Micro Seiki ma-505 x is a low or heavy tonarm?

That arm has a 14.5 effective mass. You can use this chart to calculate if the cartridge that you want to use would be compatible with that arm!

http://www.mh-audio.nl/Calculators/RF.html

Thank you so much for the link-)

one question : Where from did you get the number 14,5 gr for ma 505 effective mass?

Here ya go!

14.5 grams

Using a basic effective mass calculator (link below), the Micro Seiki MA-505 has an effective mass of 14.5 grams. It needs another ~16 grams to achieve a new effective mass of about 30 grams. To add 16 grams to this tonearm, more weight at the headshell and more counterweight has to be installed.

https://www.micro-seiki.org/denon-103-project

MA-505 Denon 103 Project — Micro

I am always puzzled by quotations of tonearm effective mass. Let’s assume 14.5g is the effective mass of the tonearm plus its OEM headshell. To this value, one has to add the mass of the cartridge and mounting hardware. In the case cited, that would be the mass (weight) of a Denon 103, which is a rather heavy cartridge as cartridges go. Furthermore, many of the many mods available to the 103 will further increase its mass. So let’s say the 103 weighs at least 10g. That’s a low end estimate. Then let’s say the mounting hardware adds 2-3g. In this case, you only need to add 3-4g to the ensemble to achieve a 30g total effective mass. Probably you wouldn’t need to add any mass over and above that of the DL103 plus hardware, because probably the DL103 weighs more than 10g. Then there’s the counter-weight.

Well, thank you again, but I found this picture

where it is said that for Ma 505 is ...; 7 gr.

Here is the link ..... I hope it is ok

No, I can not  add this link .... any way it is a micro seiki brochure where it is said that ma 505 effective mass is 7 gr.