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.

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

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.

Thanks Kevin. I went back and double-checked the schematic. You're right, each 300B does have its own separate filament supply. In this way, would it be possible for me to use four individual 2A/3A DC-DC modules to power them separately? BTW, how’s the D&G project going? I remember it requires something like 10V at 8A or even 10A for the filament supply. That’s already more current than a typical household socket can handle for just the tubes! I’d love to hear more about the D&G if you’re open to sharing. I got the chassis at the end of 2024, the link was expired for a while but i do believe you can find it somewhere else. For GG building guide, you can browse the GG thread, it has all the info and issues you may like to know.

You should easily find it on eBay or somewhere else. Regarding the file, it is not mandatory for GG. You should complete the build by populating the GG boards only. Otherwise, you need to figure out the drilling distance to perfectly make the center board fit well, which is unnecessary.

This post is a bit long, but if you're interested in alternative filament supply ideas for DHTs, feel free to read on. I know @kevin gilmorehas already mentioned and implemented a solid filament supply solution for 300B tubes using five modular switch-mode power supplies from Traco. One 12V unit is used for controlling soft start, and the others supply 5V for the 300B filaments. It’s a good and proven design. However, I believe it may take up more space than I can afford in my chassis, so I’ve been working on an alternative. Summary of My Design Concept: 1. Inspired by @JoaMat approach using rectification + DC-DC regulation, aiming for a similar or smaller footprint. 2. Native soft-start behavior with no need for external resistors to set ramp-up current or output voltage. 3.Up to 5V @ 3A output per module. 4. Molded inductors for reduced EMI 5. Adjustable switching frequency (to avoid audio band). 6. Dual-random spectrum modulation for further EMI suppression. 7. No heatsink required. I’m currently working on the PCB layout based on my schematic (slow progress – I’m not a professional). Any suggestions, corrections, or feedback would be greatly appreciated! 1.Design Rationale (skip if you’re only interested in the result): There are many DC-DC converter chips available, but my criteria were: Very small footprint Adjustable switching frequency (to avoid 20Hz–20kHz band) Integrated soft-start, over-voltage, and over-current protection (to safeguard DHT filaments) Output current ≥2A @ 5V Low noise and ripple Adjustable output voltage (to support both 2.5V for 2A3 and 5V for 300B) After some research, I settled on the TI LMQ664x0 series, which offers 1A/2A/3A variants. To leave some headroom, I chose the LMQ66430, rated for up to 3A output and 36V input. Datasheet:https://www.ti.com/lit/ds/symlink/lmq66430.pdf This chip is extremely compact – only 2.6mm x 2.6mm – though I’m still unsure if I can hand-solder something that small.☹️ 2. Circuit Topology (text version for now): Although I’ve drawn a test schematic, the files are on another computer. I’ll post them later if needed. In general, the topology is quite typical: Use a transformer with a secondary in the 9–16V AC range, Rectify the output, Feed it to the LMQ66430 module. Key connection notes (based on the datasheet): PIN 1 and PIN 3: Rectified DC input PIN 6: GND PIN 7: Output through an inductor (preferably molded) PIN 8: NC PIN 10: 1μF to GND PIN 11: Feedback resistor network for setting Vout (see page 29) PIN 13: Set switching frequency (tie to GND for 2.2MHz; see page 14) PIN 14: NC PIN 15: GND For layout reference, check Page 39. Also, combinations and example circuits are shown in Page 9.2 Disclaimer: I haven’t verified every detail yet. In this way, there may be changes before I finalize the layout and perform live testing. I have some concerns if two modules can work in parallel. In Megatron XL, the two 300Bs per channel share the same filament supply. If I use two separate LMQ66430s in parallel, I’m unsure whether uneven current draw might become a problem, since these chips don’t have a master/slave current-sharing mechanism. I have another separate question, please bear with me for bringing this up here: If I substitute 300B with EML 20B-V4, I understand that I no longer need the pair of 3300Ω balance resistors and should instead use a cathode resistor connected to the center tap (CT) of the filament. But I’m unsure about how to treat the G-stage resistor R4 (680kΩ). Should it connect from the grid to the cathode, or from the grid to -450V? My guess is that the G-stage connects to -450V through the 680k, and the cathode bias is set independently via the CT, not the filament ends. I admit I’m not very confident when working with DHTs – I often get confused by how CT-based cathode bias works when combined with DC filament supplies. Any insight would be really appreciated!😊

C3M0350120D can be a substitution. 1200V instead of 1700V. Should be good in this role. Otherwise, the new 1700V next generation Sicfet will be released soon as Wolfspeed said will be in stock Q3 2025.

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!

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!

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?

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.

Working on a shrunk CFA3 layout this week, here is what it looks like. Haven`t tested it yet. The dim is about 8.6 inches * 3 inches per channel. Not small enough, but I bet someone can make it better by SMDing.

The TO-220 version of the 10M90S is readily available, so there's really no need to consider alternatives for now. Better to stick with what’s proven to work.

That 10M90S decided it was done with this world and took a leap of faith! Looks like it couldn’t handle the heat.☹️ Did you check this section? Looks like it was shortened by the solder.

I didn`t check the schematic. However, the input resistors(Signal input) shouldn`t be a probably if you go with 15k or 20k.