How can this be best prepared? Urea 3g Nabenzate 3g Tetracycline HCl 4g Beeler Base AD 80g

You have seen right. There is an incompatibility between Tetra and nabenzoate but this is desirable for the formation of tetracycline base. It is essential that all tetracycline is precipited. Therefore, I would replace the nabenzate with 3 G disodium phosphate 12 aq. This is known to be the right amount to convert all tetracycline HCl into tetracycline base. Because of this conversion to base, Tetra does not exhibit any incompatibility with nalauryl sulfate! As a preservative I would then add 0.2% sorbic acid. It would be useful to prepare 100 g and put 20 g aside to see how quickly discoloration occurs!!

PS: would be easier to work with Teracycline base but I doubt this product is still available in Vrac. Inquire with your wholesaler.