Hi Artsi,
Found this post below, which represents somebody's thoughts on the best TS 6SL7GTY. BTW, do you have one from 1951?
"I have given this type some notice in other threads from time to time, but I think it deserves more attention, since it isn't nearly so well known as its most common electrical near-equivalent, the 6SL7GT.
I've been using it in projects since 1967, and it was always Big B's front end tube (five of them in the current version). These are the three in the power amp chassis front end. Two of them ordinarily wear homebrew copper tube shields.
The 6SU7GTY is one of several Tung-Sol proprietary winners. The familiar 5881 and 6550 are two other Tung-Sol winners, and two that made it into manufacture by other companies. Tung-Sol took the 6SU7GTY through several marks and construction styles, but apparently no other company picked it up. The closest thing to it, so far as I can see, is the 5691.
In fact, the Hytron 5691 shown in this view appears to be a duplicate, and I think it's a rebranded/relabeled 6SU7GTY (1951 mark).
The 6SL7WGT (Sylvania) is probably the next closest equivalent, and two of those, with their brown bases, can be seen in the view above, in the rear row of tubes behind the 6SU7 trio.
Over the years, I've come to believe that the 6SU7GTY is to the 6SL7GT what the 7025 is to the 12AX7. In fact, I think it's not an exaggeration to call it "the octal 7025."
It features a low-loss micanol base (this helps improve the noise and signal loss factor). It carries Aquadag electrostatic shielding on the inside of the bulb. It is also tested with AC on the heaters for a low hum-and-noise characteristic, and it is tested for balance between its two triodes.
I first got acquainted with this tube during high-school years, via my former boss, on whose bench I had apprenticed for two junior-high summer seasons. He was an old Signal Corps vet, and he had many boxes full of WWII surplus tubes (wish I had them now!
). A number of these 6SU7's were among them. I was able to get half a dozen to start with, and I built my first project with these tubes in 1967.
Since then, I have experimented with several marks of this tube (from World War II vintage through the late 1950's) over the years, but the mark you see here (1951 vintage) is far and away the best-performing version I have encountered.
It is somewhat microphonic if the micas are not snug against the glass, but earlier and later marks can be far worse.
They also tend to show up much worse for hum and noise. The 1951 mark seems much slower on average to develop heater-cathode leakage than other production runs (some later 1950's editions developed it fairly quickly!).
In this 1951 mark, it seems, they got everything as good as it gets with this type:
For some years, these weren't hard to find, and they weren't pricier than ordinary 6SL7's. I found a number of them at RARSfest in the late 1970's, and got several more from Fair Radio at about the same time. Since then, I've been lucky enough to find a few, here and there, in mil-surplus lots that turned up at RARSfest and elsewhere. About 15 years ago, I found and snapped up a huge batch of those Hytron 5691 tubes that appear to be (and act like) rebranded 1951-mark 6SU7 tubes, going for next to nothing at Durhamfest.
These days, of course, things have changed. These tubes have been "discovered," as one look at any mark-1951 6SU7GTY's on "that auction site" will show you (the asking prices are egregious these days!)
. I'm just glad I discovered this tube when I did, and stocked up on it long ago.
Guys with large lots of mil-surplus tubes should search them for this type, especially the 1951 mark. If you find any, treasure them and put them to work if you can, for you have some cherries that are definitely worth attention."
