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.

[Shawn]

The latest version of Megatron XL comes with 4 pots each 300B.

What are those pots adjusted for(cycled in the pic)? I'm not familiar with the DHT tube.☹️

I assumed they are for Grid voltage adjustment.

[MLA]

Yes; gives roughly +/-4V of adjustment range in my build. Can be upped if larger range is needed.

[Shawn]

7 hours ago, MLA said:

Yes; gives roughly +/-4V of adjustment range in my build. Can be upped if larger range is needed.

Thanks for your help, MLA.

Another thing I’d like to confirm: based on the 300B datasheet (see chart below), I’m planning to use a 400V B+ supply. According to the curve for Eb=400V, it looks like a plate current of 70mA corresponds to a grid bias around -85V.

Would you consider this a good operating point?

[MLA]

I'd say that's a high operating point for this amplifier; the math of push-pull output stages is different from single ended and as long as the transition is smooth you can pick much lower operating points. See for example:

https://www.valvewizard.co.uk/pp.html

Sorry, I should have remembered; Megatron is NOT push-pull. 

Megatron (XL) has about 22-23mA through the EL34 current sources. I've tried up to ca 30mA but not beyond that.

Edited 3 hours ago by MLA

[Shawn]

24 minutes ago, MLA said:

Megatron (XL) has about 22-23mA through the EL34 current sources. I've tried up to ca 30mA but not beyond that.

Thanks for the feedback! I wasn’t aware of the differences in operating point strategy between push-pull and single-ended, so I’ll definitely take the time to study the article you linked—much appreciated.

Also, if it’s not too much trouble, could you share what grid voltage you typically use for the 23mA setting? And when you went up to around 30mA, did you notice any significant sonic differences?

Thanks again for the quick and helpful reply!

[simmconn]

The Megatron final stage is CCS-loaded single-ended output working in class-A. There is no problem selecting operating point that way. However the final stage is self-biased. The grid bias eats up part of the B-, in other words B- is not equal to Eb in the tube datasheet. 20 to 30mA is more than enough for an estat amp.
 

Since 300B has a low mu (2.85), which is about 1/3 to 1/4 of a triode-strapped EL34, the undistorted output voltage will be less than with EL34. But the DHT fame and the aesthetics probably more than compensate for that.

[Shawn]

9 minutes ago, simmconn said:

However the final stage is self-biased. The grid bias eats up part of the B-, in other words B- is not equal to Eb in the tube datasheet. 20 to 30mA is more than enough for an estat amp.

If the output tube (like 300B) is self-biased, should I measure the voltage between the cathode (K) and the control grid (G1) to determine the actual grid bias (Vgk), and then adjust the trimmer resistor accordingly?

I understand that in a fixed bias setup, you’d measure between GND and G1, but in a self-bias configuration, G1 might be at 0V and the cathode elevated — so the true bias would be relative to the cathode, right?

Just want to make sure I'm adjusting based on the correct reference.

Thanks in advance!

[simmconn]

Ok, I stand corrected. The Megatron XL uses a 'hybrid' biasing scheme for the 300B. The grid is adjustable between 0 and 0.99% of B- ('fixed'-biasing), which cancels out less than 10% of the bias generated by the cathode resistor (self-biasing).

The voltage between the cathode (K) and the grid (G) is the actual grid bias (Vgk), regardless of self-bias or fixed-bias. In a fixed bias setup, you’d measure between GND and G when K is grounded.