Preamp cutting out on left side

Not sure why you are in this forum but oh well, you are far from the only one. It could be your balance control. They get dirty and the contacts stop working. Try to gain access from the inside (power everything off and unplug first) and use a swab with high grade 99% isopropyl. While you are at it hit the input switch contacts in similar fashion. Chances are 25% or so that this is your problem. 

The reseating of all the tubes has seemingly solved the issue “for now”…it’s been 2 weeks and no cut outs , I’ll report back after more time passes , thanks for the help . 

Tube equipment these days tend to be very reliable. The non-tube stuff is usually rock solid in high end gear. Tubes are pretty reliable as well, but it never hurts to have an extra set of tubes around. If something happens, then swap tubes.

just to put a period on this problem ...it was the selector switch of my Manley Jumbo Shrimp Preamp that went bad and was causing a short on left side...replaced , all ok .