Chordes

Do not worry about the extra length of the cello strings. Just cut the rest you do not need. For the viola is about O.K., you need to cut a few cm.Any case, the new strings will not change the air resonance. You need to find somebody to open a little bit the sound holes.

Thank you very much! You answered many of my questions.
Concerning the string length of my lyra I made a big mistake, I measured the w h o l e string. Measuring from the upper to the lower kavalari it is 28,7 cm. So, if I order the strings from PYRAMID, are they not much too long or is there something which I do not understand? So far I played with the strings that were on the Lyra and I was not concerned about obtaining new ones. But now with the sound problem I would like to try the string solution first, because I am not familiar with wood work.
Thank you.

The complete unswer of all questions is impossible here, all are treated in the book and some in the site context.
I will try to answer these possible in short comments:

1. Which holes do I have to enlarge? And how do I do it? And what is Helmholtz (air) resonance?
The "holes" are the sound holes on the belly of your lyra. You can try to enlarge them (if you are used to work with wood using a cutter and a file, otherwise find a friend that can do it), subtructing any time a tenth of a mm from the wood in the round part (not the right central area), up to the point that the rissonance is mooved out of the 329 Hz. In the internet you can find a lot of articles regarding Helmholtz resonance, try them that study the violin air resonance.

2. String tension:
For Pyramid, there is no string tension chance. The C 15,5" (e.g. 39,4 cm active string length) has the code 139 104, and for the Cello 1/2 (active string length 58.6 cm) La is 170 101

3. String angle or bridge angle:
This is the angle of the string on the bridge, in degrees, the angle between the extention of the active string line and the rest of the string after the string: this determine the tension applied on the belly from the string:larger the angle (higher the bridge), larger the string force on the belly.

4. Active length of Cretan lyra strings:
This must be between 28-29,5 cm. I cannot understand what kind of lyra you have with a 40 cm active length. If this is your active string length, everything changes! This is not a Cretan lyra.

Thank you very much for your reply and your recommendations. Since I am still quite a beginner in learning to play the Cretan Lyra I have more questions concerning of what you said:
The easiest way to change this is to enlarge the holes aiming to arise the air resonance a few Hz, to move out of the Mi frequency.
Which holes do I have to enlarge? And how do I do it? And what is Helmholtz (air) resonance?
As for the strings itself, in the shops there are 3 kategories of tension mentioned, low, medium and high. In the description of the strings it says that medium is the normal. They don't kategorize in kg.
What is a string angle (15 degrees)?
What measure is 15.5 (for the Viola strings)?
The strings (Dogal) that are on my Lyra now are about 40 or 41 cm long (energo mikos.

Sorry that I write all this in English, but my Greek in this subject is not very good, for the technical terms.

Thanks a lot!

The range of the string tensions used is reported in the relative table (Το όργανο-The istrument/ Χορδές-Strings). I cannot quarantee any "ideal" tension, but, for maintaining the "character" of the lyra sound, I suggest (for string angle 15 degrees) for La 11-12 kg, for Re 8.5-9.5 kg, and for Sol 5.5-6.5 kg.
Any case, the instability in the sound of your instrument, at the 329 Hz (E4), is not related to the string quality! It is some kind of wolf tone, most probably related to the Helmholtz (air) resonance of your instrument: if you check the air resonance of your lyra most probably will be very near to the 329 Hz. The easiest way to change this is to enlarge the holes aiming to arise the air resonance a few Hz, to move out of the Mi frequency.
Returning to the strings question, apart from those suggested in the table, you can test also the Pyramid strings, a much cheapest and working solution: try the Viola 15.5" C-4th string, for Sol, and the 1/2 Cello A-1st and D-2nd for La and Re.
A notice: the next weeks I hope to have active also the English version of the site!!

I have red the article about lyra strings in the book. I have problems with the MI (on the RE string), when I push, which trembles in an unpleasant way. I thought to buy now the adviced strings, for SOL the DO string for Viola from the serie Chromcor/Pirastro, for RE + LA from d'Addario Prelude for Cello 1/4. When I look up the artikles in the Internet, one has to tell which t e n s i o n is wanted. And I am not sure which tension is the ideal one. Can you please comment.
Thank you
Kosmas