Shawn

I actually kept the trimmers on the back side of the board. The PCB is mounted upside down near the top of the chassis, and I didn’t drill dedicated holes in the top cover for adjusting the trimmers. So each time I need to fine-tune them, I just tilt the whole chassis on its side and yes a bit inconvenient, but still manageable. A better solution would definitely be to make access holes in the top panel.

Paralleling a 3.3k resistor with the original 1.5k gave an effective resistance close to 1k, which I expected to behave similarly to using a single 1k resistor. After doing that, the offset increased significantly from around +12V to about +60V. So in the end, I reverted back to the original 1.5k setting resistor.

Thanks for the correction. I actually paralleled a 3.3k resistor with the existing 1.5k CCS set resistor, which gives an effective resistance of around 1031Ω. Based on the measured voltage drop, that gives me roughly 24–25mA of current. I believe the difference might come from my cathode resistor and grid voltage setup — possibly the cathode current is already limited before reaching the CCS. That might be why the current didn’t rise as much as expected even with the lower set resistor.

Quick update: it’s 6 AM here in California and I just wrapped up some late-night tweaking. Someone PM’d me earlier asking how I solved the offset and balance issue, and I can finally say: done. Offset is now around 0.3V and balance is about 0.2V. If anyone else is running into similar issues, here’s what worked for me: 1.CCS Set Resistor: I kept the CCS set resistor at 1.5k, which gives around 22–23mA static current. You can try lowering to 1k if you want around 25mA, but I found 1.5k sufficient. The more plate current you have, the higher the offset you will get. 2.Cathode Resistor: Changed from 3300Ω to 7k–7.5k. A larger cathode resistor gives you a deeper grid voltage. I could only find suitable 7k wirewound resistors on Mouser – they’re a bit bulky but still fit. Note: Larger values will increase the offset, so some trial and error is expected. 3. Grid Pot: I replaced the 10k pot with a 50k, which gave me a much wider range to trim both offset and balance. Final bias: around 24–25mA static current, Vgk ≈ -75V. Hopefully, this helps anyone else dialing in the Megatron XL and happy tweaking!😊

A quick update on this build. Regarding the low-frequency distortion at high volume that I mentioned earlier, I did some troubleshooting. First, I checked the GRHV output for the +300 V rail. It's extremely stable and consistently holds at 300 V, so it doesn’t seem like any current limiting is kicking in. By chance, I noticed that the D3 LED (next to the 2SA1968 and I'm using STN9360 in this spot) flashes during loud low-frequency passages. That seems to suggest the current mirror may be going into over-saturation. This might be related to the two 150 kΩ resistors labeled R8A101 and R8B101 in the schematic. In the original Megatron, there is actually just a single 150 kΩ pull-down resistor instead of two. That difference might be causing the issue. Also measured the anode voltage of the 12AX7 tied to the current mirror. Under loud, bass-heavy signals, it shows a clear voltage drop from about 355 V down to 345 V momentarily. I’m planning to try shorting one of the 150 kΩ R8 resistors to see if that improves things. Updated: Turns out the current mirror itself was fine. The culprit was actually one of the PSVANE 12AX7 tubes. I suspect there may be a reason these were on sale at TubeDepot. At least the one I got had issues under load. Anyway, swapped it out and the problem is now completely resolved.

The grid’s pull-up resistor is R5 (680 K), and the pull-down side is two 500 K resistors. The 10 K trimmer (pot) has a very small effect, just about 1–2 V of offset adjustment range. With the current 7500 Ω cathode resistors, Vgk is roughly –76 V. To reach the WE datasheet’s recommended value, the cathode resistance would need to increase further, but that would push the offset even farther out of range. So the grid trimmer really needs to have more authority. The 10 K value is clearly too small here; something like 50 K or even 100 K would likely be a better choice. I’ve noticed that at this: 75 V operating point, there’s slight low-frequency distortion at high volume. I’m not sure if that’s because Vgk is too negative (too “deep”) or not negative enough. Yep, it’s a real hassle. At the very least, having to remove and reinstall the PCB every time is incredibly annoying.

Today the 7.5 kΩ resistors I ordered arrived. To swap all eight cathode resistors I pretty much had to remove every screw on the chassis. Long story short, the offset is much improved, going from –100 V to about +10 V. Big thanks to @JoaMat for the help! For me, this voltage is acceptable, but if someone wants to get even closer to 0 V offset, I think 7.0 kΩ might be the sweet spot. Also worth mentioning: the G-grid adjustable pot on the board offers around 2–4 V of Vgk trimming range. My 300Bs are running at ±400 V, since I feel 450 V is pushing things a bit too much. I think the main reason(Correct me if I'm wrong) for the original –100 V offset was that the cathode resistance was too low, allowing too much current. This made the anode current too high, and because the EL34 CCS isn’t perfect, it compensates by lowering its voltage output. With the original 3300 Ω resistors, I saw the B+ drop from 400 V to 370 V under load. One more note: after increasing the cathode resistor, Vgk sits around –80 V. For safety, I’d recommend replacing the original 220 uF/100V cathode bypass caps with 160V rated ones. More pictures will be posted later.

Last night I did the first power-up. I used some very inexpensive 300Bs just in case anything went wrong and to avoid risking the expensive tubes. Balance was actually very normal. About 5 V on one channel and 3 V on the other. But the offset was around –100 V on both channels. I’m guessing this is the “–100 V output” issue you were referring to? Let me know if that matches what you’ve seen. I haven’t had time to swap the cathode resistors yet. Measured Vgk is about –42 V, which is clearly not deep enough. I’ve ordered some 7.5 kΩ resistors, which after calculation should bring Vgk closer to –90 V. Also, for anyone interested, here are the LMQ66430 output voltage setting resistor positions I used on my board: M20-8760542 is a header pin jumper. 1–2: reserved positions 3–4: 6.3 V 5–6: 5 V 7–8: 2.5 V 9–10: 1.5 V Hope this helps anyone planning similar setups.

I’m planning to use ±400 V for the supply with an anode current around 23 mA. I noticed that on Kevin’s PCB layout for the 300B, the cathode resistors are shown as two 3300 Ω parts. Do you think I should replace those with 3600 Ω instead? Also, for filament voltage switching, I’m using the pin header setup shown below(MPN: 0010897120). Each row corresponds to 2.5V, 5V, 6.3V, and 12.6V, and I also left one extra row reserved in case I need it in the future.

I used an inductor from Coilcraft, MPN: XGL6020-222MEC. The manufacturer claims it has the lowest DCR in the industry for its class, and it’s also very compact at around 6.71 x 6.71 mm. For the output voltage selection, I set it up with a few jumpers to switch between different resistor values for 2.5V, 5V, and 6.3V outputs. If you need help calculating those resistor values, just let me know. Also, I’m curious. When you switch from 300B to 2A3, did you modify any surrounding circuit parameters, or can you drop them in directly (aside from changing the filament voltage)?

I took a look at the datasheets, and their Vgs vs. Id transfer curves actually seem fairly similar in the linear region, so I think they should be directly swappable. But it's probably best to measure the Vgs your circuit runs the C2M at, and then check if that voltage still lands in the linear/controllable region for the C3M. That's my understanding for using them in something like the GRHV, though I haven't factored in the overall feedback loop behavior here. So not sure if I'm missing anything on that front. I have ordered a pair of C3M and have a Carbon on hand, A straight swapped can be easy. Will report it later.

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

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.