GEESH ART - If ya got questions don't be afraid to ask! (Art & I have exchanged a half dozen e-mails the last two days!
Early T-A's were a dry clutch operated planetary gear reduction unit with an over-running clutch located between the engine clutch & transmission, in fact the small T-A's the output gear was the constant mesh and 5th gear driving gear inside the transmission.
As Paul R said, the planetary gave 33% speed reduction while increasing pull power 45% according to IH. The over-running clutch used was a "ramp & roller" design so when the T-A clutch engaged there was no lurch against the already loaded gears in the trans & rearend unloading them then quickly loading them again in the low range. The T-A shift was a seamless increase in gear reduction, no break in power because of that over-running clutch. The newer style T-A's use what they call a "Mechanical Diode" but I've never actually seen one so I can't really describe their operation.
The first T-A's in the SM-TA had a bolt-together housing and ALL later T-A's used a more durable 1-pc casting. The same basic T-A unit (with the exception of that case) was used in EVERYTHING from the SM-TA (1954 vintage) up thru the 686 last produced in 1980 regardless of tractor size & HP.
Most problems with the T-A's were with that over-running clutch. As the ramp & rollers wore the rollers got flat spots and the ramps wore where the rollers ran and they would not "Catch" which allowed the tractor to just stop when the low side of the T-A was engaged. Shifting fast, no feathering into & out of T-A, allowed the rollers to securely seat themselves. Also allowing them to free-wheel by engaging T-A and letting the tractor coast accelerated wear on the O-R clutch. Always best to ONLY pull the T-A back when under load.
The later BIG frame T-A's were a totally different design and only had a 22% speed reduction. They were built to transmit 100+ HP also.
When doing field work in varying soil conditions and hills the T-A let you use the gear that gave you the best speed in good conditions and still pull thru the tough spots & hills or slow down for turns. A real productivity improver. Normal operation was to let the engine pull down from full load RPM, SM-TA, 400/450 ran 1450 RPM so about 1000 RPM and pull the T-A back and the engine speeds back to 1450 as quick as it can and picks up the load, 300/350's ran either 1750 or 2000 RPM so either 1150 RPM or 1300 RPM. Once the tractor cleared the hill or tough spot and the engine was running at speed and not laboring, squeeze the release handle and let the T-A lever fly forward and the tractor smoothly accelerated back to full speed for that gear.
The operator's manual had pretty explicit instructions for adjusting the T-A clutch and engine clutch. Those instructions should be followed. The T-A linkage effected the disengagement of the engine clutch and lots of times pulling the T-A back was required to reduce the bind the engine clutch was causing on the sliding gears in the transmission. As with all mechanical things, wear in the linkage pivots after 50+ yrs of use can make adjusting them difficult.
From what I understand a good new H-D remanufactured T-A costs between $500-$600. Bad part is to R&R a T-A requires splitting the tractor between the clutch housing & transmission. Best way to keep them healthy is to USE them and adjust them as needed.
Wyatt & I had MANY laughs watching the PD #1 video, I was caught on tape reaching for the T-A lever on my CC #72 to slow down at the end of my first round up by the barns. Some 40 yr old habits die REALLY hard. We talked about how a T-A could be built for use in a CC. I might just try that some day! It would NOT have the O-R clutch like production T-A's had, it'd be more like the prototype T-A's IH built in a couple H's back in 1951.
