Mellotronists

Upon listening to the three mp3 samples, My machine (M-400 #1017) 
does the same thing--to a somewhat lesser degree, but noticeable 
nonetheless.

Now, I'm no 'tron techie, but the first thing that crossed my mind 
was the flywheel. The new MkVI's have a more massive flywheel that 
provides more inertia, thus reducing the likelihood of slowing down 
when many keys are depressed at once. Then comes the troubleshooting 
that may separate the warbling from the drooping sounds when 
multiple keys are played in particular, such as the motor 
controller, the current through the pitch knob, lumps/imperfections 
in the drive belt, voltage fluctuations, the tape frame out of 
square, tension, glazed/stretched tapes,--so many factors.

I had kind of figured that in my situation, the slight warbling may 
have incurred in the transfer from the original masters to my set of 
tapes. I was at Kean's house when I had both sets made from the 
original masters on what was at the time equipment around 25 years 
old...and it may have been very likely that since it was at his 
house--voltage fluctuations indeed may have occured like a 
refridgerator in the kitchen or the air conditioner kicking on in 
the living room while the transfer was under way. Highly unlikely, 
but barely worth mentioning. That would have only affected maybe a 
note or three and not every one, however...and not account for 
warbling through an entire set.