I want to swap out my tubes with EL34s to fine tune my amp. How do I do that?
Even though the EL34s will plug right in and work, we do not recommend them for two reasons. First, they draw more heater current and will cause the power transformer to run a bit hotter. The extra heat probably won’t damage the transformer because we design our transformers with some extra margin (overkill). But why risk it? The second reason is more important: The output transformer impedance match is not correct for EL34/6CA7/6550/KT88 type tubes. It was designed for the 6L6/5881/7027/KT66 family of tubes. You won’t hurt the transformer at all with the EL34s, but the efficiency and performance will be less than ideal. The screen resistors will also have to be changed or your tube life will suffer.
If you are looking for a different sound, try different speakers. Unless you run the amp wide open on the master volume all the time, the changes that you will notice by changing tubes will be minimal because, until they reach clipping, 5881s and EL34s sound and perform exactly the same. Even if you are running at full power all the time, changing speakers is a better way to fine tune your sound than changing tubes.
If I change my tubes, do I need to re-bias my amplifier?
Most often, the answer to this is “No.” If you’re buying stock tubes, such as 5881’s or 6L6’s, there won’t be any problem. For other applications, such as a using KT66 or 7581A tubes, call Soldano for re-biasing recommendations.