I was looking at the Nostalgia Air tube substitution page and it shows that a 6S7 and 6K7 can be substituted for each other. On the 6S7 page there's an asterisk next to the 6K7 that means "parallel filaments". On the 6K7 page there is NOT such an asterisk next to the 6S7. Any idea what "parallel filaments" means? 73 Mark K3MSB