Can the secondary impedance of Lundahl output transformers, such as 4 ohms, 8 ohms or 16 ohms, be selected via taps?

With most Lundahl output transformers, this is unfortunately not possible without reconfiguration. Lundahl has usually chosen a fixed configuration in favor of better coupling and better utilization of its individual windings. This results in significantly better bandwidth and lower distortion, which benefits the sound. For the LL1620, LL1623, LL1627 and LL9202, we have attached an extra PDF datasheet showing how to switch between two impedances with three jumpers or three relay contacts per channel. If you want to commit to one setting in your DIY project, you should usually choose the lowest impedance for tube amplifiers, in this case 4 ohms, as this is often more advantageous sonically and the difference in power is negligible in most cases.