Tanglewood resonator won't stay in tune

I'm using strings 9-42 as I have always used light "bendable" strings.
However, when I brought it back to the shop to complain about the tuning problem, they said I MUST use 11-strings, not 9 and that's why it's not staying in tune.
This sounds like complete nonsense to me as there should not be any problem in tuning, or staying in tune with, lighter strings.

Take a look at this old thread discussing string guages on resonators.

Resonators like heavy strings.

I'm wondering if maybe the lighter strings are not "loading" the cone enough. I normay like thicker strings( I run 11's on my Les Paul) and I run 12's on mine, so I might not be of much help. Have you tried a set of heaver strings just for testing? Might be worth the $7 just to elemate that from list of possable issues.

Thank you for both replies.
No, I haven't yet tried heavier strings but perhaps I will when I get back home after a month away sans guitar.
However, both replies, as well as the link in kent_eh's post, refer to the sound rather than to the tuning difficulty that is my main problem.
As I mentioned in my first post, even after 5-10 minutes playing (with quite some string-bending) one or more strings are sure to go slightly out of tune.
I am most reluctant to believe that the very light gauge strings I use are in any way responsible for the tuning problem. Therefore, using a heavier gauge will not resolve my problem.
Comments welcome.

Thank you
Paul

I am most reluctant to believe that the very light gauge strings I use are in any way responsible for the tuning problem. Therefore, using a heavier gauge will not resolve my problem.

That may be true, but without doing the experiment, you'll never know for sure.

As the other Paul said, $7 for a new set of strings is a cheap experiment to confirm or eliminate a possible cause.

A theory, based on nothing but my thoughts:

I could see how it might be related, though. The cone is pushing up against the strings with a certain amount of force, and if that force is much stronger than what the string tension is providing, the movement of the cone might "overpower" the string tension. Heavier strings will "push back" harder, and be more of a "balanced" system.

On a strat, when you change string gauges, you often have to adjust the spring tension. On a reso, you can't (I don't think) adjust the "strength" of the cone's push.

I know on mine the cone/bridge will drop down quite a bit when I am bringing the strings up to pitch, and it seems for a few min after. Also just to jump to the basics..... Are the strings binding in the nut?

One last thing.../ have you tried diferent brands of strings?? Some strings just don't "like" some brands

Thanks again for the replies.
Yes, I have used strings of different brands (D'Addario and coated Elixirs) but similar performance with both.
No, I don't believe the strings are binding at the nut, particularly as the strings I'm using are quite a bit lighter (thinner) than those for which this dobro was designed. Nevertheless, next time I string it, I'll put a little graphite in the nut grooves.
However, there might well be something in the comments about the cone dropping with string tension. Yes, I think I will change to a heavier gauge (11's or 12's) when I get home again at end September.
Incidentally, the 12th fret intonation was a little out for the lower three strings which I was able to rectify by adjusting the bridge angle to a minor degree.
Since then, the extent to which the strings slip out of tune, although still there, has seemingly reduced to a degree.