Yup, it takes time to get our heads wrapped around the whole sound change, controls, and tube burn-in progression timing, the TA-26 is similar to the TA-20 in many ways, yet very different in so many other ways.
There are a bunch of threads here on head-fi covering
the 6SN7
"
The Reference 6SN7 Thread" 1st post listing has a ton of useful "6SN7 Reference" Info
https://www.head-fi.org/threads/the-reference-6sn7-thread.117677/post-1380036
Here is the first part, the quick description list of 6SN7 tubes:
6SN7GT* -
First designation of the 6SN7 tube type. ‘G’ = glass body & ‘T’ = short bottle.
6SN7W* / 6SN7A* / 6180* - 10000hrs minimum life. Often labeled with JAN-CHS (equals Sylvania-made military stock) The A is generally a taller bottle with the mica above the metal band while the W has the bottom mica a bit below the metal band. But it's common to find both labelled as each other also.
6SN7GTA* - 'Upgrade' of 6SN7GT, max 450Volt Ua, 5W per system and 7.5W total. Hence tube better fulfills special requirements for TV sweep applications, like higher peak power and better impulse capabilities.
If your amp is designed around this tube, you MAY NOT be able to substitute in other 6SN7 versions due to the higher ratings for this tube – check with your manufacturer.
6SN7GTB* - Same as GTA, but with faster tube heat-up (11 seconds only).
If your amp is designed around this tube, you MAY NOT be able to substitute in other 6SN7 versions due to the higher ratings for this tube – check with your manufacturer.
6SN7GTC* - Metal envelope type 6SN7 [ie. glass envelope replaced by a metal cap]. Not well regarded. From Audio Asylum:
”South American trash can style 6SN7, 1970's vintage, pure rubbish, I wouldn't even count on it really being a 6SN7 as the South American tube makers were fairly...'liberal'...with their use of tube numbers.” –Robert H
If your amp is designed around this tube, you MAY NOT be able to substitute in other 6SN7 versions due to the higher ratings for this tube – check with your manufacturer.
6SN7GTY* - Only found on 6SN7GTs with brown ‘low-loss’ Micanol bases. RCA (US - lighter brown) and Brimar (British - darker brown) made GTYs.
6SN7WGT* - Military version of the 6SN7GT; 10,000hrs minimum life
(up for debate). “IMO, while tubes (like car engines or light bulbs) may be rated for a specific lifetime, a LOT depends on the actual usage type and pattern the tube will see. E.g. Switch your amp on/off 10 times a day? Don’t expect 10,000hrs.” -Author
6SN7WGTA* - Military version of 6SN7GTA.
6SN7WGTB* - Military version of 6SN7GTB.
VT-231* - Military Part number for 6SN7. Part number used until 1945 - hence presence of this designation is usually a guarantee for real old tubes.
JAN-XXX-6SN7* - JAN= Joint Army Navy. Factories conducted stress tests (above normal 'civilian' specs) on samples of standard tubes. If they survived, the whole lot would be approved (batch approval), and marked with the prefix JAN. The samples that are actually used for the stress tests are discarded.
Do remember that the stress tests conducted were on SMALL samples. “ Very interesting thing nobody talks about - especially dealers who have made a living buying old military stock that occasionally pops out of warehouses. What did the government do with tube shipments they received where their spot testing results were not acceptable? Well, in some cases, they sent them back, whereupon the manufacturer sold them into the secondary markets. A lot of those are still floating around today. OR - the government stuck the shipment into a warehouse, and in typical bureaucratic fashion, forgot about them - until some SoCal slickster approaches them with a great offer. Bingo! They are back on the market as super-duper cold war commie beating top grade mil-spec tubes - and sold to you and I, who don't realize that they probably failed the spec test. The VT or JAN designations have too often been assumed in tube folklore to be better tubes. In reality, they may be the rejects. I have occasionally found FIELD STOCK in foreign countries left behind after a war. This is the primo stuff. Not the stuff sitting in a US or UK warehouse.” –Robert H. [PHOTO HERE]
5692 - Military part number for a 'shock-proof' tube. Introduced 1948. The famous RCA "red base" is a 5692. Weight is much higher, and tube construction differs from 6SN7s. 275V maximum plate voltage & maximum 1.75W per section.
[PHOTO HERE]
33S30A* / 33S30B* - Swedish military versions of the 6SN7. Very
very rare and sought after.
ECC31, 32, 33, 34 - Popular MISconception: these are
NOT equivalent to 6SN7.
Especially the ECC32/CV181 - close, but the 50% higher current draw will probably fry some transformers. Grid voltage, gain and transconductance are all different for these tubes. Ask your amp manufacturer first before substituting these in.
CV181/CV2821 – Alternate designations for ECC32/ECC33 respectively.
The popular Mullard CV181 is NOT a drop-in replacement for 6SN7s – 50% more power draw means that the tube will probably send your transformer up in smoke.
1633/6042 - 25V/0.15A heater version of the 6SN7.
6N8S* - Old Russian designation for the 6SN7. Some Eastern European and Chinese tube factories still use this designation.
6H8C - Russian designation for a 6SN7 with a higher plate voltage.
[PHOTO HERE]
13D2 - Europe-only 'special quality' version of the 6SN7GT. The tube is reinforced for additional mechanical rigidity.
[PHOTO HERE]
CV1986* / CV1988* - Both are the UK Government Common Valve designation for the 6SN7GT.
[PHOTO HERE]
CV2627* - UK Government Common Valve designation for the 6SN7W.
B65* - 6SN7 replacement made by Marconi/Osram/GEC.
Extremely rare and sought after.
[PHOTO HERE]
7N7 - Electrically IDENTICAL to the 6SN7 tube. This tube has a loctal-base. To fit a 6SN7 socket, a loctal-socket-to-octal-base adaptor must be fashioned. Why loctals? These tubes were created (and strongly advocated by RCA and Philco) for use in high-frequency radio applications, where the longer wires present in normal 6SN7 bases were causing problems by picking up interference.
[PHOTO HERE]"
There is a ton more info in that 1st Post, and the whole thread, check it out here:
"
The Reference 6SN7 Thread" 1st post listing has a ton of useful "6SN7 Reference" Info
https://www.head-fi.org/threads/the-reference-6sn7-thread.117677/post-1380036
I read up, see photo's, and post in the "
6SN7 Tube Addicts" thread:
https://www.head-fi.org/threads/6sn7-tube-addicts.479031/post-16772217
https://www.head-fi.org/threads/6sn7-tube-addicts.479031/post-16772903
And, for the other TA-26 tube - the 6AS7G - here is the "
For 6AS7G tube rollers here ....." thread, and it's first post has a great list of 6AS7G variants!
https://www.head-fi.org/threads/for-6as7g-tube-rollers-here.410326/post-5441008
"6080 - low gain @ 2- great vocals, wide sound staging
7236 - medium gain - great bass, fast and dynamic. Almost SS sounding
5998A - medium gain in straight bottle. Plate construction like 6AS7.
5998 - medium gain at 5, full-bodied sound, same current load as 6AS7
421A - same as 5998 but with matched plates/sections
6AS7G - low gain, the standard tube for most, plentiful
6AS7GA - low gain, like the standard 6AS7G in straight bottle
6520 - premium 6SA7G, sometimes with 5998 plates
6H13 - Russian 6AS7G equivalent. nothing magical with these
ECC230 - European 6AS7G equivalent. haven't seen one in person
6528 - equiv to 5998 but very high gain at 9, twice the current load. Make sure you amp can handle it
6336 - like 6528 but low gain at 2, still twice current load. Make sure you amp can handle it"
