(via email response) For anyone else too embarrassed to ask on-thread why I
use left hand drill bits. This week an amp, as received from a well
respected UK maker, had a knob effectively seized on. The owner had never
touched them. The grub-screws were brass and overtightened so one nib of the
2 nibs, making a screwdriver blade slot, had broken off at assembly.

Tip
To free seized equipment knobs
For the situation where the knobs are seized onto the shaft by rusted
grub screws,especially where the screw penetrates the shaft;
after you have butchered the grub screw slot try this.
Make up some guide tubes,small enough to just slide into the hole
in the knob containing the grub screw,these tubes drilled on a lathe with
a clearance bore to take a drill bit. This drill bit usually needs to be
extended by brazing onto a longish rod (so the chuck of the
drill misses the face of the equipment).
Use some cutting oil and drill into the grub screw.
Ideally use left hand drill bits and left-handed power drill
rotation, such drill bits are available from specialist suppliers ,
other suppliers may kook at you as though you're trying to wind them up
(anti-clockwise).
To convert a right hand drill bit well enough for this use grind the
cutting face back on the opposite rake angle, swarf clearance
is not relevant here. Often the bite into the drill bit
into the screw or the localised vibration or heating is enough to
shift the screw.
Now use a small "easi out"(maybe this is a UK trade name),but consist
of a coarse left-handed cutting thread on a coarse taper.
Wind into the hole in the grub screw and hopefully extract.
If this fails repeat the first procedure with larger diameter drill bits
until nothing remains of the grub screw,retap a larger hole and use
a larger grub screw for knob reuse.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/