Megatron Electrostatic Headphone Amplifier

[Shawn]

10 hours ago, JoaMat said:

Nice idea. If you can implement an auto detect function as well – like Elekit has done with TU-8900. 

Power module…

It's a great idea. I’ll likely move forward with the current plan for now since I’ve already ordered the PCB prototypes, but I definitely want to explore how that auto-detect function works.

My guess is that it detects the difference in cold filament resistance between 2A3 and 300B tubes, but I’m not exactly sure how it’s implemented in detail. I’ll need to dig into that more.

Also, from the image you shared, you reminded me that I should probably add a few small ceramic caps for filtering (like C9, C8, C7 on the right side of U1). Thanks for the heads-up.😅

The test PCBs should arrive this weekend. If I’m able to populate and test them successfully, I’ll report back with results.

BTW, do you know what kind of DC-DC chip on the picture? Too blurry for me to read.

[JoaMat]

I don't what chip it is. 

Marking might be???? Very hard to see.
26QH
288
A3HL

Including in curcuit is also dual comparator 2093B and  .05R sense resistor.

Good luck and looking forward a report.

[Shawn]

11 minutes ago, JoaMat said:

26QH
288
A3HL

Find it. I will share the info here anyway if you are curious. The switching freq range is between 900 to 2400(kHz). Lower than the LMQ, but shouldn't have any problems.

https://www.ti.com/product/TPS62913

[JoaMat]

That was fast! Thanks a lot.

[simmconn]

It’s not hard to do a 1:1 clone of the PCB. It’s also a great interview question for a junior electrical engineer to come up with a circuit that does this. However not a lot of them would probably know how a vacuum tube filament/heater behaves.

[Shawn]

Good news: It works!
Measured output is 4.9V for the 5V setting, 6.1V for 6.3V, and spot-on 2.5V for the 2.5V setting.
I haven’t tested the soft-start function yet — if anyone has good suggestions on how to choose a suitable load resistor for testing, I’m all ears.

Bad news: I spent two days figuring out how to hand-solder with a heat pad and heat gun the QFN package. Even when using a stencil, too much paste tends to remain on the pads, which makes them very prone to shorting.
As a result, I ended up ruining two boards and some components. At this point, I really don’t feel like assembling the remaining three boards myself. I think it’s best to leave the rest to a proper assembly house.

Updates:

Soft start works. About 3.5ms rising edge time for 6.3V.

Edited June 9 by Shawn
Updated for soft start testing

[sorenb]

3 hours ago, Shawn said:

if anyone has good suggestions on how to choose a suitable load resistor for testing

I'd use 300b or 2A3 tubes as load
be aware that NOS tubes may have different results from new production tubes

[sorenb]

7 hours ago, simmconn said:

It’s not hard to do a 1:1 clone of the PCB.

Maybe harder than you anticipate
Recently I was sent a not working Blue Hawaii BJT.
Turns out the PSU PCB is a clone of Kevins layout ... just flipped upside down, with components firmly mounted on top of the traces ...a year down the road, it went boom, with lifted traces and some literally burned totally.
@kevin gilmore  has been kind and provided a modified version of his layout to match the mounting studs in the chassis.

 

[JoaMat]

Thanks to @Shawn, who found the device used in Elekit filament supply module, I now have a working DHT switched mode filament regulator ”prototype” (in red circle).

Board size 23mm x 15mm (0.9in x 0.6in)

Soldering looks awful, but remember my eyes are 69 years old and my hands are not that steady.

Edited 20 hours ago by JoaMat

[Shawn]

9 hours ago, JoaMat said:

I now have a working DHT switched mode filament regulator ”prototype” (in red circle).

Great to know that. You could further reduce the PCB size by using a compact bridge rectifier like the CSPB40K-HF. It has a much smaller footprint compared to standard rectifier packages and still handles decent current.

Like the way how you fix and test the 2A3 tube.

 

9 hours ago, JoaMat said:

Soldering looks awful, but remember my eyes are 69 years old and my hands are not that steady.

Recently, I purchased these soldering tweezers from JBC to perform SMD work. Nice and decent.

[JoaMat]

 I used a Mini Hot Plate when soldering the buck converter and inductor. First a thin layer of solder and then up on the plate. 150 degrees Celsius for a minute before up to 250. Rest of components are hand soldered.

Thanks for the bridge tip. I have some small Schottky bridge rectifiers (CDBHD240-G) I intend to try first.