Let's start at the beginning with the tube's glass shell which holds a vacuum. A
vacuum is needed to keep the electrodes from burning or oxidizing--the same
reason a vacuum is
needed in a light bulb (a light bulb is actually a single electrode tube!). The vacuum also keeps the electrons in the tube from
colliding with air molecules which would generate noise.
Edison discovered while working on the light bulb that if you
put a wire in a vacuum tube and heat the wire (light bulb filament) electrons
would "boil off" into the vacuum. The kinetic energy of the hot,
in the hot wire actually knock electrons off the wire into the tube vacuum. He also discovered
that since opposite electrical charges are attracted to one another he could put a positively charged '
inside the tube to collect the free electrons and create a current from
the hot filament ( cathode) across the gap to the plate. This is the
'Edison Effect,' which acts as a one-way electronic valve--heat the filament and electrons flow,
remove the heat and the flow stops. It's a one-way valve because the plate is
not heated so electrons can't flow from the plate to the filament. This is why
vacuum tubes are referred to as 'valves.'
Later someone discovered you could put an electrically charged
metal screen, or '
grid,' between the cathode and plate to block the
electron flow. Because like charges repel, a negative charge on the grid (lots
of excess electrons on the grid) repels the negatively charged free electrons
trying to flow from the cathode through the grid to the plate. But a positive
charge on the grid (scarcity of electrons) allows the free electrons to pass
through the grid. If you fluctuate the electric charge on the 'control grid' you
fluctuate the current flow between the cathode and plate. This effect is used as a
signal amplifier by applying a low level guitar signal to the grid which controls the
much larger flow of free electrons between the cathode and plate (the plate is also
called the anode--next time you want to insult someone call them an 'anode' :D ).
With 400 volts on the plate the electrons move at about 24 million miles per
hour from cathode to plate (yea, wow).
At this point the tube is acting as a current amplifier.
on the tube's control grid are amplified into large current changes on the
tube's plate. Remember voltage is 'electronic pressure' and current is the
amount of electrons flowing. We will discuss later how the tube's load resistor transforms the
circuit from a current amplifier to a voltage amplifier.
leak resistors 'leak' off unwanted DC voltage created when free electrons from the
cathode hit the grid. Ridding the grid of these electrons prevents a grid
voltage buildup which would affect the grid-to-cathode bias voltage.
An important concept to understand with tube electronics is that
the plate brings in high voltage DC to power the tube and simultaneously carries
the amplified AC audio signal out. The AC audio signal 'rides on top' of the DC.
This is called 'DC offset.'
AC Audio Signal With DC Offset
The AC audio signal rides on top of the tube's high voltage DC.
How the Load Resistor Works
Once again referring to the
Vacuum Tube Preamp diagram above the load resistor
is shown connected to the tube's plate.
The load resistor changes the amplifier circuit from a current amplifier to a
voltage amplifier. The load resistor allows a small flow of current from the tube's plate to make a
large change in voltage thus creating a
voltage amplifier. The load resistor restricts the flow from the 250v DC power supply
to the tube's plate. When the tube flows negatively charged electrons to the
positively charged plate the voltage
drops across the load resistor and between the load resistor and plate. If you slow the flow of electrons from
cathode to plate the
voltage drop across the load resistor will decrease causing a voltage rise on
the plate. Stop the flow
completely and the voltage on both sides of the load resistor will equalize at
250 volts. If there's no current flowing there will be no voltage drop across
the load resistor.
One way to
describe how a tube is used as a voltage amplifier is with a water hose analogy.
Think of the high voltage power as water faucet pressure and the wire running from the
power supply to the tube plate as a water hose. You can simulate the load resistor
by clamping the hose partially shut. Water flows through the clamp but at
reduced pressure. The water pressure is the voltage, the water flow is the current
and the clamp is the load resistor resistance.
You have high
pressure from the faucet and when water is flowing the clamp reduces the
pressure beyond the clamp. There is a pressure drop across the hose clamp. Now
put your thumb over the end of the hose--your thumb
simulates the preamp tube's control grid. Close off the flow with your finger
and you have no water flowing and the water pressure will rise and equalize on
both sides of the clamp so there's no pressure drop across the clamp. This simulates the control grid stopping the electron
flow from the cathode to the plate--no flow = no voltage drop across the load
Now allow some water
to flow by releasing thumb pressure and the
pressure will rapidly drop in the hose between your finger and the clamp.
This simulates the control grid allowing electrons to flow to the plate.
Add more thumb pressure to decrease the water spray and the hose pressure
increases, allow more to flow by your thumb and the pressure drops. The
partially clamped hose causes more pressure drop when small amounts of
water are released. Without the restriction of the clamp
much more water must be released to get the same pressure drop. The load
resistor causes much more voltage drop when small amounts of current are
released by the control grid. Without the load resistor much more current would
have to flow between the cathode and plate to get the same voltage drop.
control grid 'releases pressure' (lowers voltage) by flowing negatively charged
electrons onto the positively charged plate. The electrons are pulled by the
power supply toward the load resistor but they stack up when they hit the load
resistor--this is what causes the voltage drop across the plate
resistor--there's more electrons on one side of the resistor. More stacked up
electrons = lower voltage on the plate. When the control grid slows the flow of
electrons the voltage rises on the plate because the power supply is constantly
pulling electrons through the plate resistor (fewer electrons = higher voltage). These voltage fluctuations
on the plate are the
amplified guitar audio signal.
Referring once again to the above Standard Vacuum Tube Preamp
Coupling cap at upper right blocks high voltage DC but allows
the AC guitar signal to pass through to the next amplifier stage.
Triode Vacuum Tubes
Single triode vacuum tube on the left, note its tubular structure. The 12AX7
middle and right is a dual triode tube--two tubes in one. Pin 1 connects to the
Plate (output), Pin 2 the Grid (input), Pin 3 the Cathode (source of electrons).
Pin 6 connects to the 'B' triode's Plate, Pin 7 the Grid, Pin 8 the Cathode.
Pins 4, 5 & 9 connect to the heater filaments. The halo like 'Getter' at the top
of the tube held the material used to create the 'Getter Flash' (mirror-like
substance on top inside of tube). The Getter Flash absorbs gas molecules to
maintain the tube's vacuum. If the Getter Flash turns from silver to ash white
you know the tube has been contaminated with too much oxygen from a vacuum leak.
This is a 12AX7 plate characteristic chart. Notice the curved grid voltage
lines? They are curved so they are nonlinear and always induce nonlinear
distortion which also causes harmonic and intermodulation distortion. This is
why tube amps tend to sound "warm". The distortion fattens up the tone of
anything being amplified by a tube circuit.
6V6GT Beam Tetrode Power Tube
Note pin 8 is connected to the cathode (K) and the beam confining plates (g ) 3
The 6V6GT is known as a
beam tetrode but in reality it
has five electrodes so it should be called a beam pentode:
cathode, plate, control grid (g ), screen grid
(g 1 ) and beam confining electrode
(g 2 ). The 3 screen grid
provides a constant, positive voltage to strongly attract electrons from the
cathode. It also isolates the control grid from the plate which reduces
parasitic capacitance between them which increases the tube's gain and
stability. Some amps have a switch to run a tetrode or pentode tube in lower
power 'triode mode' by tying the screen grid and plate together which allows the
screen grid's voltage to fluctuate with the plate. 6V6GT Beam Tetrode
A beam tetrode's "beam confining electrode" (top
left) is tied to the
cathode and directs electron flow into a beam and onto the plate. The beam
confining electrode is the tube's fifth electrode. The
heater wires are shoved up inside the cathode, which is placed inside Grid NO.1
(control grid), which is placed inside Grid NO. 2 (screen), which is placed inside the
beam confining electrode, which is placed inside the plate.
6L6GC Beam Tetrode Power Tube
The cool blue glow is caused by electrons striking gas
Non-beam power tubes such as the EL34 and EL37 don't have a beam
confining electrode but have a
grid as grid 3 (g ) which is tied directly to the cathode and keeps
high speed electrons from bouncing off
the plate upon impact. 3 EL37 True Pentode Cathode and Grids
In this true pentode the Suppressor Grid (g ) 3 replaces the beam
tetrode's Beam Confining Electrode (also called g ) 3 .
Take a look at this page with excellent
detail shots of an EL37
true pentode's insides which clearly shows the control, screen and
So to review, the filament heats the cathode, the cathode gives
off electrons, the plate and screen attract the free electrons but the grid
controls the flow--the control grid is the valve that controls the flow of
electrons through the tube.
Place a guitar signal voltage on the control grid and the
guitar signal will control the flow and the tube's output will be the amplified
How to Draw Tube Load
Lines for more insight into how tubes work.
Amplifier Overdrive for specific information on how overdrive distortion
The Rectifier Tube
a one-way valve that combined with the power transformer acts as an electron pump to convert
325 volts RMS AC into 360 volts DC. This high voltage AC from the transformer is
often called HT (high tension) voltage. Note that when you measure between
ground and either of V3's AC input wires you'll see
325 AC volts, but if you measure between the two AC input wires you'll see 650 AC
volts. An amplifier's rectifier pulls electrons out of the amp circuit to create
positive voltage (electron scarcity = positive voltage). V3 is different from the preamp and
power tubes in that It has two plates, two cathodes, no grid and its heater filaments are directly
connected to the cathode to keep the heater-to-cathode voltage low. rectifier tube
325 volts of
alternating current (AC) from the power transformer is connected to the
rectifier's pins 4
and 6 which connect to the two plates. As the positive half of the AC wave
(+325V) charges pin
4's plate positively, pin 6's plate is charged negatively. The pin 4 plate's
positive charge attracts electron flow from the cathode generating a positive DC current
on the wire attached to the cathode's pin 8 (pulling negatively charged
electrons out of the B+ wire connected to pin 8 creates a positive voltage on the
Nothing happens to
the negatively charged pin 6 plate.
Then as the negative half of the AC wave enters the tube (-325v), pin
6's plate is charged positively and attracts electron flow from the cathode
while pin 4's plate is charged negatively and does nothing. Therefore both
halves of the AC wave are converted to DC which makes V3's 5Y3GT tube a 'full
High voltage AC flows onto the plates connected to pins 4 &
6. High voltage DC current flows from the cathode out pin 8. Note the 5Y3GT on
the right has a directly heated combined cathode/filament where the GZ34 on the
left has a separate cathode and filament that are electrically connected.
Both positive and negative voltage is used to create pulsing DC in a full
wave rectifier like the 5Y3 and GZ34.
Because the cathodes are directly heated by the power
transformer's 5v AC supply the cathode has both 5v AC and high voltage B+ DC on
it at the same time. This is why the 5v heater supply doesn't have a center tap
like the 6.3v supply--the B+ DC voltage would be shorted to ground through a
center tap. This 5v AC is on the B+ output wire but is filtered out at the first
Most fixed bias amplifiers use a single diode to rectify the 50v
DC fixed bias voltage. The single diode functions as a half wave rectifier and
generates a very lumpy DC voltage that must be filtered by a resistor and relatively large
capacitor (RC filter).
The cathode and filament are combined. The Internals of the JJ GZ34S Rectifier Tube
The GZ34S's two big metal plates are hollow in the center
so the two cathodes (hollow tubes) will fit up inside them. High voltage AC
is brought in from the Power Transformer through Pins 4 and 6 to charge the
plates. The fit is tight but the cathode and plate do not touch each other. The
hollow cathodes have filament heater wires running their entire length inside
them. The hot cathodes emit negatively charged electrons. The free electrons are
pulled to the positively charged plates. Notice the thick power conductors that
connect both cathodes to the output pin 8. Electrons are pulled through these conductors
from the B+ wire attached to pin 8. Removing electrons from the B+ wire creates
a positive charge in the wire. Conventionally we think of positive DC power flowing from the
cathode and out pin 8 to the Standby switch when in reality the electrons are
flowing the opposite direction. Photos by Rob
The plate horizontal and vertical supports hold the plates
and cathodes in place. The Getter Halo's (below)
only function was to hold the 'getter flash' until it was flashed onto the
inside of the tube (the silver coating on the top of the tube). This silver
coating or "flash" absorbs oxygen molecules to keep the tube's vacuum oxygen free.
JJ GZ34S Rectifier Tube
Photos by Rob Robinette.
Double folded cathode heater filaments are coated with thin
electrical insulation and shoved up inside the hollow cathode.
By Rob Robinette
Suggested follow on reading:
How Tube Amps Work
It's easier than you think. How to Read Tube Amp Schematics
How the Fender 5E3 Deluxe tube
5E3 Deluxe Modifications
How to Draw Tube Load Lines
from the Woodie
to the 68 Custom Deluxe Reverb Reissue. Deluxe Models
Fender 5F6A Bassman Information and
How the Fender blackface AB763
Deluxe Reverb Works
AB763 blackface Modifications
difference between a blackface Vibroverb and Vibrolux? AB763 Model Differences
How the Marshall JCM800 works
It looks like a simple tone circuit but it's not. How the TMB Tone Stack Works
Bright, Normal, High, Low: it's an elegant circuit. How Fender Input Jacks Work
RCA Receiving Tube Manual,
, 2nd Edition.
Designing Tube Preamps for Guitar and Bass
, 4th Edition.
, 2nd Edition.
Vacuum Tube Circuit Design: Guitar Amplifier Preamps
Robert C. Megantz,
Design and Construction of Tube Guitar Amplifiers
How the 5E3 Deluxe
Works ] [
Deluxe Models ] [
My 5E3 Build ] [
The Trainwreck Pages ] [ Fender Input Jacks
B9A Prototype Boards ]