathode voltage, electrons being
drawn from the cathode get attracted to the grid rather than the anode causing a forward
grid current into the grid. This current causes a voltage drop across whatever resistance
happens to be in series with the grid, making it hard to drive the grid past 0V. The more
we attempt to make the grid positive the more current flows into it and prevents us from
doing so. It is actually the grid signal that is being clipped, while the tube continues
to amplify what appears on its grid normally. Grid current
limiting(bottom of the output waveform is clipped) occurs when we try to drive
the grid positive, beyond 0v. The more resistance in series with the
grid the greater the voltage drop caused by this grid current, and the harder and more
abrupt the clipping. Biasing a tube closer to Grid current Clipping results in more
power dissipation and is called warm biasing. Centre biasing offers maximum headroom
before the signal is clipped.
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A Static Anode Characteristic Graph with a 91k and a 120k load line.
The difference between plate load resistors is the smaller load resistance produces a
steeper line and more harmonic distortion. While a larger load resistance will increase gain and voltage swing.
91k load line, B+ 270v.
Total Tube Series Resistance 95k, 270v/95K = 2.85mA
Gain 210v-152v = 58
Cathode resistor = 1.75v / .8mA = 2.2k.
Voltage Swing = 270v-88v = 182v
Bias @ 196 volts
Second Order Harmonic Distortion = 108-74/2x(108+74x100 = 9.3%
120k load line, B+ 270v.
Total Tube Series Resistance 125k, 270v/125K = 2.15mA
Gain 203v-140v = 63
Cathode resistor = 1.75v / .67mA = 2.6k
Voltage Swing = 270v-72v = 198v
Bias @ 187 volts
Second Order Harmonic Distortion = 115-83/2x(115+83)x100 = 8.1%
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page info resources from a highly recommended book
Designing Valve Preamps for Guitar and Bass By The
Valve Wizard
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