Updated: Apr 5, 2020
Part of a blog series Tube Amplifier Circuits Explained
There are several options for how a pentode tube can be operated. This kit is designed for ultralinear mode, and so I will not go over a circuit for a standard pentode design in much detail. But I will cover a few basics so you understand the options and how ultralinear mode fits into those options.
First, and simply, you can take any pentode tube and operate it like a triode (“triode mode” or “triode strapped”) by simply connecting the screen to the anode, and the voltage of each will always be identical. Both the anode and screen will swing together in voltage across the output load as a signal is applied and current passes through both of them. Output power will be lower, and the tube characteristics will generally have more lower order harmonic distortion—some consider this a desirable “sound” for a tube amplifier, though obviously perception of sound is highly subjective. The grid-curve chart shown earlier for the EL34 is not in triode mode and would look different if the tube were triode-strapped.
Second, to operate in pentode mode, you would put a DC voltage on the screen at around the B+ level or somewhat lower, and then you would also bypass the AC by using a capacitor to ground (not shown) so that your screen stays at a constant voltage. Only the anode will swing in voltage as the signal is applied. Pentode mode will have more power output possible, and generally produces more higher order harmonic distortion—some would consider a less pleasing sound, again a subjective generalization. (I know I have just mentioned a few types of distortion and have not discussed this topic much yet; we will come back to it soon in more detail.)
Ok, these are two fine options. But some clever folks in the 1950s made a discovery of a third option that has very interesting results. You could think about triode mode and pentode mode as two ends of an extreme with respect to the screen voltage. On the triode end, the screen voltage is entirely determined by the anode (plate) voltage which means it changes with the audio signal, and on the pentode end the anode voltage is completely independent and the screen voltage is held at a constant level.
Each of these modes has their own form of non-linear distortion, but they tend to curve in opposite directions. If we were to take a position part-way in between these two extremes and put that on the screen, there’s a sort of magic that happens at the right spot where we can get a more linear operation (less distortion) and still get most of the benefits of pentode mode for power. By putting a special tap on the output transformer, we can vary the screen voltage in proportion to the anode but not as much as in triode mode. It was found that a tap around 40-45% of the primary is the ideal spot. This was called “Ultralinear.” It could be considered a form of negative feedback.
There is deeper theory of ultralinear operation you can study elsewhere. It’s used in our case to have optimized performance: good power output and low distortion. Critics of ultralinear might believe triode mode sounds better or pentode performs better, but in my opinion this is a good option given design objectives of low distortion, reasonable cost and sufficient power output from the selected components.
You can re-wire this kit to operate in triode mode by connecting the screen to the plate and tying off the UL tap of the transformer. You would still want a resistor on the screen pin (not illustrated here) for stability reasons to prevent oscillation, similar to a grid stopper resistor we will also put on the control grids. In our circuit we use 1.2k ohms to handle both the stability and also to pull down the voltage just a bit more to keep the screen at a safe level. The screen is a sensitive part of the tube, and the datasheet will tell you the maximum voltage and power dissipation of the screen. Most EL34 tubes are rated for around 425V on the screen and about 8W of power—notice that there is some current passing through the screen as some electrons will be hitting it, even while most are passing through and on to the anode.
We have spoken of something several times through these posts, so next let's really get into the topic of understanding distortion.