Upgrades
Level Five Revision A
After one year of listening the Level Five amplifier I decided to play around with the operating point of output tubes over again. As my colleague J.C. Morrison said: «Run your triodes hot!». So I tried to increase the idle current through the 2a3 tube.
The only thing that needs changed in the circuit it is the power supply and the bias. The biasing resistor R8 was reduced to 500 Ohm, and R2 resistor to 51 Ohm. These changes set the plate voltage at about 231V with the idle current 75mA. The 2A3C plate dissipates about 17.3 Watt, but this is ok for Shuguang 2A3C monoplates.
As B+ voltage dropped down, I had to rise it back.
This can be done in two ways. One may is to increase the value of the first PSU capacitor (C1). Another way is to replace kenotron by a different model with smaller voltage drop, e.g. 5AR4. In this case R1 dropping resistor becomes useless and may be removed. I picked the second option with 5V4GA rectifier tube. But it was not enough, voltage drop was still more than expected and I had to add one more capacitor in parallel with the first one, 1 uF value. Warm-up time is about 30 seconds which is greater than the warm-up time of the 2A3s.
I'd like to try US made 5AR4 in the future, this kenotron has a little bit more voltage drop comparing to its European 5AR4 counterpart (current production), but still less than 5V4GA. It would allow to remove extra capacitor (C0).
Also I replaced the Audio Note AN-160 choke (10H@125mA) by a Lundahl LL1673 (10H@200mA). This replacement gave me the desired headroom for idle current draw, that protects the choke from the saturating.
The results are excellent! The sound is more tight and dynamic, but at the same time it remains gentle and smooth. Measurements showed that the linearity of the output stage has improved more than twice!
Test Conditions:
Input = CD (2V RMS), adjusted for 1 W output.
Test frequency = 1 kHz.
As a result I got:
• 0.7% with 8 Ohm dummy load
• 1.1% with with Klipsch RF-15 speaker
I'm very happy with the sound improvements and consider the modifications definitely worth doing. If you ever plan to assemble Level Five, use this revision, it is really better.
Stay Tuned!
