That's an interesting idea to consider, but quickly looking over the ones available in 10uf, I would have to get into more spacing acrobatics than I'd like to for this build. Do you know what using a smaller capacitance one would do? The 4.7uF ones look small enough to fit.
Go down in smaller capacitance, and you'll lose bass response. That said, Pete probably has a safety factor built in, so 4.7uf ones might work. However, Wima makes MKS film caps with lead spacing of only 5mm. It will take a bit of acrobatic lead bending (if you don't want to just squish them in there), but they'll probably fit easily enough. Almost ANY film cap is going to be better than an electrolytic in the signal path. C13 and C14 AC-couple the FETs to the NuTube, so there might be quite an improvement.
I'm in the "why not?" category here as to making things complicated by diversifying the BOM and looking at different brands, as well as the mixed opinions on how much audio grade really matters. Cost isn't much of a factor for only a few caps. It's been a fun learning experience to question these things. I guess it would be better to phrase my question "is there any downside to using power caps in the positions that are not coupling the signal path?" I plan to keep C3, C6, C13, and C14 as audio grade (being the coupling caps in the signal path), but could save some space and get higher endurance with some power caps for the rest. Seeing your use of the UPW for C5 and C11 earlier in the thread sort of sparked this question.
No, there's no downside with using good quality power caps in power positions. I would argue, though, that Panasonic FM and Nichicon UHE are better than both of the ones you mentioned.
Here's the thing: without trying to be critical, the NuTube Millett Hybrid uses a switching AC adapter. It's a good quality adapter, to be sure, with only about 150mV of ripple. However, you can hear improvements in ripple down into the single digits of mV in some headphone amps. A high quality linear-regulated adapter (Jameco has some nice, inexpensive ones) might be down around 50mV of ripple, or at least three times quieter than the supply Pete spec'd for the NuTube. Maybe the lower one makes a noticeable difference in sound quality, I don't know. We'd have to ask Pete to find out. Switching adapters very easily adjust themselves to a wide range of line power (120 or 240 without a conversion plug). Linear-regulated adapters can't do that. So, the switching power supply ensures that the NuTube MH can be built all over the world without hassle and great results. It's just an educated guess on my part why the switching power supply was selected. Again, we'd have to ask Pete to know for sure.
However, if you can hear the effects of the ripple (not the ripple itself, necessarily), then that means the power supply is partially heard through the amplifier circuit. In that case, it might be more beneficial to use audio-quality electrolytics for the power supply, too. IOW, the fact that they may sound "good" may be more important than the fact that they have more ESR and less ripple current rating than a good power cap.
I have these 3 types lined up as my possible choices for the 4 signal path caps, or all of them if I decide not to go with the power caps elsewhere. Elna's are currently the front runner in the "why not?" upgrade due to their recommendations in many posts. The 470uF ones should fit as they are Ø16, which I see fitting in earlier pictures in this thread, but the 1000uF ones are out (only FG comes in Ø16 and they're out of stock right now - I'd prefer to keep the 35V rating). For the 10uF caps, I'm unsure if I could squeeze in Ø8 for the 50V ones to match stock ratings, but the 35V ones match the stock FG size and are noted as "upgrade" on Pete's BOM.
A further note from the above answer on film caps - many film caps' leads are quite easily adjusted to different spaced pads on the PCB; electrolytics, not so much.
For the power caps, if I go that route, I am looking at UPW's. I like the higher stats on the Panasonic FR's, but for some reason the 10uF ones are listed as $56 on Mouser right now and are out of stock. I could get the B version and deal with wide lead spacing though.
That $56 is probably a Mouser error. I've seen that happen before on other parts. You might just call them if you really want that cap. While not quite as good, the Panasonic FC is perfectly fine.
You might just have to read up on the spec sheet, though - or compare with DigiKey to make certain, because this one: EEU-FR1H100B is only 33 cents. The only problem is that Mouser says the lead spacing is 5mm, but the diameter is 5mm, too - that's impossible. So there may be a good chance that the lead spacing is actually 2mm on that cap. It's worth investigating by studying the datasheet on the Mouser page for that cap.
If you want a great power cap, look at both the ESR and ripple current rating. I think actually that ESR (less is better) is more important in digital circuits (DACs), whereas the ripple current (more is better) is better for power applications.
Thanks for your response.
One thing is probably certain above all else - this is way too much analysis/thinking for the NuTube. It's inexpensive enough to buy some of all of these parts and then try them out on your own to see what differences you may notice, if any.
