Welcome to Team STR JHC! If you're using 2A3s and 866As I'm guessing you've got a STR-104. I agree you ought to try the STR in both the amp and preamp, but you'll need to use the 866As with the UFO, the 2A3s don't have enough juice. I use my STR-1002 with my (non-Decware) pre/headphone amp and couldn't be happier. You've probably already read through this thread but if you haven't I encourage you to do so, there is a lot of interesting experience here.

FWIW the 2A3 is a power triode, similar to the the 300B but lower power. It has a 2.5V filament, there is also a 6A3 tube that is the same except the filament is 6V. They aren't common, but usually cost less than 2A3s and will work with the STR set to 5V if you are interested.

Please keep us updated as you get more experience with the STR in your system.

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I have indoor cats and (so far) have been unwilling to try any of the mercury vapor tubes so I don't have any direct experience. But it seems like everyone who tries them really likes them.

My system includes two UFO25s and I've toyed with the idea of getting a pair of 104s for them so I can use more of my collection of rectifiers. But that isn't happening any time soon, if ever.

When I used my STR-104 with my Torii Jr.  I settled in with my favorite 2.5v tubes in the Str-104 being a RCA 866a in the rear socket and a 3B22  or a 2A3 (2nd) in the front socket.  My favorite 5v was a 300b paired with a type 83 Mercury tube or 2 866A with adapter.  Go back through this thread 3-6 pages and I posted my experiences with different tubes.
Fyi - Al told me not to use 2-2A3or 2-300b tubes at the same time.  They may sound light and the tubes life would be very short due to driving them too hard. A good sounding great value 300b tube is a Linlai 300b.  $60-70 on ebay or Aliexpress. IMO a high quality expensive 300b  tube would not be appreciated in the STR like an amp or pre.

Just to add to this piece, I spoke with AL recently, and he put it this way:

"...if you are using the STR with a power amp, then putting 2A3 x 2 or 300B x 2 will be more stressful to the tubes. In case if you are using the STR with a pre-amp, buffer, DAC or something not demanding power, then it should be fine and nothing will go wrong."

I've tried one 300B at various times, but in general I have been so pleased with sound from two 872A tubes, that I tend to stay put. Not having kids or pets minimizes my risk.

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Very interesting. Because of our indoor cats I've been unwilling to try any mercury tubes but I'm not worried about xenon. I've also said I'm done buying tubes for my STR, but these look like they are practically free compared to the 10V DHTs.

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+1000. Transformative is the word I keep coming back to. Now I'm looking forward to GroovySauce getting a DynaGrid Rectifier, aka DGR, and doing an external rectifier shootout.

I fully agree with the xenon tube suggestion. In my case, the 3B22. Right now it is paired with an RCA 1616 and it sounds really good. As others have mentioned, its a good pair, mixing the qualities of both tubes. For now, I am happy with these tubes and will continue mixing the 3B22, 1616, and 866A to see what the best combo is.

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It's on a SpaceTechLab QA-114. Great amp, lots of tube options with that as well.

The 4B32 appears to be the xenon version of the 872A so I'm curious about your experience, Tony.

Adamaley, thanks for the review and clarification. Visually, the tube I ordered appears to be the one you are recommending. It should arrive next week. I will follow with interest your thoughts on using a pair of the 4B32s. In the meantime, I went back to a pair of 866A and a 300B in my STR 1002 and enjoyed the sound. Tonight, I will try a pair of 3B22s in place of the 866As as it has been a while since I have given them airtime.

It looks like I ordered the "wrong" 4B32s but they were cheap so even if they are horrible (or DOA) it isn't a big loss. They are supposed to arrive Saturday so I'll know soon enough. In any event, typically I need to tune the other tubes in my non-Decware preamp to which the STR is connected to optimize the sound anyway.

Yesterday, I received an unused 4B32 that I placed with a pair of 3B22 in my STR and was very impressed with this combination. This is a 4B32 that visually matches Adamaley's preference. I thought the sound was excellent right out of the box and matched the SQ of my 872A pair that has been my favorite throughout. The 3B22 pair has been another favorite of mine, so they may also be contributing. I will have to compare this combination against two 4B32s. I should be able to do next week as I was so delighted with the first 4B32 that I ordered a second, but this will definitely be the last tube I purchase ...,,ever.

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My feelings exactly. Mine arrived in Arizona this morning so I doubt they'll get delivered tomorrow, but I'm happy to wait. I also ordered a pair of the "preferred" 4B32s that should arrive early next week.

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Me too. Pass the candle.

OK, Xenon for the win! After about 4 hours these tubes sound fantastic. Huge 3D sound stage and just the right amount of tube liquidity without excessive warmth. And for pennies on the dollar for what I paid for the 10V DHTs I say run, don't walk to get some. My Cetrons were (in theory) brand new and presumably need some breaking in, but they sound great after 4 hours.

I also briefly tried a pair of the other version, that I'll call "hooded." They were also very good. They were used, so (in theory) didn't need any break in. They had an even bigger sound stage and were a little more aggressively treble forward in my system. I'd probably spend some time tweaking the other tubes in my system to rein that in. But I went back to the preferred "hoodless" to get some more hours on them.

One other thing, I'm not looking up the old post but I believe someone said that AL advised the 4B32s run hot. My experience is exactly the opposite, these are the coolest (in a thermodynamic sense) tubes I've ever used. I didn't think they actually powered on. After 4 hours they were literally the same temperature as when I took them out of the shipping box. The STR is cool as a cucumber as well. Anyone care to confirm or deny my experience?

P.S. If you're going from 10V tubes don't forget to flip the voltage switches. I was so excited to get mine powered up I totally forgot about it, but caught it after a few seconds and no harm done.

Where are you getting these open 4B32s from, cant seem to track any down?

Thanks for reporting back. Give the 4B32s some burn in time
I started to feel their magic after a few days of listening.

On my end, I've gone deeper down the rabbit hole and ordered a pair of Philips DCX 4/5000 tubes. They are electrically similar to the 4B32 and Xenon filled.

Which 3B22 do folks prefer with their STR?

That's what I have. I was hoping for impressions on other 3B22s