Been where you are about a year ago. Can't say I found such a thread, so I might as well do a quick writeup
Just a quick introduction to R2R to get you kickstarted on consumer grade reel machines.
Reel Sizes and deck sizes
The consumer types of R2R use 1/4" width tape, known as quarter inch. There are two "sizes" of machines, the ones that will take reels up to 7" and the ones that will take up to the 10.5" NAB standard. The 10.5 reels (metal ones) typically come with a hole in the center and need a "NAB Hub Adapter" to be mounted on a deck. There are however several makes of plastic 10.5 reels that don't need a hub adapter as they mount directly on the deck's reel shafts. Although the metal NAB reels with hub adapters certainly look "cool", I personally prefer the plastic, hubless, ones for two reasons: First, they are lighter, making acceleration and stop easier for the deck (see the paragraph on brakes). Second, I have yet to find a hub adapter / reel combination that is perfectly centered. Slightly decentered reels in FF will cause vibration, uneven packing of tape and probably wear to the deck's motor bearings.
Tracks, heads and speeds
The decks, depending on model and heads installed, will be recording either 2 channel stereo (aka half track) or 4 channel stereo (aka quarter track). There are also some 4 channel models recognizable by the 4 VU meters on the front panel.
The 2 track models only record on one direction, 4 track models can be used to flip over the tape and record both ways.
Typically, a R2R machine will have 3 heads. One for erase, one for Record and one for Playback. The Rec and Play heads are not the same and each is optimized for it's task (one of the reasons R2R performed better than the early 2 head cassette decks). Rec heads require a relatively large gap and high output for better recording while play heads are better with a narrow gap to pick up small signals and high frequencies.
There are various types of heads depending on material. These are scaled in terms of performance and strength. Permaloy heads are known for being easy to wear and Ferrite - Glass heads are the toughest. Wear is caused by the friction of the tape against the heads.
The R2R decks typically run faster than compact cassette decks. The standard speed for the cassette deck is 1 7/8" per second (1 7/8ips). The typical "slow" speed for R2R decks is 3 3.4ips and the typical "fast" speed for them is 7.5ips (double and quadruple of the cassette speed).
Semipro decks also allow even faster (15ips) speed and special purpose versions allow slower speeds. Revox has several such versions of the B77 deck (another very well known and regarded deck). The Low Speed version runs at 1 7/8 and 3 3/4ips. And the Super Low Speed (SLS) version runs at 15/16 and 1 7/8ips. These decks are reportedly built for phone tapping and communications monitoring purposes to allow maximum unattended recording of communications or broadcast - they are not for music reproduction and enjoyment.
Although there is no rule about this, typically a deck will provide two speeds. 4 track decks offer 3 3/4 and 7.5ips and 2 track offer 7.5 and 15ips. There are the Technics and Otari that spring to mind that offer 3 speeds as do some of the professional Studers and others.
Also, the Technics RS-1500 and Otari 5050 offer exhchangeable headblocks where the user can switch from a 2 track machine to a 4 track machine and vice versa.
Transport and directions
There are also auto reverse machines (4 track stereo) where a sensor (typically conductive) will sense the end of tape and beginning of leader by means of a conductive foil splice there. Obviously there is no point to make a half track machine with auto reverse.
Additionally, there are differences in the transport design. There are single capstan machines and dual capstan machines. Easily recognizable by the one or two pinch rollers. On single capstan machines, the capstan "pulls" the tape and is typically found on the right hand side of the unit, after the heads in the tape path.
Dual capstan machines utilize one capstan (and pinch roller) on either side of the heads. The Right Hand Side (RHS) capstan pulls the tape as usual and the LHS capstan rotates a hair slower to tension the tape better over the heads
Dual capstan machines can be available as autoreverse machines. In such cases, the deck has provision for switching the capstan speeds so that in reverse the LHS capstan rotates a hair faster. Clever ideas have been used in this, like uneven flywheel diameters inside and belts changing place depending on rotation direction, or simpler ideas like direct drive motors.
Auto reverse machines need more heads than single direction ones. Since SideB tracks are located slightly offset from SideA tracks, the reverse play head needs to be offset accordingly from the forward play head. Some decks allow reverse recording, so they have 6 heads.
There are some other variations of the above two categories: I've seen at least one AKAI model with a single capstan and a huge pinch roller between the heads. This arrangement (with a set of head on either side of the capstan) allows for auto reverse with a single capstan.
Thre is also the so called "ISOLoop", where a single huge capstan and a reversing hub are used with two pinch rollers. The best known example of that design is the Technics RS-1500 family of decks.
Of tension and brakes
One of the issues a R2R deck needs to address is that the tape is large, heavy and the reels, especially the full ones, are of substantial weight. Stopping a full reel doing Fast Forward is not a simple task as the inertia caused by rotation is significant. Additionally, during play or record, the tape needs to follow a precice path to pass over the heads at a specific point, be tense and move precicely.
All this means that the tape path must be well defined and precice. For that reason R2R decks utilize a mixture of guide posts, pulleys and spring loaded tension rollers. The function of all these is to make the tape path as stable and precice as possible. Typically, the more such guides a deck has the better the design.
In order to create tension, when a tape is played, the supply reel motor gives a mild reverse torgue to the reel. This doesn't allow the reel to empty uncontrolled but causes a mild breaking action. The other reel, the take-up reel, rotates in the correct direction with torgue to take up and tension the tape.
In addition to guide rollers, all decks use one or another sort of spring loaded "Tension Arms". These are used to ensure the tape is adequately tensioned over the heads. Although dual capstan machines use the first capstan/pinch roller to achieve this, they typically also have tension rollers as part of the "tape handling" mechanism to cater (for example) uneven wounding of tape on the supply reel.
The tension arms in many cases double as a tape present sensor using a contact switch when the arm is tensioned by the tape loaded over it. Most decks won't even start if this tension lever isn't moved to the range where it "senses" tape.
Better and later models don't use spring loaded tension arms but servo controlled ones that sense the actual tension and adjust reel tension accordingly. For example, when the deck senses that the take up tension servo arm is slacking, the take up reel motor speeds up to avoid slack and as soon as it senses the arm moving back to the "safe" area, it reduces the take up reel motor's torgue to a lower one.
In order for a deck to actually stop during FF or Rewind, it is not enough for the motors to stop receiving power. That will ensure uneven stop and the tape will either be spilled (if the supply reel is fuller than the take up reel) or stretched and cut (if the take up reel is fuller).
Many decks use a solenoid that engages and disengages breaks applied on the reel plates. Breaks are typically felt pads mounted on arms or foils, wound around the reel plates and pulled or released by the solenoid.
Better and later decks utilize electronic breaks. When the deck needs to stop from FF, the supply reel's motor provides a high torgue to that motor while the take up reel's motor provides a mild torgue. That eventually and gently stops the reels. When the tape senses very little tape speed it engages the mechanical breaks that in this case act like "parking brakes".
About tapes
There are different types of tapes depending on formula and thickness.
The typical tape used by almost all tape decks is the Ferric oxide tape, similar to the Type I tape on cassette decks.
There was also a FeCr tape produced by Sony and if you can find a reel, get it and put it on a shelf for display. It's rare.
And, during the last years, there was the high bias tape produced by Maxell, TDK and others. Only a few decks can use this tape so it is also a collector's item.
Then, there is plain tape and back coated tape. Back coated tape is better mechanically and on other factors, like print through.
And then there's the thickness. There is normal tape, double play, triple play and - I've read - Quad play. The difference is in thickness. Thinner tape allows more to be spooled on the reel hence more play time.
Normal play will give you 1200ft on a 7" reel and 2500ft on a 10.5 reel. It is common to find 3600ft tape on 10.5" reels and I have found there are no problems caused by thickness. I've also tried 3600ft tape on 7" reels and they are fragile and ultra sensitive. Not a tape to FF or REW.
One thing that needs to be mentioned about tape is that some makes and formulas made by certain companies over a time period are known to shed. The tape absorbs moisture and the bonding agent holding the ferric oxide on the substrate is released in the form of a sticky substance. This ruins the tape eventually, sticks on the heads and guideposts and is a pain to remove. These tapes should be avoided but if one needs to be played (e.g. for transfer) it can be restored to good condition by mildly baking it at 40degrees Celcius for a couple of hours.
There's lots more to add to this, but I will leave that for later or others.
