> Can another tube such as 5Y3 or 5U4 be substituted for > 5Z4? Have you done it? 73, John, K5PGW The 5Y3, 5Y4, 80 and 5Z4 have 2 amp filaments. The 5U4 and 5Z3 have 3 amp filaments. The Z3 and U4 are equivalent except for base and socket connections. Don k4kyv