Example 1: Lets say we are standing at the shore of a
completely smooth fresh water lake that is 10 feet deep all the way
across. Now lets say a huge 500 pound fat guy jumps off of a dock at the
other end of the lake. He creates a wave that is 5 feet higher than
the lake surface. The wave also dips down below the lake surface 5 feet.
You are on the other side of the lake and you can see the wave coming
towards you but you can only see the top 5 feet of the wave. This wave is
actually 10 feet high if you consider that the valley of the wave is
5 feet below the lake surface and the wave peak is 5 feet above the lake
surface. This wave is 10 feet high peak to peak.
The lake surface is like a constant DC voltage in an amplifier
power supply that measures 10 volts DC above chassis ground and the wave
is equivalent to a 10 volt peak to peak AC signal voltage riding on top
of a that steady 10 volt DC. The AC signal fluctuates the smooth DC voltage
level up and down to a 5volt DC low level and a 15volt DC high level.
(Theory). In a nutshell:
Take a look at the Typical 12AX7 stage in the diagram above. A tube/valve
amplifies a voltage by taking a very small AC voltage that is present on the
input control grid (pin 2) and makes the current fluctuate through the tube
from the cathode (pin 3) to the plate (pin 1). This fluctuating current
flows through a 100K plate load resistor and is turned back into a much
larger copy of the AC control grid voltage on the other side of the plate
load resistor. When a fluctuating DC voltage comes up against a coupling
capacitor the fluctuating AC level appears on the other side of the
capacitor but the smooth DC voltage component is blocked. Capacitors will
block a smooth DC voltage but will appear to pass the fluctuating AC voltage
component.
Details: The input
control grid is able to repel/hold back or attract/allow electrons to flow
across the vacuum gap between the cathode and the plate. The input control
grid is between the cathode and plate physically. The electrons are repelled
back to the cathode or attracted away from the cathode by the control grid
voltage. When the electrons see the higher voltage of the plate, they are
attracted to the plate, not the control grid. They pass right on by the
control grid and make a bee line to the plate.
The higher the negative voltage
level (more negative) at the control grid the more it can hold back or repel
the electron flow from cathode to plate. The lower the negative control
voltage is (less negative), the more the electrons are allowed to flow from
cathode to plate. Of course the AC control grid voltage is always
fluctuating up and down and so the current flow through the tube is
always fluctuating up and down following the sine wave of the control grid
voltage.
A DC current flows up through the 820
ohm cathode resistor from ground and into the vacuum tube, when the control
grid allows current to flow. The DC current flowing through the 820 ohm
cathode resistor produces a voltage drop across the top of the cathode
resistor. This makes the top of the cathode resistor some positive DC
voltage level with respect to the control grid and this sets the bias
point of the tube. If the cathode is some positive DC voltage then the
control grid is going to be a negative value with respect to the cathode
voltage. The control grid is tied to ground through a resistance. In our
12AX7 example above, the control grid ground resistance is part of the input
jacks. If you measure from pin 2 to ground you would get a resistance
reading of 1 meg. This is only if an open dummy jack is inserted into jack
number one.
If the DC voltage on at the top of
the 820 ohm cathode resistor is +1 DC volt, then we could conceivably
introduce a 2 volt peak to peak input signal to our negative control grid
without making the control grid go to some positive value. The control grid
should not go to some positive value technically. If the control grid is
positive with respect to the cathode, it will start to attract the electrons
that were on the way to the plate. If the control grid attracts too many
electrons, it will start to flow a current. Control grids are not meant to
flow current, they are only there to present a voltage potential. A control
grid that flows too much current will fail and the tube will fail.
A cathode biased amplifier stage has
the added benefit of being self biased. The more current the tube flows, the
higher the voltage drop across the cathode resistor. The higher the cathode
voltage, the more negative with respect to the cathode the input control
grid is. The more negative the input control grid is, the more it repels
electrons back to the cathode. This is kind of like a governor on an engine.
Example 2: The input control
grid and resulting tube current can be compared to a person that is holding
a garden hose that has a spray nozzle on the end of the hose. It takes just
a little hand pressure to squeeze the nozzle and this let lots of water flow
out of the hose. The nozzle handle is the control grid, your hand is the AC
voltage that is present at the control grid and the water flowing out of the
hose is the current flowing through the tube. I don't have an example of
what happens next. I guess the water stream could turn a wheel and be made
to do some work. This would be a small pressure from you fingers controlling
a much larger force.
The fluctuating current
through the vacuum tube flows from the cathode, through the vacuum and then
out the plate. It then flows through the 100K plate load resistor. The
fluctuating DC current appears on the other side of the 100K plate load
resistor as a varying DC voltage drop.
The .022 coupling capacitor is
connected to the plate load resistor. Capacitors do not pass smooth DC
voltage through them but an AC voltage component can appear on the other
side of the capacitor. The voltage is developed on the other side of the
capacitor because it is referenced to ground through some sort of
resistance. In our 12AX7 example above, the .022 coupling cap is connected
to a 1 meg pot that is connected to ground. There is a constant 1 meg
resistance from the .022 coupling cap to ground. The amplified AC control
grid signal is now ready to be passed onto another stage of amplification. |
