Older stepped attenuator with exposed SMD resistors and knurled shaft
Newer stepped attenuator showing the dust seal band and "D" shaped shaft
The VTA stepped attenuators for the VTA ST-70 and ST-120 amps came out in May of 2010. The earlier ones through the summer of 2010 had exposed SMD resistors and a knurled shaft. The newer models seen in the second photo have been used since September of 2010. The newer models now have the SMD resistors and the gold plated contacts on the inside sealed against dust. All the newer models also have a "D" shaped shaft instead of a knurled shaft. The black knob that comes with the attenuator fits over the "D" shaped knob and is held in place with a set screw . The reason for going to the "D" shaft is that sometimes a knob on a knurled shaft which is placed on the shaft with a "press fit" will (in time) grab the shaft so tightly that it is difficult to remove. Did you ever try to pull the bass, treble, balance etc. knobs off a vintage amp/preamp and have them feel like they were "welded" on the knurled shaft? The black knob was chosen because it matched the front black tube sockets and front black input jack backing board.
Bob
Newer stepped attenuator showing the dust seal band and "D" shaped shaft
The VTA stepped attenuators for the VTA ST-70 and ST-120 amps came out in May of 2010. The earlier ones through the summer of 2010 had exposed SMD resistors and a knurled shaft. The newer models seen in the second photo have been used since September of 2010. The newer models now have the SMD resistors and the gold plated contacts on the inside sealed against dust. All the newer models also have a "D" shaped shaft instead of a knurled shaft. The black knob that comes with the attenuator fits over the "D" shaped knob and is held in place with a set screw . The reason for going to the "D" shaft is that sometimes a knob on a knurled shaft which is placed on the shaft with a "press fit" will (in time) grab the shaft so tightly that it is difficult to remove. Did you ever try to pull the bass, treble, balance etc. knobs off a vintage amp/preamp and have them feel like they were "welded" on the knurled shaft? The black knob was chosen because it matched the front black tube sockets and front black input jack backing board.
Bob