MLA

I realise I was a bit unclear above. The Q12 IXTH6N100 is indeed a current limiter and as I read the schematic, it's there to protect the whole psu should the amp board develop a short of some type. So what I did above was "steal" my Q12 by re-wiring it for start-up duties instead (placing it between pre-reg caps and Q8). However, that also means that my boards no longer have the Q12-based psu protection in place. So in the future, I will probably put a second IXTH6N100 on the board (in the Q12 position) to restore that protection. Also, you don't specifically need the out of production C2M parts for the psu. The next generation of those SicFets, specifically the current production C3M0900170D that also replaces the old C2Ms on the amp boards, work just fine.

Picture from work in progress (if there ever was anything else). I can now start the Super-Carbon without using the Variac. Initially, the PSU SicFets blew up during startup when doing a full power on, very similar to what SimmConn reported here: Presumably my SicFets ventured outside their SoA during startup. I'm not using the Cree C2Ms here (though it's a similar spec'd SicFet) and I have the current sources set for 25mA which might be pushing things a bit, so this particular problem may very well be unique to my build. However, since this forum has a lot of clever participants , the solution was at hand in the same thread. As per @Kerry's advice, I wired in the depletion mode MosFet current limiter directly after the pre-reg caps instead of having it after the post-reg caps. Also, I added a Vds clamping Zener as per @simmconn's mod. Not skilled enough to tell which mod did the trick or if both are necessary (and one can of course never exclude the possibility that it now works for some other reason that I have no idea about), but anyway it now starts up without anything blowing up and I can move on to the "oh, I'm supposed to listen to it?" bit. Special thanks to Kerry and SimmConn (in addition to Kevin designing these things in the first place); I would not have figured this out without you! Apologies to all UK members; haven't gotten around to finding a new picture host yet.

Since it sits across the 10k to ground from the cathode, it acts as a cathode bypass cap. Many things are said with emphasis in tube amp forums about how such caps influence the sound and whether they should be there or not if one wants further reading. However, if you choose to have one, the important bit is that a small value like 0.47uf will reduce the bass frequencies quite a bit. Typical values applied to avoid that are 47uf and 100uf.

Definitely 47uf. I put .47uf there by mistake when I built it; took days to figure out what I'd done wrong :).

Somewhat ironic - since I'm not in the UK, your post is showing me exactly the same amplifier picture I posted earlier :). I see the problem though and I think it's been brough up before in this forum. I was hoping for Imgur to resolve this, but if it persists and/or spreads I might look for another image host.

Almost, but you also need to connect to GND on the amp board, so: First, connect GND on the amp board to PSU1+ and PSU2 -. This is now your relative ground point. For the PSU1 board, since it has + connected to GND, it's negative terminal will sit at -550V relative to GND, so connect that to B-. For the PSU2 board, since it has - connected to GND, its positive terminal will sit at +550V relative to GND, so connect that to B+.

Maybe more like a power up-Carbon ; same design but with higher PSU voltage and new current sources, using current production sicfets.

Low precision writing on my part. No LEDs directly in CS circuit itself, but on the boards I have, two LEDs sit across the incoming CS power rail, so they see 15V.

Test build of Super-Carbon up and running to see what is what: PSU angle brackets are recycled from a previous, very tight, build, where those cutouts made perfect sense . Two small issues worth mentioning on amp boards. First, TTC004b and PZTA42 have opposite pinouts (ECB vs BCE) so if using TTC004b, transistor front needs to face heat sink. Second, you need to add some current limiting on the CS LEDs if you intend to use them. Running at +/-550V. No listening yet; need to build a more permanent CS supply solution first and also tidy up in general.

Very nice! A new JoaMat "interesting twist"-project in the works I believe ?

It's GRHV with either 325V or 330V secondaries (don't remember exactly), and I lowered the current limiting resistor to 3.6R I think.

I see no drop under load at +/- 400V, so you might be just on the edge of your current limiting. I did run my XL at 33mA for a while which worked (and sounded) fine, but then changed back to 23mA to test some other stuff and right now it's dismantled for fitting of 2A3 heater supplies, so can't help with data points.

Once properly warmed up for an hour or so, the transistor legs sit at around 110, transistor casing at 100 and hottest part of plastic tab at 90 (all in degrees Celsius and with at least +/- 5 degrees or error possible; the IR-gun I use is not the most sophisticated). Definitely too hot to touch, though still a bit cooler than the EL34s . So a bit of margin on both die temperature and what PLA Pro can withstand, though I guess the latter will age quite a bit faster than it would at ambient temperature if I would leave the amp turned on 24/7. I normally don't post pictures because my case work is rather poor, but I found a long exposure mode on my phone which after some experimentation captures the tube glow without leaving the rest in darkness, so I thought I'd make an exception. Pardon the somewhat battered front panel. Should also add that it sounds great.

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.

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

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:

I can report that JoaMat's new mini T2 boards are working as designed/intended. This is with EH 300b and Amperex 6DJ8 and same filament regulators as JoaMat has shown before. Have only listened to it in mono (which sounded great) since I managed to blow channel nr 2 by re-using a 1N100D that I thought was fine but which wasn't. Lots more current through one of the lt1021's than designed for and it blew after a couple of minutes. Spare parts are on order so expecting to get back to a stereo setup by the end of the week. If you intend to build this, get yourself some 1N100D now. The only place I found with some in stock before next year is TME and it's dwindling fast (was 115 this morning, is 83 this moment).

^ What he said. Live transistor tab and risk of considerable voltage on heat sink, so for testing only and to be treated as an open lid amplifier. The original question for this testing was whether Joachim's latest current source can match the EL34 in performance sonically. After quite a few hours of listening, my conclusion would be yes; a Megatron modified with this solid state CCS is a really great amp (just as the regular Megatron is, btw). Designing a pcb for it where the CCSs can be mounted inside the chassi on proper heat sinks would be a great next step. Let's see if we can convince Joachim to take this on... .

These are really nice; drop in replacement for the EL34s in the current source positions. Just plug it into the tube socket and then spend some time comparing tube to solid state CCS sound. Joachim was kind enough to provide four of these for a Megatron we're experimenting with in Gothenburg. The current source itself is really nice and in comparison to the EL34's, might have an edge when it comes to transient reproduction (completely subjective opinion of course).

all good, but output capasitance 60pF vs 11pF Noted that and am wondering for this application; how much does it matter when used as a current source? I don't know enough of the theory to figure that one out. Help... ?

So, the golden rule for Kevin Gilmore boards seems to be to always follow the board. Schematics are sometimes earlier drafts rather than the finished design, which can be confusing. I have build both the 1.8 GRHVs and the .6 Carbon boards which you have with success. If you go for +/-400V rather than 450V, SimmConn has a nice post on how to push the pzta42 back into a more linear region. The BOM excels and links to Mouser carts posted by mwl168 in the first post of this thread both open and download without issues for me. Should be a good place to start.

GRHV does not fail gracefully... most or all of the active parts might need replacing. Sören B posted a good guide for trouble shooting somewhere.

Next step is usually swapping the tubes between boards to see if it's the boards or the tubes. In the process, cleaning the tube pins might be a good idea; if some have oxidized and have poor contact, hum may occur.

Indeed. Damn, I actually thought I had figured something out

Really nice! From picture, it looks like Ecc99s or similar replacing the ksa1156s?