The KT77 was the MO Valve version of the 6CA7, or the EL34.
The KT90 is a tube that was developed by the Ei tube company in Yugoslavia in the late 1990s to be a more Powerful Octal based Beam Pentode.
The B+ for the 6CA7/EL34/KT77 is considerably lower than for the KT90, and while the KT90 has the same Pin-out, the operating conditions are different.
Hope that this helps,
Happy Listening
The KT90 is a tube that was developed by the Ei tube company in Yugoslavia in the late 1990s to be a more Powerful Octal based Beam Pentode.
The B+ for the 6CA7/EL34/KT77 is considerably lower than for the KT90, and while the KT90 has the same Pin-out, the operating conditions are different.
Hope that this helps,
Happy Listening