Some Marshall transformer trivia…
Many will be aware that Dagnall replaced Drake as the supplier of iron for the 100W amps in 1967. The C1998 output transformer was introduced in August, with a 1.5” stack and grain-oriented steel. Possibly because this change roughly coincided with the move to ‘lead’ specs (split cathode on V1, 33k/500pF tonestack etc.), this OT is often described as the perfect ‘rock’ OT - in contrast to the narrower-bandwidth of the Drake 1202-119s and -132s which preceded it.
The C1998 was phased out in 1971, replaced first by the C2373, then by the C2668. As with most Marshall parts changes, there was a period of transition for the changes to universally take effect, and there were ‘72 amps which came with C1998s. The easiest way to distinguish an original C1998s without removing bell covers is by the distinctive cloth-covered (‘self lead’) secondaries (although there was a period around its introduction when flying leads were used).
The C2668 has a completely different geometry to the C1998, and always used a nylon bobbin and alternative insulation. It was clearly a cheaper transformer, for better or worse, and the sound is correspondingly different.
The C2373 is a different matter. There are both paper- and nylon-bobbin variants, but a well-known transformer winder has told me that the paper C2373s have the same winding pattern and insulation as the C1998s, with the only exception being an extra layer of paper between the primary and secondary to prevent arcing. As such, they are essentially a more robust C1998.
I’ve had both on my bench and the cross-sections seem to corroborate this. First is a C1998, second a C2373.
