Dating cts speakers
This was a vast improvement and resulted in fewer blown speakers. This is probably due to the reality that the D series had been discontinued and the product code was convenient.And since the new cone design would retrofit the D series, there really was no reason to change the code.With the introduction of the K series, the wattage remained at 100W RMS, with the same 4" diameter edge wound aluminium coil.Because of improvements in the cone material and slightly increased thickness, distortion was "pushed back" making the speaker work cleaner at even higher volumes.
At full throttle, it breaks up very nice, with the compression of the 6L6 tubes helping keep that distortion nice and warm.In the first image, the D series speaker, you can clearly see the surround is actually a continuation of the cone paper itself, out to the mounting lip and under the cork at the edge.It is not as sophisticated, and therefore weak by design, and prone to blow out at higher volume or square wave abuse.While it is cleaner than most of its contemporary competitors, Jensen, Celestion, CTS, Emminence, et al, cone "break up" could still be achieved before ear splitting volumes were reached.
This is part of what made them so desirable in Fender amps, they stayed cleaner, and so drove the sound better into the higher volumes.
D series speakers were commonly grey framed for the general market and Fender amps (Deluxe, Showman, Twin, et al) were initially outfitted with the grey framed D series speakers, usually by special order.