Dating drake transformers
The output of the third prototype is supposed to have exceeded 100 watt (Doyle, History of Marshalla big Hammertone Radiospares power transformer (with taps for both 350VAC and 425VAC).
According to tech reports, these early amps have around 470VDC on the plates of the KT66s (indicating that the 350VAC tap was used).
The early ones had ohms switches mounted to the transformers, whereas the later ones had ohms switches on the backplates like later amps had.
The transformer was the same Drake 784-103 used in the 1965-66 JTM45s, although two transformers was used for a quad of KT66s.
Putting capacitors in series like this will increase the voltage handling but decrease capacitance (two 32uf, 450V capacitors in series gives 16uf, 900V). Today RIFA and BC Vishay 33uf are popular replacements.
The head cabinet on these amps use two narrow Vox type vents (used until late 1966).
The Radiospares choke is rated for 20 henry and 70m A, whereas the Drake and Dagnall are rated for 3 henry and 100m A.
The first 100 watt amps known today as JTM45/100s used JTM45 plexi faceplates and white Super 100 Amplifier back-plates. The main stages in the evolution of these early 100 watt amps are Whos Pete Townsend.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.This high resistance gives more voltage drop across the choke than with the later chokes.When running the amp hard, screen voltages (and preamp voltages) will sag more, giving a softer response with more A key element to element to the sound and response of these amps lies in the power supply, especially the filtering (smoothing).