I have measured the charging voltage and it has been 13,6-13,8V ....... that's what I mean by "on the border"
The charging voltage must, what I have learned, be at least 13.8V for the battery to charge.
This is not true. A modern battery is a 6 cell lead acid battery and since all cells are 2.2 volt nominal that adds up to 13.2 volts nominal maximum. Charging systems should charge anywhere between 13.4 and 14.4 volts. It will vary depending on load and engine/alternator revs. So your 13.6-13.8 volts is perfectly fine. You can see the reason why it must be 13.4 volts minimum charge so that it is at least 0.2 volts above the battery possible maximum and will have a positive charge.
Now there are some new batteries that have a silver-calcium content and specifically are used on many Fords and require upto 14.8 volt charge, so the newer Fords are tweaked to charge at that higher voltage. If you fit a calcium battery to a car not designed for them, then you could have a problem with the battery not getting enough charge.
The fact that the charging voltage has been so low may well indicate that one diode is defective, it will lose approximately 30% of the charging voltage if so.
Now the charging voltage have dropped to 13,3V at 3000 r / min and the charging light has started to light. This would then perhaps indicate that two diodes are defective?
Not sure why you think it has to be one or two diodes, but faulty regulators are more common than diodes, especially with these Paris-Rhone/Valeo alternators. A faulty diode usually either stops it charging altogether or allows it to drain as it allows a reverse current flow. In my experience these alternators are highly unreliable (compared to good ones like Bosch and Hella). Since any Murena is old, and you have a new regulator, have you check all the other contacts? I have found the wiring and contacts inside the rear of the alternator either badly corroded or even broken. This is where I would start. Don't forget the earth side too. Check alternator to engine and engine to chassis earth.
When you are checking the charging, always test the voltage at the battery terminals. If you take readings anywhere else be prepared for a drop as the circuits are notorious for loss of voltage owing to poor connections. Modern normal lead-acid batteries cannot stand voltages over 14.4 volts so you couldn't use one on a Ford designed for upto 14.8 volt charge. Modern batteries only have lead 'sponge' plates unlike the older batteries which used to have proper lead plates and had a higher internal resistance. This was the reason old cars with dynamos used to charge around 15-16 volts and if you fit a modern battery to an old 50's or pre-war car you have to adjust the regulator down otherwise it overheats the battery.
Does anyone know where it is possible to buy the diode bridges for the Paris Rhone A13N10 alternator?
It is now classed as a Valeo so try searching under that name too, but I always found spares for these a problem since they were not common in the UK in the past. However a quick check shows lots of Valeo diode packs for sale at about £28.
Roy