Dating marshall transformers

Two original factory schematics show 560V at the plates (and 559V at the screen).

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Some of these amps have the same power board layout as the previous amps (#2), whereas others had the later layout, although filtering was kept the same.

Some use the round yellow indicator, although some use the square red indicator seen on later plexis. Serial numbers covers the range from 6.600 through 6.800.

The second prototype reportedly used two output transformers, two GZ34 rectifiers, and four 6L6 output tubes, whereas the third prototype used KT66 output tubes.

Ken Bran and Dudley Craven at Marshall eventually dropped the GZ34 tube rectifier, increasing power and reliability.

The first amps used the rare block logo, the later ones a standard Marshall logo. It was recreated by Marshall as the 40new output transformer, the Drake 1202-84, was introduced (2 stack; 4K primary; 8 and 16ohms secondaries as well as a 100V secondary for PAs).

The indicator was the round yellow Radiospares lamp used on the JTM45s, and speaker cabinets had Celestion silver alnico speakers. This transformer was designed for a quad of KT66s, placing the two 784-103 used earlier.These amps run at much lower voltages than the later KT66 amps, producing less power while being easier on the tubes.Also, it should be kept in mind that the voltages drop or sag significantly under load, since the power transformer was only rated for 200m A (by comparison, a Twin transformer is 450m A).However, the 100 watt PA amps used JTM100 faceplates. First, the plexi Superlead and Superbass backplates were introduced, then the so-called Black Flag J. The first prototype is reported to have used four 6V6 output tubes and a single GZ34 rectifier tube.It probably used only one Radiospares Deluxe Output transformer, reportedly giving an output of 60 watt.The early ones had ohms switches mounted to the transformers, whereas the later ones had ohms switches on the backplates like later amps had.

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