How can I reduce feedback in humbuckers without covers?
If the pickup has 4 screws on the bottom to hold the bobbins in place, I would carefully tighten them and not strip the plastic in the bobbin. Make sure the pole pieces are not loose and if they are, carefully put a small bit of silicone around the threads if the plastic is stripped out. Some bobbins have holes the size of studs that the adjustable poles go through and they will need extra work. I ve often put a small amount of silicone on the magnet if the pickup hasn t been wax potted. Wax potting can be done for extreme volumes and should be done by a qualified and reputable repairman. Wax potting needs to be done under a controlled environment and with proper equipment and ventilation. I ve put rubber bands around the ferrous spring steel heights adjust springs to keep them from feeding back under loud conditions. Jimmy Hendrix also plucked his springs on the back of his Stratocaster to get great tones especially when using his vibrato arm.
