Mono questions

I have a couple of mono questions on a stereo system:
1.  Will playing a mono recording be the same as engaging mono on the preamp?  For example, if I played the Beach Boys "Pet Sounds" stereo, with the mono button engaged, would it sound the same as playing the mono version of "Pet Sounds."

2. I have read that while playing a mono record, I should also engage mono on the preamp?  Is this true?  Why?  If the source is mono, the preamp should reproduce what is fed into it regardless of mono setting on the preamp.  Thanks for helping out a mono newbie.
1. No. The mixes for the stereo and mono versions are usually completely different
2. Engaging the mono switch can remove hum that is often endemic to mono pickups. For example with my Miyajima Zero I get hum pickup that is completely removed by using the mono switch on my preamp 
Folkfreak is correct in noting that back in the early 60s, studios frequently staged two sessions, in one of which they recorded in mono and in the other, in stereo. In those cases, the two recordings can never sound exactly the same. But I don’t think that this is your question. I think you are asking about whether or not you "need" either a mono cartridge or a mono mode on your linestage to get the most out of your mono recordings. In that case, the answer is a resounding yes.

On mono LPs, audio information is recorded only in the lateral plane, whereas in stereo, both horizontal and vertical motion of the stylus are sensed by the stereo cartridge and converted into two channels of information. If you play a mono LP into a stereo circuit, the cartridge will reproduce vertical modulation of the stylus velocity as noise. If you then depress your mono mode button, there will be phase cancellation of the noise thus produced. This really does work to improve the listening experience, in many ways. If you likewise use a mono cartridge, there will be a similar positive effect.

One can of worms is that SOME mono cartridges are merely stereo cartridges in which the two channels are internally bridged to produce a mono output. In theory, this is identical to engaging the mono mode button on your preamp. Some other mono cartridges are "true mono", built such that there can be no voltage output in response to vertical movement of the stylus assembly. Whether using such a cartridge is audibly superior to using either a mono mode switch or a "bridged stereo" cartridge with mono output is open to debate. And believe me, it’s been debated to death. Finding out which mono cartridges are true mono vs bridged stereo types is also tricky. You have to read between the lines of ad copy to deduce what’s up. There is also a vociferous group who believe that a mono mode switch is all anyone ever needs to get the most out of mono LPs.

One more thing, as regards the comment on Miyajima mono cartridges.  The hum from some Miyajima cartridges in some set-ups is not endemic to ALL mono cartridges. It actually arises BECAUSE the Miyajima cartridges are among those few on the market that are truly mono in build. When there is some potential difference (voltage) between the grounds of a stereo preamplifier receiving the ground leg of the signal from the Miyajima, that small voltage can be converted to hum.  This is curable.  I don't think (but I am not sure) that this would happen with the more common "bridged stereo" type of mono cartridge. Hum is not an inevitable consequence of mono reproduction.

Great comments fellas, but allow me to make one correction. What studios did in the 60’s (and not just the early 60’s, but all the way through the decade, stereo LP’s not becoming standard until ’68 or so), was not record two separate sessions, one for monaural and one for stereo, but rather simultaneously recorded each session on both a monaural recorder and a multi-track (whether 2, 3, 4, or 8). Doubling the recording time would have been far too expensive to do otherwise. The best studio musicians are paid up to triple scale, and a room full of them adds up to a lot of money! Not to mention the studio time itself (even in the 60's, more per hour than most people made in an 8-hour day), as well as the engineer, assistant engineer, and the recording tape itself, which ain’t cheap. A 10-1/2" roll of tape running at 15ips (the standard in the 60's) records only about a half hour of music, at 30 ips half that.

The resulting monaural and stereo LP’s in some cases can sound like different takes, as the stereo mix may contain parts not included in the monaural mix (mono mixes are done "on-the-fly", not in post-production). Even the mono and stereo versions of the same recording of some Beatles songs contain different guitar parts, making them sound as if they are different takes. They aren’t. Now, some of the 1960’s "live-in-the-studio" Jazz albums DID contain different versions of some songs on the monaural and stereo editions of the albums, but that was a result not of mono/stereo sessions, but only of different takes from the same session.

Odd. I wonder why I have original Capitol issues of "Something Cool" with June Christy, both the mono and stereo versions.  It takes 5 minutes of listening to know that these are two entirely different recording sessions.  She doesn't even attempt to sing the tunes exactly the same on the two versions (at least in most cases).  These are circa 1960 recordings.  Granted, I don't know how common this was, because I don't own enough mono and stereo pairs to judge.  But it certainly must have been done with some regularity, because June Christy was a very successful artist but no Sarah or Ella. Perhaps, as you say, these are admixtures of "different takes from the same session".  That would also explain what I hear, but why choose one take for the mono version and another for the stereo?  Does that not imply that in one case they set up the mikes for mono, etc?  I am not claiming to know the answers to those questions.
The answer is simple , Lew. Mistakes! The guy who is sent to the tape vault to retrieve the master when it is time to press the LP may or may not grab the correct reel for both the mono and stereo pressings. Those kinds of mistakes are more common than you might imagine. The original 7" 45 RPM pressing of Bob Dylan’s "Positively 4th Street" actually contains "Can You Please Crawl Out Your Window"---a different song! The 45 was immediately recalled when the mistake was discovered, making it an expensive collectors item. The song "Can You Please Crawl Out Your Window" was later released as a 45 on it's own, but the version on that 45 is a different one than the one on the mis-pressing! Dylans band on that song, by the way, is The Hawks, later known as The Band. Great song, with a blistering guitar solo.
BACK TO MONO said the mad genius. Who am I to disagree. Playing mono records on a stereo system? Another way to go is a Tru Mono cable. Google it. It's an ear opener!
onlyqualityhifi, how is the Tru Mono cable different than 2 into 1 connected to 1 into 2 RCA cables which have been available for decades, although for a bit less money?

I suspect that is the same as an "internally strapped" mono cartridge such as Lew mentioned, or most configurations with a stereo/mono switch on a preamp.