Pre-phase inverter master volume and cold biased "cold clipper" gain stageĪnd tweaked the resulting circuit for an aggressive but sweet overdrive tone. They cascaded the Plexi's bright channel into the normal channel, added a Introduced in 1974, the "Master Volume Lead" series amps were Marshall's first true high gain amplifiers. The Birth of the High Gain Amplifier 2203 Master Volume 100 Watt, 2204 50 Watt and JCM800 Lead Series This is one of the "secret" reasons theįender 5F6-A Bassman, Marshall JTM45, Plexi and Master Volume amps sound Upstream gain stage adds a unique overdrive effect that limits clipping of both It is the first triode to go into overdrive and its interaction with the Users do not want the volume to drop when they roll off some bass.īut the Cathode Follower does much more than feed the tone ![]() Keep the tone controls from affecting the amp's output volume too much. The Cathode Follower supplies a low impedance signal to the tone stack to Tone stack from loading down the high impedance guitar signal coming off V2A's The primary purpose of the Cathode Follower is to prevent the Marshall employed a 470pF Bright Cap or "treble peaker" around the Bright or "Lead"Ĭhannel's Mixer Resistor to preserve those high frequencies. Of Valve 2) Miller capacitance to roll off some high frequencies. The large 470k Mixer Resistors interact with V2A's (the A half 022uF coupling cap also helps preserve bass frequencies. Its very large 330uF bypass cap boosts all guitar andįrequencies for a rich full bodied tone. Signal is amplified by V1A (the A half of Valve 1) which has a warm bias from its 820 ![]() 0022uF coupling cap also limitsīass frequencies and facilitates a cleaner, leaner overdrive tone. Mids and highs that sound good when later stages push into heavy distortion.īecause of this design the Bright or "Lead" Channel responds extremely well to boost andĭrive pedals. High frequencies which reduces the Bright Channel's lower frequencies early in the circuit so it can focus on 68uF bypass cap boosts the gain of mid and Signal is amplified by V1B (the B half of Valve 1) which has a cool bias from itsĢ.7k cathode resistor. V2B cathode follower, Tone Stack and Phase Inverter. Both channels parallel oneĪnother and both go through two preamp gain stages before hitting the no-gain Pickups often sound better through the Low jacks. Impedances can color the tone from the guitar's pickups so you should always tryīoth the High and Low inputs to hear which sounds better with the guitar you're Impedance is 1 megaohm while the Low jack's is 68k. Voltage divider with the two 68k Grid Stopper resistors. The Low inputs have their guitar signal cut in half by forming a Got the idea for the 2203 Master Volume preampĮach channel's High and Low jacks function just like the Fender V1B before hitting the third V2A gain stage. A common mod with these amps was to change theĬhannels from parallel to series so the guitar signal went through both V1A and These two channels parallel one another and meet up at the V2A The "Plexi" came with two distinctly voiced channels each Signals that are 180 degrees out of phase to feed to the Power Tubes. The Phase Inverter creates two mirror image After the Tone Stack the signal goes into the Phase Inverter. The tone stack with a low impedance signal to keep the tone stack from loadingĭown the guitar signal. ![]() Through DC coupling (no coupling cap) to the V2B cathode follower which supplies The signal isĪmplified by the V1B preamp stage then flows through the Volume I pot to The Plexi's topology is still very similar to theīassman. The Marshall Plexi guitar amplifier was an evolution of their very successful Fender 5F6-Aīassman copy, the JTM45 but with just two preamp gain stages it can't Table of Contentsġ959 Super Lead 100 Watt and 1987 Lead 50 Watt If after looking at this page youįeel you need some background information on how tube amps work see my To do with the year the amp was introduced. ![]() Also be advised that Marshall model numbers have nothing Substitutions and "upgrades" to these long production run amps so many parts,Įspecially capacitors' component values can differ from these Keep in mind that Marshall made a lot of part How the Marshall Plexi, 2204 and JCM800 Amplifiers Work
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