Pentode preamps are conceptually different from their triode cousins and their multi-grid characteristics can seem daunting to those of us who live in a 12AX7 world. We can't directly use a single plot of plate characteristic curves, for example. With a little effort we can translate the published curves to a desired screen voltage, but how do we choose the screen voltage in the first place? The screen grid represents a huge monkey wrench that evokes havoc on the classic "draw a load line on the datasheet" and "set the operating point" procedure for creating a guitar amplifier preamp.
The classic gain equation for a triode preamp, which is based the tube's amplification factor, its plate resistance, the plate load resistor, and the load to which the stage is connected, suffers a serious breakdown when the amplification factor and the plate resistance hit really big and fuzzy numbers. Such is the case for a pentode. So how does screen voltage affect preamp gain?
The issue boils down to our basic definition of gain: output voltage swing divided by input voltage swing. For a pentode preamp the output signal voltage, which is represented by the plate voltage, swings between between a minimum of nearly zero and a maximum equal to the plate supply voltage. The maximum output swing is thus defined by the plate supply voltage. This, it turns out, is independent of the screen voltage.
Maximum plate voltage is at cutoff. Minimum is when the grid reaches 0V. So the voltage gain of a pentode preamp is roughly these two extremes divided by the grid voltage swing needed to create them. This is where screen voltage plays a key role.
Here is a plot of plate current versus grid voltage for an EF86 pentode.
The plot corresponds to a plate voltage of 250 volts, but it doesn't change much for other plate voltages. The screen voltage, which has a much greater influence on plate current, is represented by 4 curves. With the screen at 180 volts the grid voltage swings from a minimum of -6 volts to a maximum of zero volts. At a screen voltage of 60 volts the grid swings only between -2.4 volts and zero. A lower screen voltage enables a substantially lower grid voltage swing to create the same plate voltage swing. This is the essence of our definition of "gain." For the large, non-linear signal swings often sought by guitar players, where transconductance, plate resistance, and other tube parameters are far from constant, a lower screen voltage can be used to increase the voltage gain of the resulting design.
A major concern with high voltage gain is input headroom. With 180-volt screens the input signal can have an amplitude of up to 3 volts without overdriving the EF86 pentode. With 60-volt screens the amplitude can only reach 1.2 volts before the pentode preamp goes into overdrive. A lower screen voltage means less input headroom.
Our thanks to Paul Reid of Rutgers University for his technical assistance with this article.
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