Megatron Electrostatic Headphone Amplifier

[kevin gilmore]

somewhere there is a mini board that is a stn9360 to to126 pinout.  but bending the pins is something i have also done.

[G600]

Wow have I ever told you that you are a bold man, Joachim?

Impressive, again!

[Shawn]

Finally, I have time to hand in the Megatron XL. Having some questions here.

What`s a decent static plate current and voltage for 300B?

For the tube rectified power supplies, can we do a solid-state rectifier instead of the GZ34/EZ81?  Assumed we have other soft-start high-voltage circuits populated.

I'm wondering if it's possible to use the Golden Reference HV supply to provide ±450for XL. Does anyone know if the current capability is sufficient?  I'm concerned the total load might be too much for this PSU.

Edited May 6 by Shawn

[MLA]

Regarding you last question, I'm using GRHV in that role so I would say it's doable, but in addition to adjusting the current limiting resistor you need to put some protective measures in place; otherwise there is a good chance the SicFet will blow up during startup (I killed 4 before understanding the problem  ). 

See SimmConn's post for great advice: 

 

[Shawn]

MLA, Thank you for the reference.

Since the C2M1000170d is ending the life cycle, I don`t want to trash them.😅

I may just go with the universal power supply which was used on Megatron before.

2n3904 current limiters needed to remove for the universal supply. I`m wondering if the 2n3904s in GRHV do the same purpose? if yes, can we remove them to increase current?

The single universal power supply worked fine on my previous Megatron build(Each El34 draw about 27ma). And I assumed each 300B draw 33mA as Kevin mentioned it is the sweet point for 300B. Don`t know if I calculate it correctly.

 

 

Edited May 7 by Shawn

[JoaMat]

2n3904 does the same thing. Can be removed. Anyhow it won’t protect you from sudden shorts.

[Shawn]

On 5/6/2025 at 10:24 PM, JoaMat said:

2n3904 does the same thing. Can be removed. Anyhow it won’t protect you from sudden shorts.

JoaMat always helps. I appreciate the input regarding the 2N3904. I've been considering a modification to handle higher output current more safely. Would it be feasible to add an IXTP08N100D2 (Depletion Mode MOSFET) before the C2M1000170D to serve as a hardware current limiter?

Drain of IXTP08N100D2 connects to the Source of C2M1000170D (main current path).

Source of IXTP08N100D2 connects to a sense resistor Rs, then to GND.

Gate of IXTP08N100D2 is tied to GND (0V) or through a simple RC filter to mitigate noise.

The current limit would then be:

I = Vgs(th)/Rs.

Would this be a viable approach to implement a basic current limiter before the C2M1000170D, or would there be better alternatives? 

 

Datasheet attached below:

https://www.mouser.com/datasheet/2/240/media-3320804.pdf

Edited May 10 by Shawn
Fixed. Lower the current limit resistor to increase current.

[JoaMat]

Did this today today – in full dayligt. Replaced EML 20B-V4 with 2A3 tubes in a Megatron with solid state constant current sources. Changes are filament voltage and cathode resistors (from 640 ohm to 3600 ohm. Only light is from blue and red LEDs and some light from the tubes themself.