A thorough discussion about pentode tubes has to start with an examination of the limitations of triode tubes.
In the talk about triodes, we learned that that inside a triode is a few metal pieces, separated by a vacuum. Now, consider what a capacitor is – two conductive surfaces separated by an insulating dialectric. A vacuum is a sort of dialectric. So inside the triode, there are a couple of ‘virtual’ capacitors, one between the plate and grid, and one between the grid and cathode. In tube tech talk, these internal capacitances are called interelectrode capacitances.
Looking at the 12AX7 datasheet, we can find the interelectrode capacitance specifications:
|Triode Unit1||Triode Unit2|
|Grid to Plate||1.7||1.7||pf|
|Grid to Cathode||1.6||1.6||pf|
|Plate to Cathode||0.46||0.34||pf|
At very high frequencies, especially radio frequencies, the grid to plate capacitance can give rise to an oscillation problem. The varying plate voltage is coupled back to the grid and re-amplified. It is kind of an internal feedback loop inside the tube. Luckily, we do not have to worry much about this, since a guitar amplifier is not designed to amplify radio frequencies.
In fact, the AX84 is designed to filter out frequencies above the audio range. Take a look again at Preamp Stage 1. We never did talk about the real purpose of R9. This 68K ohm resistor (called the grid stopper) forms a low pass filter when combined with the capacitance between V2B’s grid and cathode. This low pass filter has a cutoff frequency well above the audio range, so it should not interfere with guitar frequencies.
The grid stopper resistor has a second purpose, which is to limit the grid current when the grid voltage is more positive than the cathode voltage. The explanation is a bit technical, but it is well worth learning.
Suppose that you are driving the amp with a hot signal, such as a high-output humbucker. As you turn up the volume control on the amp, the AC voltage applied to the EL84’s grid increases. At some point, the positive peaks of this AC voltage will exceed the cathode
voltage. As I mentioned earlier, normally there is very little current that flows out of the grid in a tube. This is only true when the grid voltage is less than or equal to the cathode voltage. Once the grid voltage exceeds cathode voltage, current starts to flow INTO the
grid. In essence, the grid and cathode behave as a diode. Almost instantly, the tube stops amplifying the signal. This is called blocking distortion. Now this condition does correct itself rather quickly if the grid stopper resistor is large enough. The grid stopper limits current flow into the grid when blocking distortion occurs, which reduces the time it takes the tube to recover from this condition.
Another limitation of triodes is that plate current depends not only on grid voltage, but also on plate voltage. For example, suppose you lower the grid voltage in a triode circuit. As we learned above, lowering the grid voltage results in an increase in plate current. And Ohm’s law dictates that the voltage across the load resistor must increase as current through it increases. Now, the supply voltage is a constant, so the plate voltage must decrease as the voltage across the load resistor increases (the two voltages must add up to the supply voltage). Lowering the plate voltage results in reduced plate current. So, gain is kept down in a triode because of this. In a preamp stage, reduced gain presents no problem because we have to ‘throw away’ some of the gain between stages to prevent
blocking distortion. However, in an output stage, interaction between plate voltage and plate current is not good – it only reduces the maximum output power of the amp.
OK, so triodes have two limitations that cause problems: interelectrode capacitance and the degenerative feedback thing. To eliminate these problems, early tube designers developed the tetrode. A tetrode is like a triode, with an extra element between the grid and plate. This fourth element is called the screen grid.
The tetrode’s screen grid solves both problems with the triode. Adding an element between the grid and plate serves to act as a shield between the grid and plate, lowering the grid-plate capacitance. And the screen grid makes plate current independent of plate voltage
(well, almost completely, but close enough for this discussion).
How does it do this? Remember our discussion of rectifier and triodes. The electrons streaming off the cathode are attracted to the plate, because the plate has a positive voltage potential compared to the cathode. You could say that the plate exerts a pulling force on
the electrons. In a tetrode, the screen grid is operated at a voltage that is slightly less than the plate voltage. Now, because the screen grid is physically closer to the cathode than the plate, the screen grid exerts more pull on the electrons than the plate. Some of the
electrons actually hit the screen grid, but most pass right through on the way to the plate. Thus, plate voltage has very little effect on electron flow in a tetrode. The steady voltage of the screen grid provides a nearly constant pulling force on the electrons. As a result,
the grid has almost complete control over plate current, regardless of changes in plate voltage.
While we are talking about electrons whizzing around inside a tube, now is a perfect time to discuss the main limitation of a tetrode. Imagine what happens to an electron inside a tetrode tube. It gets knocked out of the cathode because of the heating effect of the
filament. Then, the pull of the screen grid makes it speed away from the cathode. It accelerates past the grid and the screen grid. The short trip is over when it smashes into the plate. This impact actually causes some electrons to be knocked out of the atomic structure
of the plate. For every electron that smashes into the plate, two or three electrons are knocked out of the plate.
We’re talking about particle physics here! Impress your friends: tell them that small particle accelerators power your amp.
This phenomenon is called ‘Secondary Emission’. Now, in a triode, the liberated plate electrons quickly make their way back to the positively charged plate. However, secondary emission can cause gain reduction and distortion in a tetrode. Suppose that a large signal
is applied to the grid in a tetrode. Such a large signal could cause the plate voltage to fall below the screen grid voltage temporarily. At that instant, the secondary emission electrons would be attracted to the screen grid, and not make their way back to the plate. This
would reduce the plate current and increase screen grid current, clipping the peak of the amplified signal.
To solve this problem, the pentode was developed. In a pentode, a third grid is inserted between the screen grid and the plate. This new element is called the ‘Suppressor Grid’. It is usually connected to the cathode, sometimes internally. This connection means that
the suppressor grid is at a voltage potential much lower than the plate. It does not interfere with the flow of electrons from the cathode, because these electrons are traveling at such a high velocity that they pass right through. However, secondary emission electrons travel at a much slower velocity and are pushed back to the plate rather easily.