LGB Tower Controller reported they cleared a helicopter for takeoff from a helipad at the same time as they cleared an aircraft for takeoff from a converging runway resulting in an NMAC.

Narrative

I was Local Control 1. Runway 26R has been closed for the last two days for painting. The complexity of the operation is difficult due to an increase in traffic using Runway 30. We also have not opened Local 2 during busy hours during the day. Local 1 has had to handle all the traffic in the air. I cleared Aircraft X for takeoff on runway 30@L4 for a left turn out. Helicopter called to fly the pattern from helipad 3. I cleared them for takeoff and completely forgot about Aircraft X taking off on Runway 30. Supervisor shouted loudly alerted my attention to the situation developing. I told Aircraft X to turn right immediately to avoid the helicopter crossing the departure path of Runway 30. I was in the middle of a position relief briefing when this event happened. The two aircraft appeared to come within a few hundred feet from each other.We have a team monitoring the operation this week. The evaluators were not in the cab during this incident. I will only speak for myself for being on edge because of the evaluation; the runway closure and the absence of a local 2 controller. I am trying to make sure all phraseology and procedures are followed exactly; even while its busy and complex. I clearly missed a situation that could have been prevented. While no accident happened; I take responsibility for this incident. I'm not sure what other recommendations I can add here.