The first Marshall JTM45 used a push-pull power stage identical to the Fender Bassman 5F6-A. Subsequent Marshall designs used EL34 pentodes instead of Fender's 6L6 beam power tetrodes to create an entirely new sound for which these amps have become legendary. The JMP50 Model 1987 also eliminates the 470-ohm screen resistor found in the 5F6-A and JTM45. While primarily intended just to suppress radio-frequency oscillation, the screen resistor value also has a significant effect on harmonic distortion at high power levels.
Pentodes and beam power tetrodes have distinctively different characteristic curves, and this translates into significant differences in performance. Let's examine some of the reasons. When pentodes operate at low plate voltages, the plate doesn't create enough of an electric field to draw electrons away from the screen and they accumulate on the screen side of the suppressor, many returning to the screen. The result is that the conditions found on the cathode side of the suppressor are very similar to those found near the cathode, so a virtual cathode forms in this region.2 These effects result in characteristic kinks in the lower left of the EL34 plate characteristic curves.
When the grid of a beam power tetrode is very negative, the plate current is low and there is very little space charge between the electrodes. This causes less suppression of secondary electrons. At low plate voltages and high plate currents, a large amount of space charge causes a virtual cathode to form between the screen and plate. As the plate voltage decreases, the tube abruptly transitions from conditions where the plate voltage has little effect on plate current to conditions where the plate voltage is the most dominant factor. This produces distinctive knees in the curves, like these for the 6L6GC.
At the left hand side all of the curves come together, demonstrating that at these low plate voltages the grid has lost its ability to control the plate current. The tube is thus acting as a diode, where plate current increases rapidly with increasing plate voltage.
Because of the space charge effect, beam power tetrodes tend to have much sharper knees compared to pentodes and clearly defined diode lines, as demonstrated by the following figure, where two 6L6GC curves are superimposed onto EL34 curves. For both tubes the screen is at 250 volts.
The characteristic differences between pentodes and beam power tetrodes at low plate voltages, especially near the knee of the zero-grid-voltage curve, affect power amp distortion at high power levels, even if the loudspeakers act as ideal resistors. Using real-world speakers, however, the tonal effects become even more significant. The reactance in the loudspeaker, which is very dependent on frequency, transforms the operation of the amplifier from a straight line into an ellipse, as shown in the following figure for a single-tube 6V6 amplifier.1 Similar effects are seen in push-pull amplifiers like the 5F6-A and JMP50.
The downward path of the ellipse cuts through the curves at lower plate voltages, where the characteristics between the two tube types differ the most.
Beam power tetrodes have precisely aligned control and screen grids that channel the electrons between the screen windings. This reduces the ratio of screen to plate current, making it less than in a pentode. This characteristic can be a double-edged sword, however, because manufacturing variances can cause greater differences in screen current compared to pentodes, up to 100 percent in some cases.3 This affects the voltage drop across any resistors placed in series with the screen. For both tube types, the screen current and screen dissipation increase rapidly as the plate voltage drops below the knee, and frequency-dependent changes in loudspeaker impedance while operating at high power levels often push the tube into this condition.
Many Fenders and Marshalls use screen resistors to prevent parasitic oscillation. In the 5F6-A and JTM45 a 470-ohm resistor is used whereas an EL34 equipped 1987 omits it entirely, driving the screen directly from the screen supply. For RF suppression the exact resistor value is not critical, but it has a significant effect on the nonlinear distortion characteristics of the amplifier. Here, for example, are composite characteristic curves for a 6L6GC Class AB amplifier with a 470-ohm screen resistor (solid curves) and no screen resistor (dotted curves).4
The curves are for grid voltages of 0, -16, -32, -48, -64, -80, and -96. Notice that they are widely spaced between 0 and -16 and between -80 and -96 compared to the region between -32 and -64. There is thus greater amplification at the extremes of signal swing, which creates 3rd harmonic distortion. The voltage drops across the grid resistors reduce the screen voltage at high screen currents, which diminishes this effect.
In summary, pentodes and beam power tetrodes are constructed quite differently, which translates into significant differences in their operating characteristics and tonal qualities. We can therefore expect the two types of power tubes to sound differently in a guitar amp operating near full power, something that is no surprise to musicians. We have also observed that the screen resistor value has a substantial effect on harmonic distortion. We can thus conclude that the Fender Bassman 5F6-A and the Marshall JMP50 Model 1987, while sharing the same push-pull circuit heritage, offer completely different sonic characterisitcs.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).
2Frederick Terman, Electronic and Radio Engineering, 4th ed., (New York: McGraw-Hill, 1955), p. 339.
3F. Langford-Smith, ed., Radiotron Designer's Handbook, 4th ed., (Harrison: RCA, 1953), p. 569.
4Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Ed., (Seattle: Pentode Press, 2009).
5Michael Doyle, The History of Marshall, (Milwaukee: Hal Leonard Corp., 1993).
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