Sinking Top

thombo · #1 07-27-2010, 06:42 PM

i have a '49 Kay M-1. the top is sinking at the inside f-holes (about 1/2 an inch), toward the top of the instrument. if that is confusing, inside refers to the part closer to the bridge; top means closer to the headstock.

1- what causes this?
2- how can it be fixed (i'll probably be going to a luthier, but i'm still curious)?
3- how urgent of a repair is this?

fdeck · #2 07-27-2010, 06:56 PM

I'm not a luthier, but while waiting for an answer, I suggest taking a look inside the bass with a mirror and a bright light, and see if the bass bar is still intact and attached along its length.

Here's my "sunken top" thread, with some potentially useful info:

Ply bass top sinking, what is normal?

thombo · #3 07-27-2010, 07:01 PM

bass bar is in in tact. thanks for the link, i did several searches and that thread never came up.

thombo · #4 07-29-2010, 03:36 PM

thanks for the link fdeck. i am still left with two questions though...

can anyone enlighten me as to what causes the top to sink?
how urgent of a repair is this?

thombo · #5 07-31-2010, 03:41 PM

Quote:

Originally Posted by thombo

can anyone enlighten me as to what causes the top to sink?
how urgent of a repair is this?

bump for the questions

fdeck · #6 07-31-2010, 03:49 PM

One thing I have noticed is that a plywood top is not as thick in the middle as a carved top, so maybe it is simply not quite as strong.

How's the back look? I would say it's in danger if it looks like the post is getting ready to go through the back.

davpal · #7 07-31-2010, 04:51 PM

Maybe the strings you are using have more tension (and thereby putting more pressure down on the top) than the bass was designed for. I believe I read somewhere that the older Kay's should have low tension strings as they were designed for the strings used commonly then - guts. I don't think it's a good idea to use high tension steel strings on these older basses.

fdeck · #8 07-31-2010, 07:37 PM

This thread may shed some light:

Bass Bar Problems

It appears that there was a time period when Kays had thinner tops -- 3 plys instead of 5. This might be possible to ascertain by looking carefully at the edge of the top.

thombo · #9 08-01-2010, 12:07 AM

thanks for all the responses!
the top is no worse now than it was when i got the bass a few years back, and the back of the bass looks fine.
the bass has had steels for as long as i had it- 1st was an old set w/ all red silks, and now it has kolstein heritage strings.
the top is 5 ply top.

William Hoffman · #10 08-01-2010, 01:48 AM

Quote:

Originally Posted by thombo

...the top is no worse now than it was when i got the bass a few years back, and the back of the bass looks fine...

now i am not sure what the problem is. if it's not the bass bar, and it's not getting worse all the while you've had the bass, is it really "sinking" (= continually sinking)? i mean, how do you know?

how is the sound post positioned?

maybe a picture would help, can you post one?

bassics1975 · #11 08-08-2010, 08:38 PM

I had a Kay many years ago (my first bass in high school) and it too had this problem. In fact, most Kay`s that I have seen tend to sink drastically in the middle. The top of a Kay is very thin compared to basses like Hofner requiring a well-fit bass bar to counter the force of the strings bearing down. I was told that the bars were originally sprung into place which is a practice that many luthiers today avoid. If done incorrectly, the spring could pull the top down rather than provide the necessary support. With a carved top, the bar is carved out and the top is reshaped with a plaster mould of the ideal profile and bags of hot sand before a new bar is fitted. I would assume that plywood would be easier to reshape. I hope this helps.

robobass · #12 08-09-2010, 05:19 AM

Quote:

Originally Posted by bassics1975

I had a Kay many years ago (my first bass in high school) and it too had this problem. In fact, most Kay`s that I have seen tend to sink drastically in the middle. The top of a Kay is very thin compared to basses like Hofner requiring a well-fit bass bar to counter the force of the strings bearing down. I was told that the bars were originally sprung into place which is a practice that many luthiers today avoid. If done incorrectly, the spring could pull the top down rather than provide the necessary support. With a carved top, the bar is carved out and the top is reshaped with a plaster mould of the ideal profile and bags of hot sand before a new bar is fitted. I would assume that plywood would be easier to reshape. I hope this helps.

Reshaping the top with a plaster mould is a very labor intensive process, so usually only done on much more valuable (or potentially valuable) basses. Replacing the bar or even adding a center inlay is also expensive, but a bit more sensible on a plywood. This will not bring back the original shape, but will prevent further sinkage, and correct some of the associated tonal problems. In Thombo's case the sinkage seems not to be getting any worse, and he reports no sound issues. I would advise leaving it alone, although having it looked at by a top bass luthier couldn't hurt. Robert Ross is in Denver, if I'm not mistaken.