It's been said that no two plexis sound exactly alike, but by 69 they had settled into a more consistant set of design specs that were not much different in actual sound and tone from the early 70's superlead amps. There seems to have been a lot variation between plexi amps especially during late 67 to mid 68, but generally the amp was becoming brighter, tighter, louder, and with more gain. There were also numerous changes in cathode caps, bright caps, tonestack's, and filtering, and transformers, made to the design. The split cathode made the bright channel a lot tighter and gainer, and made jumping the channels more effective. One was the change to E元4's and the other was the change from a shared cathode on V1 to what "plexiologists" call the split cathode. Two important modifications occured during the period between 1967 and June 1969 when the plexi era ended. The earlist 100 watters used the JTM45 pre-amp with a 100 watt power amp that used KT66 tubes. At the end of the plexi era the amp had evolved to the point that it was rather bright, but it also had more gain. The shared cathode load is the ‘tail’ and is critical for good balance. The second triode in this arrangement, with its grid grounded, gets its signal from the cathode, meaning it does not invert the output at its anode. early 70's "Superlead"?īasically one has the plexi panel and the other has a metal panel if we are talking a 69 plexi and 70 metal panel. The first triode in the circuit can be viewed as a grounded cathode amplifier, inverting the signal it sees at its input. Whats the difference between a late 60's Marshall Plexi vs.
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