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).
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.
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