OK time has come to post a few sentences about two tube types I've stumbled upon in my quest to find some "new" tube types which are a)good/great performance wise b)preferably cheap(ish) c)rare.
Big big portion of game related to those two tube types I did with a help and in cooperation with @marty
and I'm sure he'll chime in particularly related to the performance of the beam tetrode tube known to outside world as 6384. Most probably not many heard about this tube type but it was produced by Bendix for military purposes to withstand ultimate operational conditions and when you hold the tube in your hand you certainly feel this "sick" attention to detail in terms of engineering. Tube is relatively small in comparison to other pentodes/tetrodes but weight wise its on par maybe even with KT170. Not to write in my words I'll just copy/paste from the text I found online:
"6384 The debate often taken up in various magazines, about which 6L6 type is "best," seems a bit silly if the 6384 is compared to the available 6L6 types. Even the Tung-Sol 5881, Mullard EL37 and Genalex KT66 look like fragile and lightweight toys next to the Red Bank tube. So it's safe to say: you won't see the likes of this tube again. The base is some kind of ceramic material, and is one of the few ceramic octal bases I have ever seen. The construction is the same luxurious style as the 6094, with all-ceramic spacers and a heater block. The pinout is the same as the obscure Tung-Sol 6AR6, a very similar rube to the 5881 physically but not quite the same electrically. Still, the 6AR6 or 6384 can be easily substituted into a 6L6 socket with rewiring or an adapter, and are rated closely enough to work well, though the 6384 makes the 6AR6 look pathetic. Extreme scarcity means we had only one 6384 to test, a new-in-the-box original Red Bank. The plate rating is 750 volts at 30 watts, which is outrageous for a 6L6 type and more typical of the infamous Mullard EL37. Yet EL37s bring $200 or more on the current NOS market, while 6384s are scarce but have nearly no value. It shows how narrow-minded the gurus can be. Simply rewire your socket or use adapter and you can use this Beastatron. Tests showed that it is indeed a beast. At 300v 50 rnA, distortion was 0.77%, lower than that of a typical Tung-Sol 5881. Peak output was 9.8V, far more than most 6L6 types and even better than a KT66. I briefly put 500v 75rnA into the cube, and it continued to work without showing red spots on the plate."
Attached is also original Bendix leaflet with all data. So I managed to secure a quad of NOS ones with several spare tubes but based on the ruggedness these will live a long life. Now since I got my H as I wrote in my previous posts I've rolled decent number of top tubes in terms of double triodes and as Marty said definitely ECC32/31 is the tube which is keeper in my system. It just shines more bright above the rest of the pack. Pentodes I also wrote about and used almost exclusively KT170 which is good option but definitely not the best as El156 and Siemens f2a were above it but due to insane prices of those I don't use them and this was a main driver of a quest to find something in this ballpark performance wise but more into KT170 price wise. 6384 fits this bill perfectly except availability but OK - Marty and myself are fine (selfish this time but sometimes you need to be
). Also Marty still prefers his Mullard el34 xf2 which is of course super cool as there are no universal solutions for different systems and ears but he can give a nice insight into differences of these two types (i don't have el34 xf2). To me 6384 in a combo with Mullard ecc32 provides most palpable musical experience with nothing in spectrum to stand out and hurt hearing apparatus but rather being seamlessly blended together with exceptional musicality. There is vast staging but still not being overblown where you can position the musicians on the stage with pinpoint accuracy. Details are there in spades while there is also great dynamics present. But to me color of the tone and musicality is the name of the game so these reminiscent tubes of the cold war era are a keepers for me.
Coming to the last portion of the puzzle is the recti and this particular one was showcased by Marty. I do have tons of different ones but the latest and maybe sweetest addition to collection is again a tube from the same era and same producer being known under the sky with the code name 6106. I won't go into tube details as details are attached as pdf. It is again a brutal demonstration of quality in a small package and all I wrote above was produced with combo Bendix/Bendix/Mullard. I'll try U52 as per Marty advice as I have it and see where this train leads me, but for time being I'm super happy with this combo.
Few pictures as always to visually support the writing