[Ron Ramirez]

There were at least three different tone control circuits used in the early (5-band) model 16.

First version:

   

This version was apparently not used very long, and was replaced with this second version:

   

This one is commonly found in 5-band 16 sets, and adds a dual .025 uF bakelite block while removing the 0.5 and 0.15 uF metal can caps.

Late in production of the 5-band 16, the circuit was changed to this third version, using three .033 uF caps inside the control and eliminating the .006 uF cap between tone control and plate of the 1st AF tube:

   

This last version was carried over into production of the 4-band 16 (codes 125, 126, 127).

There may have been another variation in between the "second version" and "third version" posted above; I don't remember right now.

Now a word regarding code numbers. All 16 cathedrals and the earliest tombstones used the Code 121 chassis. Code 122 chassis were strictly intended for consoles, running a higher B+ voltage and necessitating a 5Z3 rectifier in place of the 80 used in Code 121. Code 123 was a chairside unit with an upside-down dial scale.

Likewise, the 4-band 16 models may be broken down as follows:

Code 125 - Tombstones
Code 126 - Consoles
Code 127 - Chairsides