Phono preamp tube rolling.

My 845 Mono's Input design is a little unusual as it has certain Tubes in Pairs that are only using Half of the Tube in the Circuit.
There is only One 6SN7 Per Monoblock and these are only using Half of the Tube in the Circuit.
It is always difficult to explain a Bespoke Built Design for the SQ, there is a separation in SQ and usually the only way to fully comprehend the designs capabilities is to be in the room with it.

It is very often the case that when dual triodes, like nearly all the tubes so far mentioned in this thread, are used in circuit, only one of the two separate triode sections is actually used.  But what is a BGRP? Thanks.

BGRP - Black Glass Round Plate. I have a pair of 12AU7s Tung Sol black glass (grey standard plates, I think) that sound fantastic, with great extension, but they are lowly 12AU7s: )

Yes, a holy grail Tung Sol 6SN7GT from the ’40s. Now listed on Ebay from $450 to $900 per pair.

It’s been a long time since I played with lots of vintage 6SN7, in a crazy headphone amp (and briefly later on in a Rogue 99 Magnum preamp). I do remember the Tung Sol BGRP sounded the most unique out of all, and the best to my ears. An extremely juicy, "technicolor" sound (like Koetsu) that was the only tube which seemed to fully deliver on the 6SN7’s purported promise as "king of tone". The Ken-Rad was my next favorite (especially BG variant), with bass 2nd only to the Tung-Sol. The Sylvania VT-231 seemed a little dry to me but was neutral and could be good in combination with Ken-Rad or Tung-Sol. I also liked the brown-base Sylvania 6SN7WGT, probably more than the tall bottle VT-231. I never liked the gray glass RCA VT-231 as much as these other makes, but it’s nonetheless a pretty good sounding tube. The Electro Harmonix 6SN7 sounded positively "solid-state" like by comparison.- a little grainy, and with a surplus of treble energy, but nice bass to balance it; honestly not too bad a tube for the money (and of course by now I’ve heard plenty of solid-state gear that is neither grainy nor bright). The Russian Tung-Sol 6SN7GTB sounds like a more refined version of the EH.

I also remember combining different makes seemed to stack only the positive attributes of each, rather than stacking or remixing the downsides (maybe this interesting quality is part of what makes 6SN7 so revered) - but if I’d ever had a full set of Tung-Sol BGRP, I probably would’ve just filled every slot with those. Problem is I kept getting used "nearly dead" ones that kept dying on me. It’s always been hard to find good ones and demand is so high people keep trying to offload duds.

For any of these old 1940s variants - GOOD LUCK finding a pair quiet enough to use in a phono stage or in the gain slot of a preamp. The old Sylvania 6SN7GTA/GTB 1950s "chrome domes" are easy enough to find quiet and in matched quantities, but aren’t as sweet sounding as the old tubes - they sound kinda like a slightly sweeter Russian Tung-Sol GTB in VAC 200iQ amps.