After a few exchanges with different people, it was time to make a few tweaks to the EF37a driver. Increasing the G3 voltage helps in squaring up the pentode curves as shown by few before. By increasing G3 to +12V or up to 20-25V can be beneficial in the long voltage swing of the pentode. There are a couple of interesting threads in DIYaudio to check out if you’re interested.
I should have fired up the eTracer and play with G3 biasing before modding the driver, but couldn’t help myself from doing this mod as it was very simple. The Screen voltage circuit I use has 2 identical versions of it in the PCB as it was designed for a stereo setup in mind. As I reused a PCB I built for other tests, I had a readily spare screen voltage regulator which could be easily tweaked to supply G3 at the levels needed: Continue reading “Mono Amp: EF37a driver (part III)”
