The 6RV was made for the Italian Government and possibly used by the military. The heater draws 150 mA whereas the electrically identical EF39 draws 200 mA. We believe all other EF39 characteristics are shared by the 6RV but we have not seen a data-sheet. Other sources suggest that the 6RV is equivalent to the 6K7GT, a type also made by Fivre. The base connections are the same as for the EF39.
The 6RV is a variable-mu RF pentode, with sliding screen characteristics, for use as a controlled RF or IF amplifier. The EF9 is electrically identical to the EF39 but is on a Ct8 base. The frequency range of operation would be up to 30 MHz.
The left hand side of the Italian Ministry of Finance paper label.
The right hand side carries the date of 21/02/1941. The maker's name is pressed into the base skirt.
The label and base lettering 'Balilla Fivre'.
The wide glass tube envelope is 28 mm in diameter and excluding the IO base pins it is 76 mm long.
References: Datasheet, & 3002. Type 6RV was first introduced in 1940.