It’s been a while since I experimented with preamps again. I settled as a permanent setup with my 01a/ER801a which I love. However, I wanted to continue with my experiments so I built a new Mule, let’s call it “Mule 2” for now. It’s based out of multiple PCBs which simplifies the construction process and reduces build time:
I wanted to revisit as a first preamp, the old and beloved RE-084/A-408 DHT which I never had the chance to share. This is a great valve. I had a decent stash of Valvo pairs which sound like angels. Here was my previous incarnation:
I used filament bias and also a source follower as was driving directly the 4P1L output stage. Performance was great as it was the way it sounded:
THD only rises to 0.03% at 20Vpp showing how linear this valve is!
I also played with an unbypassed cathode resistor instead of filament bias:
As you can see below, the frequency response is impacted at LF with the increased reflected impedance at the anode due to the unbypassed cathode resistor. Gain is also reduced a tad. This can be sorted with the increase of the gyrator capacitor C1 which I left at 100nF:
My latest version is very similar and went back to filament bias, but with SiC diodes instead. I love the sound of these diodes. I biased the valves at lower voltage but will likely push them up to 6mA/150V as before:
I tested the new mini-gyrator PCBs based out of SMD devices. They work brilliantly!