Maarten,
 
> Even with this program as a help, a major retune costs me over 2 hours, and a
> minor a small hour.
 
If you're new at tuning a keyboard instrument, I can understand that
it would take that long. When I just finished building my Zuckermann
five octave unfretted, double strung, clavichord, a tuning job took me
about 6 hours! But I wasn't using any devices except a single tuning
fork to get me started, and I had to learn about hearing beats (i.e.,
no prior experience tuning). Within a few months of regular tunings,
the time settled down to around 45 minutes for a complete job, and has
remained there. I use rubber mutes to quiet one string of each unison
while I tune the other, and moving those around takes as much time as
tuning each note. I wish it were like on a harpsichord where you can
turn off the whole set at once.
 
My instrument needs tuning very often, I think unusually often. After
a complete tuning, I will hear an occasional note that has moved from
where I put it immediately upon starting to play. I can touch up the
"quick drifters," and then get a couple hours use out of it before I
notice an annoying amount of notes going sour. After a day, it's still
playable, but a lot of notes will be wrong, and it'll take maybe 10-15
minutes to put everything right again. What's weird is that I can let
it go *forever* and it'll never get unplayable. Some notes will go way
wrong, and I can fix those quickly, but besides that the whole range
will shift around mostly together, so it's almost in tune relative to
itself (but still nothing's perfect). When I was taking lessons and
playing regularly, I touched it up every day before practicing, but
even then it needed the daily work. I haven't been playing regularly
recently, so that's how I know how it behaves over long periods
without tuning.
 
This tuning behavior has been consistent for six years through all
weather conditions, so I doubt I'm being struck by sudden humidity
changes that are out of the ordinary. I mean, it probably is humidity
that's doing it, but that's normal on this planet. (I've had the
instrument in San Diego and Pittsburgh, with the same tuning
behavior.)
 
In my letter to David Way back when I finished the instrument
(actually I wrote him about a year after it was done), I mentioned
that I had to tune so often. In his response he simply said that "your
instrument should stay in tune for a long time." Hmm. Not very
helpful.
 
I was surprised to see David Calhoun's comments that a clavichord's
sound blurs up a bit due to the double stringing, and thus can
tolerate being a little out of tune better than a harpsichord.  That's
how I understood his message. I'm surprised because I find the effect
of the double stringing to be just the opposite. Tuning has made me
very sensitive to the two strings of a single note being slightly off
the unison. I'm a lot more sensitive to unisons being off than to
other intervals being off. So I can tolerate a fifth that isn't quite
a fifth, etc., but as soon as one of the unisons starts beating much,
I go nuts. Then again, maybe I've become too picky. I've never heard a
piano (live) that didn't have some beating unisons.
 
David, as a Zuckermann rep, do you have any comments about my
clavichord going out of tune so quickly? I'd love to be able to do
something about it. Or have any comments anything else I said?
 
Chris