Hammer Bubbling

This is the second part of a two-part post.  The first part is here.

The Baldwin grand was unplayable.  All I was supposed to do was tune it, but I couldn't even do that.  Every note was bubbling.  When the hammer heads hit the strings, rather than making clean strikes, the heads bounced against the strings, striking multiple times.  That is called bubbling.  Usually when it happens, it happens with a few notes, or with most of one section, but in this case it was over the entire keyboard.

What causes bubbling is that the keystroke is not matched properly to the hammerstroke.  When you press a key, it moves a certain distance - this is the keystroke.  The movement of the key causes the hammer to move from rest to contact with the string - this is the hammerstroke.  If there is too much hammerstroke, or too little keystroke, or too much free play between the key and the hammer, bubbling will result.

There are some quick fixes:  you can eliminate the free play (called lost motion), then increase the let-off, or raise the hammerline, or increase the key dip, or raise the key level.  The only problem is that each of these corrections may introduce distortion and unevenness to the action regulation.  Slight distortions are tolerable, but past a certain point you simply trade one problem for another.  Ultimately, bubbling indicates that the action needs a more thorough regulation.

Six months ago I had tuned this piano, and its action seemed fine.  Why was it so far off now?  I checked my notes, and found that I had had to eliminate some bubbling then.  I had also noted evidence of past technicians making quick, ad hoc adjustments to the action.  My plan was to regulate the action thoroughly this summer, when time and budget permitted.  But I had to do something right now.

There was no lost motion to eliminate.  Normally this is good, but I was hoping for a quick solution, and this is the first thing to try.  The keystroke felt too shallow, as if the entire key level needed raising.  Unfortunately, that's exactly what I did not have the time and budget for now.  At a glance I could see that the hammerline was all over the place; that could be quickly adjusted.  The let-off was fine, so I decided not to disturb it.  Nor did I disturb the front rail punchings under the keys.  Then I thought of something else.  One reason the regulation would change so drastically from one season to the next would be if the keyframe was not sitting securely on the keybed.  There are special bolts called glide bolts for ensuring this, and I made a quick guess that they hadn't been regulated in a very long time.  You can sometimes feel the poor seating in the action, a kind of bounciness, and it will change with humidity.

Regulating an action is like solving a Chinese puzzle - each move affects other moves.  Some moves need to happen at the beginning, other moves must wait toward the end, and a lot of moves will need to be repeated.  Typically the end moves are done regularly, but the beginning moves will be neglected because they necessitate moving through the entire regulation, for which there is often no time and insufficient budget.  Regulating the glide bolts is such a beginning move.  But today I decided to gamble on an unorthodox move; I would screw down the glide bolts far enough to actually raise the key level just a little.  It's a hack, but a creative one.

I adjusted the glide bolts, raising the key level just enough to help with the bubbling but not so far as to cause other problems.  Then I raised the hammerline as far as necessary to eliminate the bubbling, even at the risk of reducing the hammerstroke a little too much.  For a handful of notes there was cause to increase the let-off a hair.  The bubbling was gone, the action felt a little odd, but it was playable, and perfectly fine for a practice piano.  It had taken a little over an hour, thus preserving the budget and not eating up too much time.  But I will have to regulate it properly from top to bottom soon - any more creative hacking will just make things worse.  Thank heavens there won't be another technician after me to say "What the hell was this guy thinking!"