Originally Posted by TheRed1 View Post
Beyond the additional Z-Matic circuitry of the AZ version, the driver stage is also significantly different. The 50-A used 6S4 versus the 50-AZ's 6CL6's. To make it properly confusing, it should be noted that there were multiple versions of the 50-A and possibly multiple versions of the 50-AZ, too.
In summary, the 50-A was produced for two model-years, 1953 and 1954. The 50-AZ was produced for the 1955 model-year and still available into early 1956. Of the 50-A there were AT LEAST three versions: the 50-A (-1), serial numbers 0001 (?) thru 1500; the 50-A-2, serial numbers 1501 thru 3000 and the 50-A-3, serial numbers 25,001 thru 26,000 (?) as well as 26,501 thru 27,500. There may have also been a 50-A-4, serial numbers unknown.
Perhaps if those lucky enough to have 50-A's or -AZ's could post their serial number along with a description, it might be possible to piece together a chronology of design changes. That is, assuming we can filter out the subsequent conversions and other alterations . . .
For the 50-A interstage transformer, (Fisher part no. 507-1104,) Sams gives 586Ω CT .050A for the primary and 1.52KΩ for the secondary. The 50-AZ uses a different transformer, (Fisher part no. 507-123,) for which no specs are listed. All of the other transformers are the same in the 50-A and 50-AZ.
Also, I just noticed the Sams 50-A service manual lists both 5V4G and 5AW4 rectifiers in the data box on the cover sheet. I guess we can conclude Fisher changed over to the new tube before the introduction of the Z-matic version. The Sams does not show any changes to the power supply circuitry associated with the tube change but the schematic illustrates the earlier 5V4G's and does not mention alternative 5AW4's.