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1. VT-231s

Hytron
Hytron was bought by CBS in 1952. Tubes made by Hytron post-1952 have ‘CBS-Hytron’ or plain ‘CBS’ labeling.
Base: black with white labels
Glass: clear
Plates: black, T-plates
Getter: bottom, rectangular, flashing rarely extends up past plastic base
Top mica: rectangular
Other significant features: top and bottom micas are identical


KenRad
Base: black, white labels sometimes written in a ‘gothic’ font
Glass: black (very common) or clear (less common)
Plates: black, flat with ‘ladder’ ridging, ‘staggered’ arrangement
Getter: bottom, flashing extending up to 1/3 of the way up tube
Top mica: rectangular with 3 spikes on both shorter sides
Other significant features: line of small holes down longitudinal midline of top mica. Protruding grid posts and plate anchors on top mica have distinctive ‘staggered’ pattern.


‘National Union’
National Union never made a tube labeled 'VT-231'. However, National Union did make a JAN-rated 6SN7GT (JAN-CNU-6SN7GT). These came in 2 flavors – (earlier) grey glass and (later) black glass. Check the US-made 6SN7GT section for more on National Union 6SN7GTs.


RCA
Base: black with silver labels
Glass: grey extending right from base to near the top, usually opaque
Plates: black (difficult to see except with a very strong torchlight)
Getter: bottom, flashing extending up past plastic base
Top mica: round
Other significant features: 2 stabilising rectangular mica ‘ears’ on either side of round top mica. Mica ears often ‘scratch’ out trails in the grey glass.


Raytheon version 1 (T-plates)
Earliest known version of VT-231
Base: black with orange-yellow labels
Glass: clear
Plates: black, T-plates
Getter: bottom, flashing rarely extending up past plastic base
Top mica: racetrack shaped with 4 spikes coming off either shorter side
Other significant features: bottom mica identical to top mica. No support rods.


Raytheon version 2 (Flat plates)
Chronologically, the next version of Raytheon VT-231s.
Base: black with orange-yellow labels
Glass: clear
Plates: black, flat with ‘ladder’ ridging.
Getter: bottom, flashing rarely extending up past plastic base
Top mica: racetrack shaped with 4 spikes coming off either shorter side
Other significant features: Bottom mica identical to top mica. 2 metal supporting rods between top and bottom micas. 2 supporting rods are ‘free-standing’ and are not attached to anything aside from the two micas.


Raytheon version 3
Unknown vintage. Rare.
Base: black with orange-yellow labels
Glass: clear
Plates: black, flat with ‘ladder’ ridging.
Getter: bottom, flashing rarely extending up past plastic base
Top mica: racetrack shaped with 4 spikes coming off either shorter side
Other significant features: 2 ‘free’ copper support posts. Bottom mica identical to top mica. Top mica has ‘spring-loading’ of plates (black U-shaped structures)


Sylvania
Base: black, white labels (rarely yellow)
Glass: clear
Plates: black, T-plates
Getter: bottom, foil, flashing extending up to 1/3 of the way up tube
Top mica: rectangular
Other significant features: micas may appear whitish. Bottom mica may be slightly smaller than top mica.


TungSol
Base: black, white labels
Glass: black glass extending from bottom of tube to ¾ up
Plates: round plates, ‘keyhole’ appearance when viewed from above
Getter: bottom, foil, very hard to see
Top mica: round with small spikes on opposite sides (earlier; 19??-42) or oval (later; 1942-46)
Other significant features: spring-loading of plates (U-shaped structures) on top mica. U-shaped structures may face same or opposite directions. Tubes with oval micas have long thin metal pieces affixed to the opposite edges of their micas to brace them against the inside of the glass. Oval mica tubes have the bottom mica identical to the top mica, sans metal bracers.