Megatron Electrostatic Headphone Amplifier

[JoaMat]

8 hours ago, Shawn said:

What specific differences do you notice after adding the 100 µF caps?

Higher gain and in my opinion better frequency response. I haven’t tried other than the 100uF electrolytics. With the capacitors the amplifier is an absolute favorite.

[Shawn]

I haven’t really followed up on this for a while, but today I did a quick LTspice simulation of the 2A3 stage.

With a constant plate current of about 23.8 mA, a 3100 Ω cathode resistor looks like a reasonable bias point (compared to 3300 Ω previously). The operating point itself is stable, and plate dissipation comes out to roughly 7.7 W, well below the 15 W maximum for a 2A3.

The resulting plate-to-cathode voltage (Va-k) is around 329 V, which is clearly above the classic 2A3 datasheet limit of 300 V. Based on older/NOS 2A3 specs, this would be outside the safe operating range. Some modern production tubes (for example, the EH 2A3) are rated for a higher Va-k (up to ~360 V), so they may tolerate this operating point without issue.

Just wanted to share the simulation results. Please share any 2A3 running data points in reality.