So I've had this 1966 Ampeg Gemini I for years, was given to me by someone like 15 years ago but I never used it much because it needed repair. Recently I decided to take it to a tech and get it fixed. Since I have decided its my favorite amp and have been trying to find out as much about these amps as I can. One thing I have yet to find an answer to is why the amp I have has been modded. The channel 1 1/4" guitar input is a knob and not an input. The tech who fixed it didn't give me any information about what the mod does. To me, it's noticable effects are as follows: On channel 2 it doesn't have any effect. On channel 1 it sort of acts as a volume knob. If the main volume knob is at 0, adjusting the mod knob doesn't do anything, the amp is just quiet. If I increase the volume on the amp then adjust the mod knob it will increase the volume of the amp, it also may be slightly more punchy with the mod knob cranked. Besides that I'm not sure why this was done and the previous/original owner is deceased. Any help or information would be appreciated, thanks!