20LF6 hey? I wonder if that amp has the tube filaments in series. If that is the case then measuring the filament voltage at the empty tube socket may not indicate the true voltage applied to the tube due to no voltage drop across the other filaments. just something to bear in mind. Drop another 20LF6 in the socket and keep a close eye on it when you turn it on. Note the color of the filament as referenced to the others and look for ANY color on the plate. There shouldn't be any.