Interested in hearing about tube rolling experiences with the Orchid and other MHDT dacs, namely, the specific comparative sonic characteristics of the various 396a, 5670 and other compatible tubes used in the output buffer. My only experience is with the WE396a that Linear Tube Audio installed in my new Orchid.
I have found that the Tesla tube is a nice sound for me. At least equal the the WE tube thus far. 
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Some folks at are using tubes from the ECC88 family. Requires a ECC88 to 6N3/5670 adapter.
How did you get the WE396a with your DAC? I just purchased one and it only came with what looks to be a GE tube. I'm hoping to try the WE396a out soon.
Waterclocker, I bought my Orchid from Linear Tube Audio. They supply and install a WE396a at no extra cost.
Unfortunately I missed out on that deal.  They were low on them and I had to buy it separately.  I got one of the last couple.  Certainly worth the cost of admission!  Seems a bit richer sounding and less digital.
I am the person who introduced the idea of using 6922/ecc88 family tubes in mhdt DACs, in the head-fi forum. As stated you need an adapter. I took a flyer on the idea after experimenting with every 5670 compatible variant tube. There were certainly great and meaningful differences among them but I evaluated the trade-offs between them as too constricting. Happening upon an adapter lit the bulb.

It's surprising that it works so well, because in the mhdt DACs, that output buffer position has the 6922 operating with a B+ around or slightly under 25v. Very unusual. But sometimes that works and in this case you give up nothing to get plenty. Going back to Havana Balanced ten years ago, through Atlantis, Pagoda and Pagoda Balanced, my preferred 5670-compatible tubes were Bendix 1964 production 6385, and Bendix or Hytron 2c51. The Tesla NOS equivalent was in the same realm. They pushed these DACs to be meaty, vivid and natural. But something was given up, a little, in elasticity and nuance.

The ecc88/6922 delivers everything good about the best 5670 compatible tubes, gives up nothing, then adds more. You'll hear big gains in soundstage dimensioning. Music is dynamically unleashed, spatially liberated and tonally more authentic still than what you expect from mhdt. Bass is deeper where appropriate, more fully realized and textured. In voice you hear more of the whole human pipe, less restriction to the throat. The whole presentation is harmonically more complete, vivid yet relaxed, natural. I use CCa and CV2492 tubes. Siemens and Valvo NOS 6922 work well too.

213cobra... Interesting post. I own the new MHDT Orchid and am currently using a Western Electric 396a. Do you know if a 5670/396a to 6922/6DJ8 tube socket adapter requires any circuit alterations in the Orchid? Petrhaps a question for MHDT. 
I have followed the recommendation made by 213cobra and have now rolled an Amperex made in Holland 6DJ8 into my Orchid.
I know my new tube has not burned in at all yet.But out of the box I am hearing what he has predicted. I'd describe the change as a more fully fleshed-out tone. I suppose what that means for me is that the already very good midrange of the Orchid has filled out even more. The female vocal I just listened to has more depth and sounds warmer and more beautiful.
Really looking forward to the sound improving as this tube burns in.@213cobra Thanks for the recommendation!
I should have mentioned that I'm using the 5670/6DJ8 tube socket adapter available on-line. It did not require any circuit modifications.
Also, now that I'm a couple of hours in, and listening to some orchestral music, the new tube continues to provide a more fully fleshed out timbre. I'd describe it as the note fundamentals are fuller and a tiny bit more realistic, though I don't know how that is possible.

RE: the link you provided.  According to the description, it's for a device that already uses a 6992. However, the adapter pins need to be for a 5670 device (i.e. Orchid), and the adapter top accept a 6992. 
He asked for 5670 to 6922 which is what I gave him, I didn't see the reverse on their website; that aside here is a link that grannyring posted short while ago under the MHDT Orchid vs Amber 3 Forum for a site that he said his buddy found the adapter at for his Orchid

I assumed he implied an adapter that would actually work in the Orchid. So, he needs a 6922 to 5670 adapter. 
The Orchid takes 5670 tubes. The adapter needs to convert 5670 pin configuration to 6922 pin configuration. What I am looking for a HIGH QUALITY pin adapter, not just any old adapter, that is, one with quality components and construction. I am aware of the adapters cited. In other words, what I am hoping for is feedback on adapter quality. Perhaps the ones cited fit the bill, I just don't know. In general, there is a lot of junk out there.
“I'm using the 5670/6DJ8 tube socket adapter available on-line. It did not require any circuit modifications...”

This is just a curiosity question. Is the adapter just “adapting” the pin outs or also compensating for the differences between the two tubes’ electrical characteristics?  According to the tubes’ data sheets, there are some differences between their characteristics that could theoretically result in different sonics but that could be at the expense of stressing other components in the circuit, or the tube itself.  I’ve heard of anecdotal examples where folks have used a “wrong” tube but liked the sonics results better. 

Maybe pm grannyring and speak to him about the quality since his buddy purchased the one in the 2nd link above and grannyring (Bill) also does mods

Better yet, grannyring's buddy just posted on the Orchid vs Amber thread - user name  rx8man ; you can get a view directly from him
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Does anyone know what screwdriver/Allen wrench I need to remove the top cover? The manual says torx/T8X50? Cant seem to find that. 
I just used an Allen wrench.  I don't know which one it was.  It was the one that fit.
Not many sets of Torx go down to the t8 size, most stop at t10.
Fortunately I had a more complete engineering set that went to t8.
I know you can buy the t8 torx  on eBay if you cannot find a set with one in it.

BTW, A 5/64 Allen work perfectly. AND the WE396A JW tube sounds amazing right out of the box!
Since I installed a 6922 adapter (from Garage 1217), I tried various flavors of the 6dj8/6922/7308 family. Hence, I'm selling my NOS 396a tubes, including WE JW396a, Tung Sol, GE and the rare 1970 Tesla Military pinched-waist flavors.
I got @pmboyd 's WE 396A. So now I'll have three tubes to play with when my Orchid arrives. Fun!
I know the 'normal' route is to throw money at a problem or spend more with an upgrade. However I found out many years ago with my Havana dac that after a pretty short break in of 50hrs or so the 6H3P Russian tube has all the clarity, bloom, bass slam, and midrange of any tube I've tried. Very cheap yet sounds like a million bucks....easily beat my Tung-Sol, Ratheon etc. They found a permanent home in both my Havana and my Orchid.

Best thing is, since they are so cheap I don't feel bad leaving them on, even when I am away.
Ok, interesting information. I have tried many different flavors of 5670 tube, some obviosly better than others, however, I will never go back to that tube after adapting the socket to emply the 6922. What a glorious difference that made in every respect compared to the stock tubes.
Best 5670 to 6922 tube adapter is made by Garage 1217 (  Superior to every other adapter that I have seen. However, please note: the pin holes are tight and therefore the tube pins must be straight in order to facilitate placement. 
gelle... Siemens made valves are from German and the European 6922 (American designation) equivalent is the e88cc. Many tube dealers, such as Brent Jessee, carry them. Top of the Siemens line is the CCa, followed by the e188cc (American 7308), the e88cc and the ecc88. Does that answer your question?
PS, Generally speaking, the earlier production Siemens valves -- like many other brands -- are superior to the later production.
@pmboyd   Thanks. Yea I have bought from Brent in the past. Got my WE 396 D getter from him. Looks like he is out of stock on most of the tubes you mentioned. He has some of the E88CC / 6922 Siemens, West Germany made, 1960s wide getter support post, gold pins in stock. Might try those. Need to see if the “ the Getter support post” is a wider tube than what I’m currently using and if it will fit (with adapter) into my Orchid. 
gelle... other dealers will have the Siemens tubes and they regularly come up on Ebay. "Farawayfinds" on Ebay has an excellent selection from the 6922 family. I have the Orchid as well and all the flavors of 6922 fit comfortably with a pin adapter. The Garage17 adapter is highly recommended.
gelle... I have several NOS Siemens e88cc for sale. PM me if you're interested.

after reading all the info on the mhdt I borrowed a friend’s solid state Constantine and went out and bought the Orchid

your sixmoons review and this hilarious review of multiple mhdt dacs where very helpful

I picked The Orchid -  a gorgeous schoolteacher. Sophisticated, tempting, and, romantic.

I just started breaking in the dac 4 days ago.

My friend found me some NOS Heintz and Kaufman 2C51 gammatrons

just breaking one of them in

thanks to everyone

I know this thread has been dead for a while, but I thought I’d share a report on my tube swap, which was inspired by landing here through a search.

I lived with my recently acquired Orchid for several weeks with the supplied GE 5670, but after discovering this thread I was inspired to try some tubes from the 6922 family. I ended up with a Siemens A-frame E88CC (using the Garage1217 adapter which works great). The Orchid sounded nice enough with the GE 5670, but the Siemens created a shocking improvement. At the risk of sounding like a generic audio review (but all true), what I’m hearing is a greatly expanded and precise soundstage, previously unheard air in voices, and elements in recordings that I had no idea were there (for example, the birds singing in the intro section of the Latin Playboys track Lemon n’ Ice). Bass is very good with no exaggerated plumpness.

The only caution is that the Siemens tube is taller than the 5670, and with the added height of the adapter it protrudes through the hole in the top of the case, maybe 5/8" or so.

I’m listening mostly to TIDAL Masters via the TIDAL app on a MacBook Pro. If anyone is curious, associated components are a Simaudio Moon SE integrated, ProAc AC 100’s (bi-wired), Schitt USB cable, Audience interconnects and PowerChord, and Mapleshade speaker wire.
I considered the garage 1217 adapter 5670 to 6922 couple months ago. I also hear some using a 5670 to 6sn7 adapter. Anyone try the 6sn7 in the MHDT. I have a lot of 6922's but even more of the 6sn7's. I'm going to order the 6922 and 6sn7 adapter after reading this and see what happens, hopefully nothing is going to burst into flames.
I tried the 6sn7 adapter with my stock Orchid and didn't care for it. I think it was grannyring that suggested you need to do some other upgrades for that to be worthwhile. 
I considered the garage 1217 adapter 5670 to 6922 couple months ago. I also hear some using a 5670 to 6sn7 adapter. Anyone try the 6sn7 in the MHDT. I have a lot of 6922’s but even more of the 6sn7’s. I’m going to order the 6922 and 6sn7 adapter after reading this and see what happens, hopefully nothing is going to burst into flames.

nothing will burst into flames... the 6sn7 adapter from garage 1217 is a quality piece, and it is fun to try many 6sn7 variants old and new... lovely sound, very airy, can roll tubes to achieve euphonic coloration or drive and transparency

significantly more variation is sq from various 6sn7’s than 6dj8s (via that adapter) and much more than 2c51/5670’s which has a narrow field of choice

just know that in all these tubes, we are talking dual triodes in a single tube - each channel runs though one side of the tube, so reasonable internal triode balance is important
Another bump to the thread. I have been auditioning some 7308 / E188CC tubes with the Garage1217 adapter. Thanks to Brent Jesse I have been experiencing an expanded soundstage, richer sound with less grain. My previous tubes are the WE 396a and my former favorite, the TungSol black plate D-getter. The WE was a little to soft for me, but I have a nice 6SN7 tube pre, which the combo might be too much of a good thing. But the 7308's are just a great addition to my modded Orchid. Thanks 213cobra for getting the idea started here.