Megatron Electrostatic Headphone Amplifier

[JoaMat]

Schematics auto switching. Not sure it's accurate, but that's what I've done.

New Grid Voltage Control boards.

One trimmer for offset and the other for balance so, both sides are controlled simultaneously. Two jumpers. For 300B/2A3, EL34 and EML 20B-V4. Works al right, quite easy to use.

The -510V is achieved with help from GRHV 110V supply. Here is one with Hammond 2VA trafo on board - 2.75in x 2in. 
 

New idea - servo controlled grid voltage.
Okey, I've tried servo controlled Megatron (with solid state CCS) before and that failed...

[kevin gilmore]

better put a 300v zener across the output of the h11d

 

[JoaMat]

Thanks a lot.

I've been thinking of a 300K resistor cross the h11d to limit the upper grid voltage.

[simmconn]

Looking at the CTR curve I would choose the operating range between 1 to 2mA of If. Since the nominal CTR is 20%, the phototransistor side would be designed around 200uA to 400uA. Adjust the resistors such that the desired normal bias voltage is centered around this range. The phototransistor could operate off the 110V supply if you don’t need the full 510V range for the bias adjustment (why would you?).

[Shawn]

On 10/10/2025 at 5:31 AM, JoaMat said:

Schematics auto switching. Not sure it's accurate, but that's what I've done.

I checked your schematic and compared it with your previous hardware photos, then corrected a few small errors(From both sides of us).

Here’s the updated layout. It’s basically reverse-engineered from the pictures you posted earlier.

I also derived the schematic from it, but it’s quite messy at the moment. If anyone’s interested, I can post that version as well.

 

Edited 1 hour ago by Shawn
Fixed the layout

[JoaMat]

 

I believe R41, FB2, C16 and C17 are connected as red line shows.

[Shawn]

1 hour ago, JoaMat said:

I believe R41, FB2, C16 and C17 are connected as red line shows.

Eagle eye!

Good catch. I’ve updated the image and added the missing R41–FB2–C16–C17 connections as you indicated. Thanks!