The Master Volume was introduced in 1976 in response to the demands of guitarists who wanted an overdriven sound at more controllable volume levels. The Master Volumes were a direct offspring of the Standard series and are nearly identical aesthetically. However, Marshall had been working on crude Master Volume designs and gained experience with this type of circuitry prior to 1976 when performing preamplifier and gain modifications to Ritchie Blackmore's 200watt Majors (Doyle, 1990). 'The preamplifiers are cascaded (wired in series) by removing one channel and connecting that channel to the remaining one so that an increase in gain (sensitivity) is created which may be adjusted by using the two volume controls as a preamplifier and Master Volume controls" (Doyle, 1990, p. 22). So was born the definitive heavy rock amplifier.
Although the Master Volume series filled a need for many players, "many feel that much of the guitar's sound quality is not quite up to that of their Standard counterparts and the warmth of those amplifiers tends to be lost in the blaze of distortion. This observation, though, is not confined to Marshall amplifiers as it is a problem generally associated with all Master Volume type designs" (Doyle, 1990, p. 11).
Regardless, the Master Volume design was a natural and logical step in the evolution of Marshall amplifier design. This basic format of preamplifier/master volume controls appears on most amplifiers, Marshall and others, that have a built-in overdrive effect to produce distortion. The Master Volume series quickly outdistanced the Standard series in popularity, so much so that the latter was eventually dropped from the Marshall line at the conclusion of the JCM 800 series and only are available in reissue versions. The Master Volume remained relatively unchanged until the radical redesign of the current and powerful JCM 900 Mark III High Gain series.
The design of the Master Volume is almost identical to the Standard series, except in appearance, and very similar in circuit design, except for the input section, preamplifier section, and minor internal changes. Except for the power/standby
switches on the front panel and the smaller input section, the front panel looks identical to the original Standard series.
The back panel features are very similar also and show only aesthetic changes in comparison. The Mark II Master Volume series remained almost unchanged throughout its 15-year history.
The capabilities of the Master Volume models, like the Standard series, are a by product of their design. The whole idea behind the Master Volume was to provide minimum to maximum overdrive at any volume level through a preamplifier
control on the front panel of the amplifier.
This feature is in direct opposition to the Standard amplifiers which only distorted
when operated at high volumes or with external booster boxes. The Master Volumes have the advantage of having the overdrive effect built directly into the amplifier circuitry and allow infinite control of the preamplifier gain with the preamplifier gain control.
The input jacks allow for a wide variety of tones similar to the Standard amplifier. Replacing the Standard amplifier's two-channel, four-input design, the Master Volume simply had two inputs: one for high gain and one for low gain. The high-gain input was used for distortion sounds and the low-gain input for clean sounds. The high-gain input sends a strong, hot signal to the preamplifier section, while the low input provides a soft, padded sound. The major capabělity advantage the Master Volume has over the Standard series is its ab ' ility to produce full-blown, distortion sounds at the lowest of volumes. The distortion produced in the Master Volume's preamplifier, however, is significantly different from the power amplifier distortion produced by the Standard series. The debate goes on endlessly about whether preamplifier or power amplifier. distortion sounds better, yet from a practical standpoint, Master Volumes are a more logical choice for those players seeking the overdrive effect wěthout highvolume levels.
The major disadvantage with the preJCM 800 Master Volumes is their inability to patch internally or link with other amplifiers like the Standard series. All these funetions must be done by an external patching and switching system (see Chapter 11). JCM 800 models did, however, have a direct input (DA.) output for linking to external destinations. All 50-watt heads will power two-speaker cabinets (the 100-watt models only carried the four-cabinet capability for a few years and dropped this feature in favor of the two-cabinet format).
Master Volume amplifiers are very versatile, probably even more so than Standard amplifiers because of their lowvolume overdrive capability. Master Volumes, both old and new, can be found everywhere from clubs and recording studios to arena rock concerts. I highly recommend them, especially the pre-JCM 800 models which have a warmer sound than their JCM 800 successors.
(The High-Performance Marshall Handbook by John"Dutch"Boehnlein)