Megatron Electrostatic Headphone Amplifier

[Shawn]

Quick update:

it’s 6 AM here in California and I just wrapped up some late-night tweaking. Someone PM’d me earlier asking how I solved the offset and balance issue, and I can finally say: done. Offset is now around 0.3V and balance is about 0.2V.

If anyone else is running into similar issues, here’s what worked for me:

1.CCS Set Resistor: I kept the CCS set resistor at 1.5k, which gives around 22–23mA static current. You can try lowering to 1k if you want around 25mA, but I found 1.5k sufficient. The more plate current you have, the higher the offset you will get.

2.Cathode Resistor: Changed from 3300Ω to 7k–7.5k. A larger cathode resistor gives you a deeper grid voltage. I could only find suitable 7k wirewound resistors on Mouser – they’re a bit bulky but still fit. Note: Larger values will increase the offset, so some trial and error is expected.

3. Grid Pot: I replaced the 10k pot with a 50k, which gave me a much wider range to trim both offset and balance.

Final bias: around 24–25mA static current, Vgk ≈ -75V.

Hopefully, this helps anyone else dialing in the Megatron XL  and happy tweaking!😊

[JoaMat]

Glad you find a way to adjust the offset.

With EL34 as CCS, +400V and offset close to zero, I believe 1.5K gives ~23mA and 1.0K gives ~34mA.

Edited Sunday at 09:37 AM by JoaMat

[Shawn]

12 hours ago, JoaMat said:

With EL34 as CCS, +400V and offset close to zero, I believe 1.5K gives ~23mA and 1.0K gives ~34mA.

Thanks for the correction.

I actually paralleled a 3.3k resistor with the existing 1.5k CCS set resistor, which gives an effective resistance of around 1031Ω. Based on the measured voltage drop, that gives me roughly 24–25mA of current.

I believe the difference might come from my cathode resistor and grid voltage setup — possibly the cathode current is already limited before reaching the CCS. That might be why the current didn’t rise as much as expected even with the lower set resistor.

[JoaMat]

After paralleling with a 3.3K resistor an increase of a few mA seems reasonable, but what happened to the offset?

[Shawn]

8 hours ago, JoaMat said:

After paralleling with a 3.3K resistor an increase of a few mA seems reasonable, but what happened to the offset?

Paralleling a 3.3k resistor with the original 1.5k gave an effective resistance close to 1k, which I expected to behave similarly to using a single 1k resistor. After doing that, the offset increased significantly from around +12V to about +60V. So in the end, I reverted back to the original 1.5k setting resistor.

[JoaMat]

Thanks @Shawn.

I have ”gathered” some info from three sets of 300B, one 2A3 and one 6A3. To me it seems that they behave quite similar and need about 7K – 7.5K cathode resistors.

Your 50K trimmer (Grid Pot) will probably make it easier with the “fine tuning”. I guess you have put trimmers on the tube side of the board for easier access.

[Shawn]

16 hours ago, JoaMat said:

Your 50K trimmer (Grid Pot) will probably make it easier with the “fine tuning”. I guess you have put trimmers on the tube side of the board for easier access.

I actually kept the trimmers on the back side of the board. The PCB is mounted upside down near the top of the chassis, and I didn’t drill dedicated holes in the top cover for adjusting the trimmers. So each time I need to fine-tune them, I just tilt the whole chassis on its side and yes a bit inconvenient, but still manageable. A better solution would definitely be to make access holes in the top panel.