The Marshall JTM50 and JMP50 use two configurations for the first preamp. One is the same design as in Marshall's Bluesbreaker and JTM45 and is identical to the Fender Bassman 5F6-A except for the tube: a high-mu 12AX7 instead of a medium-mu 12AY7. The 250uF cathode bypass capacitor CK is sometimes increased to 320uF or 330uF with very little impact on audio frequencies.
In the plate circuits RL=100k and RK=820. For the high-gain inputs RG=1M and RGS=34k. For the low-gain inputs RG=RGS=68k. The output coupling capacitors are CG=0.022uF versus 0.02uF for the Bassman, which is an insignificant difference that resulted, most likely, from parts availability.
Marshall's substitution of a 12AX7 triode into the 5F6-A preamp circuit affects mostly the overall voltage gain. Using our Preamp Output Impedance Calculator we get an unloaded voltage gain of 62 for the Plexi, versus only 35 for the 5F6-A. Not all of this increased gain shows up at the grid of the next tube, however, because the output impedance of the preamp rises to 38k, compared to only 20k for the Bassman.
The Marshall JMP50 Model 1987 splits the bright and normal channels into two separate circuits.
The JMP50 plate load resistor is RL=100k. The cathode resistor RK is 820 ohms for the normal channel and 2.7k for the bright channel. For the high-gain inputs RG=1M and RGS=34k. For the low-gain inputs RG=RGS=68k. The output coupling capacitor CG is 0.022uF for the normal channel, but is reduced by a factor of 10 to only 0.0022uF for the bright channel, increasing bass attenuation. Our Coupling Capacitor Caclulator shows attenuation of almost 3dB at 82 Hertz, which is the lowest note on a guitar with standard tuning.
The cathode bypass capacitor CK is either 250uF or 320uF for the normal channel, which fully bypasses the cathode resistor for all audio frequencies. It is only 0.68uF for the bright channel, which bypasses the cathode resistor only at midrange and higher frequencies. At low frequencies the negative feedback from the cathode resistor reduces the gain. When driving a 1M load our Cathode Bypass Capacitor Caclulator shows a gain of 28 at 82Hz, rising to 59 at 1.6kHz.
For the 5F6A-style preamp the cathode resistor carries the DC current of both triodes. For the DC operating point this means that the effective resistance for one triode is 1.64k, double the value of the resistor. With the two cathodes separated, however, Marshall leaves the cathode resistor for the JMP50 normal channel at 820 ohms. This raises the grid line, causing it to cross the load line at a much higher point. The net result is that the normal-channel triode operates closer to saturation and further from cutoff compared to the Bassman-style circuit. In contrast, Marshall increases the bright channel cathode resistor in the JMP50 from an effective value of 1.64k to a value of 2.7k. In terms of DC plate current we can think of the Marshall JMP50 Model 1987 normal channel triode as operating "hotter" and its bright channel "colder" than in the Bassman-style circuit.
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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