General view from above General view from above 6SL7 high-mu twin triode 6SL7 high–mu twin triode 2A3 direct-heated power triode 2A3 direct–heated power triode

The Circuit

With schematic design I foreknew only the one thing: it must be Single–Ended Direct–Heated Triode Amplifier. Of course it must run in pure class A. This is the only efficient way to get pure sounded amplifier. Yes, I'm purist. There are many good solutions in the DIY community, but nothing seemed satisfactory to me. After looking at different circuits in the Internet, I have decided to calculate my own circuit design.

At first I assumed J.C. Morrison's «Fi Primer» schematic as a basis. However I was not sure about direct–coupled input/driver stage in there, and lately my doubts were confirmed: driver stage gave a lot distortion, and I have changed it for timely–tested SRPP stage.

The SRPP is the optimal choice because it offers low output impedance and high gain output with minimum THD. I have designed it using 6SL7 high–mu twin triode. The tube is very linear and have good sound reputation. I took the Russian 6H9C NOS 1975's for its quality and availability. For the beginning it is very good sounding option. In the future it can be changed to NOS Sylvania version. The SRPP operating design works like a push–pull amplifier that uses the current flowing through the bottom 6SL7 triode to define an inverted signal to drive the top 6SL7 triode.

For output tube I took 2A3 direct–heated triode. The 2A3 tubes have been very popular for their gentle, transparent and detail sound. This tube in not so sweet as 300B, but has rather similar smooth and at the same time dynamic sound. Reasonable price was also a plus. As a biasing type I prefer cathode–biasing. I found optimal operating point with plate current of 49mA and plate–cathode voltage of 225V (75mA and 231V in revisited circuit version). It is about 11W (17.5W revisited) dissipation and gives 2W output power. With 3.5K transformer it produces extremely low distortion level at the output.


Level Five 2A3 Schematic