Dating drake transformers
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).
These 100W amps are basically like a dual JTM45 but with a solid state rectifier, giving slightly higher voltages.
Whereas a JTM45 puts out 30-35 watt at the point of breakup, these early amps put out around 70 watt at the point of breakup (and more than 100 watt at full breakup).
The choke (inductor) on these early amps appears to have been the Radiospares 20 henry choke often seen on the early Marshall schematics.
The Radiospares choke is different from the Drake and Dagnall chokes used later.
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.
Attempts to reverse engineer these amps, including the transformers, have given similar voltages, although users of the originals amps report everything from 530V to 625V.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 preamp used a single 16uf capacitor (for V1 and V2) like the JTM45s; whereas the phase inverter (V3) used 32uf (a JTM45 typically has 16uf).However, screens filtering is only 16uf for 4 KT66s (a JTM45 has 32uf for two KT66s) and mains filtering is 32uf (like a JTM45).