metaruss

♦ Back ♦ Description ♦ Construction ♦ Performance ♦ Pictures ♦

All of my previous tube amps were of push-pull design, mainly because I felt I needed the power output. Since getting back into tube audio, I have designed several projects based on that assumption and have collected most of the components needed to build some of them. They have all sat in boxes waiting for me to have the time to commit to them. I also set a personal goal to complete my preamp before building any of them. The result of that goal has been that the only amp that has been in service for the past 8 years has been my Thomas amp. 8 watts of medium-fi audio that sounds pretty good at reasonable volumes through my mediocre speakers (it runs out of breath when it tries to rock hard, though).

I have always wanted to build a big single-ended amp, but over time I have come to the realization that I don’t need 100W of power to be happy with one. I really wanted to give a good SE amp a try, so I hunted around for a simple design that would give me at least 8W. I eventually came across this design by George Anderson. He calls it the “Simple SE”. The whole thing lives on a single PCB, which makes building one to try a cinch…at least until you go overboard with the surrounding chassis.

The amplifier is a two-stage design with a single 12AT7 serving as the input stage to both channels and a pair of EL34/6L6/6550/KT88 type tubes for output. Pretty much anything that is pin-compatible with these will work with some minor tweaks. Each of the input stages’ plates are loaded with a constant-current source, which helps provide the necessary gain. The output tubes are cathode biased, which not only simplifies the design but also allows for optional local feedback to the output stage for driving difficult loads and/or helping push through cheap output transformers. If the output transformers have an ultralinear tap, the board supports that as well as triode mode. The B+ power supply uses a 5AR4 rectifier, but the PCB also supports solid-state rectification.

This Simple SE build allows all of the above options to be selected via a series of toggle switches on the top plate. It also allows 1 of 6 different cathode resistance settings so that just about any tube type can be setup at its most optimal operating point. That is controlled by the knob near the external capacitor. To ensure that the resulting cathode current on a particular set of tubes is as expected, a set of banana jacks are tucked-away behind the capacitor.

More about the build-up of this amp is documented on the construction page.