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Author's notes (John Bessa):

The material below easily explains one of the project amps, the Moviola "Squawk Box," which according the University of Denver tube experts, is "by the book." These experts, Tuttle and Calvert, also supply an old-school RCA reference model high-end amp from the late 50s that is free of controls, and therefore simpler to analyze. (Of course balance and bass/treble control will be necessary for a stereo that would grow from it). Far more troublesome are

  • the basic workings of tubes and amps, especially how control grids work, and how the amp works as a "whole systems model," and
  • understanding and fixing the recently-deceased and sorely-missed late-model Sherwood 8000 amp and tuner.

The latter is so complex that it may become a tear-down to be rebuilt more simplified using the basic amp.

(I just got an email from the U of D stating that I can use their material here, so more images will appear soon.)

Still on the agenda is figuring out how these actually work, and, from what I have seen, I honestly don't think that any single person has the full picture. I believe that the amps were created purely based on experimental data, and that people have been gluing on theories that seem to explain phenomena, but don't predict it... not to mention that explanations is so full of personalized technicaleze and obfuscated with math and graphs that we don't need because we can hear the benefits.

Figuring it out[edit]

The perception I am getting of tube function is that there is a very highly-charged piece of metal (filament or anode) that actually "evaporates" or "boils" electrons so that they can travel to another piece of metal the anode. The boiled electrons are allowed to make this trip by a grid that is hooked to input, a microphone, for example. To create this effect, the metal has to be very hot (thousands of degrees). In between these two pieces of metal is a "grid" that can stop the passage of electrons when it has the charge of the boiling piece of metal, which is negative, and allow the passage when it has the charge of the other side, which is positive.

The boiling side, or cathode, has to be hot, but in most tubes, the heat comes from a separate piece of metal source making the filament separate from the cathode, on its own power source.

So a high charge is applied to the cathode pin to heat the filament and the opposite of this high charge is brought back to the power supply through the anode pin. The other pins connect to the grids, and very fancy charge control is going to have to be applied to these to grids to achieve actual rather than reverse amplification as the tube will do in its simplest form, and in this charge control lies tube engineering (any clues?, anybody?)

The Triode tube in an amp[edit]

Triode amp
6sn7 triode open output (possibly pre) amp
Triode power amp
12b4 triode power amplifer, note that the layout is exactly the same except there is no output; power flows up the anode past the transformer that powers the speaker.

This picture of a triode amp is valid, as all input tubes are triodes to allow linearality. The 6sn7 is described as 1/2 of the common 12ax7. It also has two diodes in it that can be used by radios (to make the radio waves in the air that are AC into DC so that they can be frequency filtered, amplified and listened to, but are not relevant here for amps).

The goal now is to explain what each of these components do, and, I imagine the flow of electrons up from the chassis through the various components back to the positive voltage at the top. The input to the left (Vi) sends in a signal (would it be positive or negative, or both?) and the output (Vo) outputs an amplified signal. The control grid is kept at the same charge as the negative ground that is on the the Vi signal input wire by a grid leak. So, assuming the signal coming in has a positive component that is directed towards the grid and resisted from going to the chassis by the resitor, then it will interrupt the negative blocking power of the grid, and allow electrons to flow from the cathode to the anode. As there is a resistor on the anode wire after the output connection, then the evaporated electrons will head down the output wire (as they experience no resistance there, and that is where they will go. (This guesswork and combines fluid current analogies with electrical reality and needs validation! But it sounds right for the moment.)

If charge to the grid blocks electrons, and loss of charge allows them through, then it may be that the input signal (that happens to represent sound) has frequencies and sends waves to the gird open and close the "gate," as it were, for the electrons with the same frequency as the sound, but possibly in reverse. So, it may be the downside of the input signal wave that is creating the upside of the amplified wave going to the output in a preamp, or, alternatively, by the power transformer, to create electricity for the speakers.

The roles of the two capacitors need to be explained; they may be there to steady the flow when charge values change.

I think that the mystery of the narrative ill-logic that plagues tube audio (AudoFOS) is derived from the misconceptions caused by conflict between the particle-flow abstraction (PFA) and the charge model (CM). Electrons flow from negative to positive in the PFA, (though I have read reference to "positrons" which fit the charge model better), and negative happens to be ground causing the "biased" charge to come from the positive side of the power supply. If the first engineers had created a positive ground (as my metal-bodied film cameras have, interestingly), then amp functionality would be more intuitive, but they didn't. So, if the grid (in a triode) is at the same charge level as the negative pole, or ground, then the evaporating electrons cannot go through the grid to the positive side of things (PST). If the charge on the signal wire (Vo) becomes like the PST and less like the negative side of things (NST), the electrons can flow to the PST. In an open output amp (OOA), or pre-amp, the extra electrons go to the PST and then go down the output wire because there is a resistor between the positive connection on the tube and the positive side of the power supply. In a transformer output amp (TOA), the electrons flow to the positive power supply, or PST, through the winding of a transformer coil. Much of that energy is transferred to parallel windings that are part of a loop that goes to, or through, the speakers. Typically, a OOA is hooked to a TOA, and TOAs typically have more grids in the tubes to combat the Miller effect, but more grids translate to more distortion (or less linearity), which is more acceptable on the output side of things (OST).

A problem here is that the signal coming from the signal output (in an OOA) will in be negative in the particle sense, an electron, and this electron will be going into the signal input of the next amp (a TOA). So perhaps there needs to be a signal particle, that is both positive and negative, which may be a frequency wave, such a music is! This brings back the question of control grid; how charge opens and closes the electron flow gate.

In the cases of TOAs, the signal flow will be to the power supply, in other words, signal particles, or musical frequencies, are being sent to the transformer or a battery and presumably to flow back to the NST, or grounded chassis, and then onto the tube's boiling plate. Maybe the positrons go to the boiling plate and the negatrons, or electrons, go to the electron receiving plate, balancing things out. But, I suspect that there is a second wire involved in the signaling which is connected to the chassis, bringing us back to the idea of a positron going to the control grid.

Since there is no material written by anyone with conceptual creativity that I can find, it may be that, after a century, this actually needs to be tested to provide a answer with isolated PST and NST to see how the signal effects the control grid so that control grid phenomena can be explained! This cannot possibly be, but maybe it is!

Pathways (three)[edit]

The diagrams provide experimental evidence of what is going on.

There are three pathways through this open-ended triode amp: the signal path, the boiling electron path, and the positron path.

  • Electron path: The boiling electrons arrive from at the electron boiling plate from the ground, or chassis, which is the negative side of things (NST), and travel through the grid "gate" to the positive side of things (PST). In this diagram, there is a resistor that blocks the electrons and sends them down the output path, or wire. This output is then negative.
  • The signal path comes in the signal input, and travels to the grid where it creates a charge different from the boiling electron plate, meaning positive as the boiling plate is giving off electrons, which (purportedly) allows the electrons through to the PST. The positive charge that came in along the signal path go where? It seems logical that, using the PFA, the charge, or charged particles as positrons would flow either to the NST or remain in the tube as capacitance. In other words they go off the signal path, and the signal path continues with the flow of boiled electrons on to the PST, and is directed over to the output wire (by the resistor in the PST wire) as power for the speakers, or more likely, a newly amplified signal for the next amp stage's signal imput wire.
  • The positron path, as mentioned, comes in the signal path to the grid and probably goes from there to the NST or is held as capacitance. The capacitance idea is supported by the existence of the resistor from the signal input (positive) to the ground, or NST, and is called a grid drain resistor, suggesting it drains the grid of stored positrons.
Pentode amp (2)
6K6 pentode power amplifier with transformer output

6sj7 pentode amplifier.JPG

Pentode amp (1)
6sj7 pentode open output amplifier

The Pentode tube in an amp[edit]

It might be helpful to look at a pentode amp now, because they seem to work well in practice for power. The first diagram shows an OOA, rather than a TOA as shown in tetrode power model diagram. The second one shows a TOA. In the first, two grids are grounded, in the second two grids are...

Split phase using three tubes
Split phase using four tubes


The positron/negatron (or electron) signal-particles-paths abstraction might be real after all! Frequency is clearly an amp "linearity" issue, as according to one document, push-pull systems send opposite ends of waves to two different tubes tp make a much "cleaner" sound.

The first diagram show how the frequency coming into the first amplifier (or stage) sends amplified signal output down both cathode and anode from to become inputs for two different tubes as input; positrons go to one, negatrons to the other based on the "vibration" of the input, or control, grid. (Input is the first grid in the center of a pentode tube on the cathode or "electron boiler" side, and the only grid in a triode.) In the two pentodes, the second or screen grid is on the NST (or grounded), and the final suppressor grid is linked to the cathode (NST), which is linked to the ground (or chassis, NST) through a "capacitor-resistor in parallel" link that always seems to link cathodes to the chssis, or ground, but not in the case of the first amp, or triode to the left. (In some cases this capacitor is polarized (+ sign on top) and other cases, it is not. And in some cases there is just a lone resistor.)

So it appears that the control or signal grid is a vibrator that sends increased signal in both directions, and that the direction of choice is out the anode (or electron receiver), but can go either way. So, perhaps the PST and NST just start oscillating and the oscillation is what causes the speaker to vibrate. What is interesting, or perhaps confusing, is that the signal input to the control grid always seems to be positive in triodes, and the other half of the signal input, negative is always grounded.

Something I have found odd is the use of the word "impedence" in tube documentation when these are obviously DC systems. What I am thinking now, is that there is a blended AC/DC electricity that is the vibration, or frequency. I can see in class A amps that the frequency is preserved, that in class B is it cut in half, and when blended, it becomes double-bumped, and class C where waves are severely hacked. The noise coming form a microphone is AC if you think about it, because a magnet is moving up and down next to a wire driving electricity in both directions. And it may become blended AC/DC if you put it in with the NST/PST scheme of things. So, imagine this, what if one were to put one mic lead one grid in a double triode, and the other lead on the other grid where the negations and the positrons scooch against each other as part of the pushing and pulling that is the AC particle abstraction. If you did it right, you might be able to get both humps in correct phase. Problem that I see is that it may actually be that control grids always have to be charged (negative?) to prevent flow and and therefore sound.

This is the part that is not explained very well, or at all (AudioFOS). And half of tube system cannot be positively charged in mirror of the other half because you need negative charge to boil electrons. Interesting thought, though.

Either way, it looks as if the input amp has to be class A as in the first diagram to preserve both humps.

Going a little farther with the particle abstractions, if the scooching back and forth of negatrons and positrons, then perhaps the particles can be unified for the AC component of the amps (which perhaps "bias" the tubes and the whole system towards the PSA or NSA) giving us signal particles, or signatrons.

This might require some electro magnetism now, because both input (microphones or phono needles) and output (speakers) both open and transformer-based, result in wires in back-and-forth motion with magnets. This I believe creates flux, which is the basis of AC current in alternators, for instance.


Note: some things in the positive side of things (PST) can have a charge of 400 volts, so it is hoped that lucidity and simplicity in this glossary will translate to safety (especially for me!).


  • Inductance and Impedence These two are AC and it is not necessarily obvious what they are doing in DC systems, but it is pretty easy. The swing of the speaker magent which is an amplification of the swing of the mic, is motion between a magnet and wire coil; what produces, and expresses, AC current. Inductance is the AC equivalent of DC amps, and Impedance is the equivalent of resistance (but with complicated math). There can be high inductance but low voltage reaching a speaker, so, even though it has low voltage, there is a lot of swinging of the magnet, especially for bass frequencies. Chokes, for example. prevent AC current, or signal frequency, from going out to the PST to the transformer, which apparently causes hum. So the choke is impedes AC current but is not a resistor as it does not resist DC electricity by putting both currents through a coil; possibly the fields created by the AC current in the coil cancel each other out (sounds reasonable. Resistors probably impede AC current -- which sounds biased to me! ;)
  • Cathode (might also be filament in some tubes) Negative electrode, heated electron evaporating plate ) / Anode Positive electrode, electron receiving plate
Cathode is where the electrons evaporate from to flow to anode to come out as an amplified charge
The cathode and filament can be separate, where the filament simply heats the cathode by being near it to make it more efficient and to allow the use of AC current for the heat. This makes one pin a heater pin then that can be AC.
  • Negatron (or electron) Particle boiled off of the cathode that flows through the grid "gate" to the anode.
  • Positron Particle that comes in along the signal wire that gives a charge different than the anode charge, or positive, so that the electrons that are boiled off of the boiling plate, or cathode, are allowed to pass through the grid "gate" onto the positive side of things (PST)
  • Signatron Combined idea of postive and negative particles to explain the "scooching" of particles in AC current that creates AC electricity. Here positive and negative depend on motion which is created by the motion of magnets back and forth across a microphone wire or expressed as the motion of a speaker magnet caused by the speaker wire.
  • Positive side of things (PST) In a triode, everything on the positive (or power supply/hot lead) side of the control grid.
  • Negative side of things (NST) In a triode, everything on the negative (or ground/chassis) side of the control grid.
  • Control grid In a triode, grid in between electron boiling plate, or cathode, and the electron receiving plate, or anode, that blocks the flow of electrons when it has the charge of the cathode, or the ground (chassis), and allows the flow when it has the charge of the receiving plate, or anode.
  • Open output amplifer (OOA) (usually a pre-amp) The output comes directly from the tube. The flow from the tube to the PST but is blocked by a resistor to prevent electrons from going to the PST power supply and instead sends them down the output wire (Vo).
  • Transformer output amplifier (TOA) (usually a power amp) The output comes from a transformer that combines winding from the PST wire coming from the tube electron receiving plate and the wire that goes to the speakers.
  • Particle flow abstraction (PFA) A way of describing the flow in tube amps in terms of moving objects
  • electrons negatrons
  • positrons negatrons moving in the opposite direction
  • signal particle pathway from input to output (in an OOA), or to the PST and onto the power supply. This may be combined positrons and negatrons (or electrons) resulting in frequency
  • Charge model (CM) -- a way of describing positive and negative charges in tube amps in terms of two sections divided by a control grid:
  • positive side of things (PSA)
  • negative side of things (NSA)
  • Two more loops (PL + SL = amp,
  • Power loop PL (DC)
  • Signal loop SL (AC) flux loop connecting mic and speaker, mic flux yields speaker flux through the media of tubes, capactors, and resistors activated by power supplies

This stuff hasn't been categorized yet

  • Screen grid
  • Suppressor grid
  • Polarized capacitor
  • Grid plate
  • Plate
  • AF amplifier
  • RF amplifiers
  • Push-pull
  • Single end (SE)
  • AGC
  • Linearity
  • Inductor (or reactor or coil) is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries.
  • Choke (inductor, measured in henries, plate and coil) Inductors called chokes are used as parts of filters in power supplies or can be used to block AC signals from passing through a circuit to prevent AC signal current from going to the power supply (PST) with the current which can create a hum.
  • Capacitor and resistor in parallel (polarized capacitor)

Tubes in ideal ampliphiers[edit]

6sn7 triode (half of a 12ax7)(pre) amp
12AX7 is commonly used triode with two triodes in it allowing for stereo in the same tube
6j5 triode amp used in example above
Tetrode symbol
plate (anode)
screen grid
control grid
heater (filament)
Pentode symbol
anode plate
suppressor grid
screen grid
control grid
heater filament


The electrode in between the filament (cathode) and plate (anode) amplifies signals. As the voltage that is applied to the control grid (or "grid") is lowered from the cathode's voltage to somewhat more negative voltages, the amount of current flowing from the filament (cathode) to the plate (anode) is reduced. The negative electrostatic field created by the grid in the vicinity of the cathode inhibits thermionic emission and reduces the current to the plate. Thus a few volts difference at the grid make a large change in the plate current and lead to a much larger voltage change at the plate, resulting in amplification. From wikipedia tubes article


The addition of a second (tetrode) between the triode's control grid and the plate (anode), known as the screen grid:

  • ameliorates stability problems and limited voltage gain due to the Miller effect found in triodes
  • extra grids increase distortion, making triodes better for input tubes, but tetrodes and pentodes better for output tubes


"Pentodes add another grid between the screen grid and the main anode, called the suppressor grid (since it suppresses secondary emission current toward the screen grid). This grid is held at the cathode (or "ground") voltage and its negative voltage (relative to the anode) electrostatically repels secondary electrons so that they are collected by the anode after all. This three-grid tube is called a pentode, meaning five electrodes" From Calvert.


Types of Vacuum Tubes by J. B. Calvert source

SPICE and the art of preamplifier design by Norman L. Koren (2001) source

Screen grids in audio and RF modulator power tubes source

Tetrode, pentode, and beam power tubes source

Electronic (tube) circuits by Tim Williams source

Grid Resistors - Why Are They Used? by Randall Aiken source (?)

Chokes explained by Randall Aiken source (?)

From Vacuum Tube Preamplifier Analysis and SPICE Simulation[edit]

Vacuum Tube Preamplifier Analysis and SPICE Simulation by Michael S. McCorquodale, 2005 source

Capacitor explained (direct quote -- will be summarized):

A capacitor behaves as a DC block. However, it will pass AC signals in general, but the transfer will be contigent upon signal frequency, type of capacitor, and total capacitance. These differences are attributed to the mechanism of current flow in each case. DC signals move charge by conduction. Electrons from one point in the circuit travel continuously to another point due to an electric field that is created by a potential difference, or voltage. Once the capacitor is charged fully by the conduction of charge to each plate, the capacitor appears as an open circuit to DC signals. In contrast, AC signals propagate through capacitors by the mechanism of displacement. Small amounts of charge are moved to and from the plates of the capacitor and the charge is never conducted through the capacitor. Nevertheless, this displacement of charge is the means by which AC signals propagate through capacitors. The AC signal is superimposed upon the DC signal, thus the composite signal can be decomposed and each component can be treated separately by superposition. For completeness, it is worth noting that this is only true to a first order since nonlinearities in the circuit will cause AC signal components to give rise to DC signal components, but this is beyond the scope of this paper. Lastly it will be shown that the DC bias of the amplifier affects certain AC properties of the circuit, such as the gain.

Grid resistance explained (direct quote -- will be summarized):

The grid bias resistor, RGB, provides a high resistance DC path to ground, thus it biases the grid to zero volts DC. Moreover, it sets the input resistance into the preamplifier. It is generally desirable for the input resistance of an amplifier to be as high as possible so the input signal is not attenuated by the voltage divider set up by the amplifier input resistance and output impedance of the signal source. The bypass capacitor, CK, provides an AC path from the cathode to ground. Therefore, the cathode resistor, RK, does not influence the AC signal since the cathode is effectively an AC ground. The grid resistor, RG, limits the frequency response of the preamplifier by interacting with the capacitance between the grid and plate to create an RC circuit with a -3dB point, or half power point, of approximately 20kHz as will be shown subsequently. This resistor limits the frequency response and damps any oscillations that may occur at higher frequencies beyond the audio bandwidth. The plate and cathode resistors simply limit the DC current in the tube, or more specifically, they are what set the bias, or quiescent, point.