The first Marshall JTM45 uses an exact copy of the 5F6-A voltage amp. Subsequent Marshalls, including the JMP50 Model 1987, increase the grid stopper resistor to 470k versus 270k. When combined with the tube's Miller capacitance, the greater resistance increases the treble attenuation at the grid.
In the JMP50 plate circuit RL=100k and RK=820, the same as the Bassman, but there is an added 0.68uF capacitor that bypasses the cathode resistor at high audio frequencies, resulting in less negative feedback in this range and treble boost. The JMP50 bypass capacitor across the bright channel volume control is 50 times greater than in the 5F6-A, providing more treble boost, but like the Bassman there is no boost when the volume control is set to maximum because the capacitor is shorted. Marshall adds a bypass capacitor across the grid stopper resistor, however, which provides treble boost in the bright channel and treble attenuation in the normal channel regardless of the volume control position.
The results are significant. The Marshall JMP50 Model 1987, compared to the Bassman 5F6-A, is characterized by a greater difference in frequency response between the bright and the normal channels, especially at high volume control settings.
1Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 2nd Ed., (Seattle: Pentode Press, 2005).
2Michael Doyle, The History of Marshall, (Milwaukee: Hal Leonard Corp., 1993).
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