Mellotronists

I have found that removing the headblock to demagnetize is not 
necessary. But, if you should do so, be sure to short across the 
contacts of the receiver plug at the back of the preamp box before 
you plug the headblock back in or you run the risk of magnetizing 
the headblock all over again... and the correct procedure is to only 
demagnetize one head at a time.

Unless of course you have a really big one (ooh missus!) like JB, in 
which case you can do the whole headblock in one swoop.