Modal & Diddley Beat?

Not sure what you mean by modal.

The Bo Diddley beat is actually a version of a widely used Latin rhythm pattern called clave (KLA-vay). THe influence of Latin music in the 1950s led to the use of clave in both blues and rock&roll. (Technically, there are are several types of clave, and the one I'm talking about here is called son clave)

Clave has five hits spread over 8 beats (two bars of four beats each).

The first bar has three hits (first beat, halfway through the second beat, and fourth beat).

The second bar has two hits (second and third beats).

The Bo Diddley rhythm is a slight elaboration of what's called 3/2 clave, or forward clave, which is played in the sequence described above.

Got My Mojo Workin' uses a version of 2/3 or reverse clave, where you lead with the second bar of the pattern.

If you get familiar with the sound of this pattern, you'll start to notice it in all sorts of places, including some harmonica rhythm patterns. One place to start is this intructional video by Michael Spiro:
http://www.youtube.com/watch?v=oDPRz0J45gc

People use "beat" to mean "rhythm" all the time and nobody gets confused unless the discussion gets technical, so no biggie.

Modal (and mode) are terms that refer to ways of using scales, whether or not there are chord changes (the idea goes back at least too the ancient Greeks, though each era treats the idea in its own way). The basic idea, at least nowadays, is that you can start a scale on any note in the scale and treat that as the "key." This make the scale sound different. It's sort of what you do every time you play in second or third position (though you also add new notes by bending).

So what you're really referring to are one-chord tunes that might use the Bo Diddley beat or some form of clave.

Hmm.

First, if you listen carefully to the tune "Bo Diddley," it *does* have chord changes. Not in the obvious way like a 12-bar blues. Brief little chord changes are tucked in to help fit individual melody notes, even though the overall impression is of a one-chord tune because you don't have a big chord-change "event" like going to the IV chord on the fifth bar.

But that quibble aside, there are plenty of one-chord tunes in the blues. Most of them pre-date Bo (who was considered a rock&roller and was much younger that Hooker, Muddy, et al) and therefore weren't influenced by him. It's the later blues artists and the blues-influenced rockers of the 1960s where you start to hear his ideas taking hold.

Each of the older artists tended to have his own take on the one-chord, partly due to strong personal identity, and perhaps influenced by where he came from in the South.

Howlin' Wolf had his own trance-like take on the one-chord tune ("Smokestack Lightnin'," "Moanin' at Midnight," "I asked for Water," etc.).

Muddy didn't seem to do so much with one-chord tunes, with a few exceptions, such as "Rollin' & Tumblin'," "Mean Red Spider," and a few others. ("I Just Want to Make Love to You" almost qualifies, but then there's that pesky bridge that goes to the IV chord and ruins it all!)

Little Walter had a few, such as "Hate to See You Go" and "Roller Coaster" and perhaps "Thunderbird" and "Flying Saucer" (not sure about those last two, so don't take my word - go and listen). However, the first version of "My Babe" was a one-chord tune with a slower tempo that was released as "Mercy Baby." It was only after Willie Dixon heard a gospel group sing "This Train" that the song got the makeover that led to the hit record.

Perhaps the king of the one chord tune was John Lee Hooker, with his signature "boogie" lick that was so successfully taken up by Canned Heat (as in "On the Road Again"). Again, a highly personal rhythmic signature that wasn't influenced by that young upstart Ellas McDaniel.

Hooker came from that old southern tradition of not keeping strictly to the structure of the 12-bar blues - when he even used it at all.

Partly, self-accompanied singers didn't need to coordinate time with anyone else. Listen to the earliest SBW #1 records from the late 1930s and you'll hear a group of players all colliding at times because they each have a slightly different idea of where the chord should change. They weren't use to playing with a group, or maybe just with each other, but over time they found a coherent Chicago sound that, while different from the later 1950s Chicago sound, had a cohesive identity.

Also, record companies such as Bluebird, in order to create a more presentable product and one that would reflect their brand identity, pressed artists to adhere to a more regular chord structure and even to a particular rhythm - the "Bluebird Beat" was a marketing phenomenon from that 1930s period. (Bo came up with his own signature beat without any prodding from Chess.)

But every now and then you'd get an unreconstructed country boy like Lightnin' Hopkins or Hooker who continued to go their own way and left others to keep up. If it was a one-chord tune, any lack of synchronization was less obvious. But as soon as chord changes came into the picture, an accompanying player better be all ears and quick on the move! (As a teen I used to play with Native American country singers who approached music this way, and it kept me on my toes!)

Muddy, as always, developed an ensemble approach to "breaking time." His band could play a 12-bar that wasn't 12 bars, either adding or clipping measures and beats in a completely coordinated fashion, like a well-oiled machine at the command of the leader. I remember when my teenage blues band started to develop the ability to just hang on the chord until the right moment that was not dictated by a template. It felt so good to be able to hang loose and yet all step together when we agreed on the right moment.

Walter is another case again. "Juke" has some moments of rhythmic weirdness that feel completely natural, partly because the ideas were flowing freely from him and partly because the band listened, hung loose, and landed together when they could tell where the "1" beat finally landed. In "Roller Coaster" Walter keepsshifting the location of the "1" and the only way you can tell is by listening to the "ding-dong-ding" guitar line that runs through the entire tune. I recently transcribed this and it was not at all easy. "Mellow Down Easy" is another one that sounds perfectly natural until you start counting time in his solo and realize that what sounds like a 12-bar blues isn't.

Listen to this. It's Gary Primich's "House Rockin' Party" from the Travelin' Mood CD (which is a great one).

http://new.music.yahoo.com/gary-primich/tracks/house-rockin-party--940735

It's a "Bo-Diddley" beat, not 12-bar, but with chord changes.