Hi Reever,
118.02500 ZLA53 Sector 53 Hi
This Sector 53, directly overhead Bullhead City and extending from FL240 to FL330, usually seems to be combined into Sector 35 on 128.075. When the sectors are combined the controller might (?) still be broadcasting on 118.025 but if you are not hearing any aircraft you will probably find them on 128.075.
Sector 36, 127.350, from FL340 and above, sits overtop Sector 53 and Sector 35. I suspect that when traffic is light or on the overnight shift they combine all 3 together ? Traffic inbound to LAX routing via Peach Springs (PGS) and the RIIVR arrival should switch over from 127.350 to ZLA 133.550 directly overhead Bullhead City.
I'm curious if you can hear the controllers on any of those frequencies dave posted ?