The 10EE's had several generations of variable-speed drives. 

The very earliest variable speed drive was a hydrostatic drive coupling. This was changed during WWII to a variable-speed system that was a AC motor -> DC generator -> DC motor, and they'd vary the field on the DC generator to change the output on the generator to vary the speed of the motor. These were also the least efficient variable speed drive systems, and they were noisy and heavy as a load of lead bricks.

These early machines are known as "round dial" machines. The speed/feeds indicator operated in a circular fashion, and the indicator swept over a beautifully machined round brass plate. It was terribly sexy from the standpoint of visual appearance.

The next variable-speed drive was based on vacuum tubes, tube rectifiers, "thyratrons," tube voltage regulators, etc. These machines now had the "Square Dial" speed/feed indication - it looked more like an early shortwave radio dial. To me, the vacuum tube control is the sexiest system, but then I'm a EE and a ham radio nerd from back in the days when I built my own tube equipment & amplifiers. To most machinists, this was the system that scared the crap out of them. If you opened up the door to the motor drive system (which was on the front of the machine, below the chip bin), you'd see this multi-colored glow out of the tubes - which were about as big in diameter as a beer can, and twice as tall. The voltages in the "WiaD" drive cabinet were high - lethally high. Those who didn't understand tube circuits were wise to open the door, lose all of their bravery, and then close the door again. It was like open a small portal into a very scary mad scientist's lab.

By 1961, I think most of the tubes were gone, replaced by solid state devices. There were several re-spins of variable speed drives based on solid state circuits.

Today, if you slap a 5HP three-phase motor into a 10EE and a VFD, you've pretty much got the power issue licked.

The HLV-H's are much simpler - and also much less powerful. I think a HLV-H has a 0.75HP motor. The 10EE has it all - precision, accuracy, gobs of power to take heavy cuts. The HLV-H is a beautiful little machine, made for taking light cuts, making absurdly fine threads, etc.

Hydrashifts: These machines are basically just a hydraulically-controlled variable-ratio pulley set coupling the motor to the spindle via a belt. If you've seen how the snowmobile transmission setup works with those two large pulleys that smoothly change the ratio as the speed of the sled picks up? It's sort of like that. So what happens on lots of the hydra-shift lathes is that the seals on the little piston that compresses the sheaves on the pulley(s) starts to leak. This is messy, and when it gets bad, you have quite the mess on the floor on a regular basis. Fixing these machines is right up farmer/mechanic alleys - it's just hydraulics and seals. Most machinists don't want to screw around with these, because they don't like tearing into their machines to fix hydraulics - because lots of machinists don't really "get" hydraulic systems.

Bridgeport Series I mills use the same sort of variable-ratio pulley sheave system, only their open/close mechanism is the speed crank on the right side of the mill head. Don't fiddle with that speed crank unless the motor is running - changing the speed on a Series I-style mill is a formula to needing to tear down the mill head and fix the pulley and possibly the belt.

Today, most of the variable-speed issue is done with 3-phase motors and VFD's. Both my Sharp lathe and mill use a VFD and a 3HP 3-phase motor. They're pud-simple by comparison.