0. Introduction & preamble FEEL FREE TO POST REPLIES TO THIS THREAD!!! Found a mistake? Have a suggestion? Got photos you want to share? Send them all to the following email address: 6sn7gt-at-gmail-dot-com (just replace ‘-at-‘ and ‘-dot-‘ with ‘@’ and ‘.’ respectively. DIE, SPAM, DIE!!!) While every effort is taken to maintain the accuracy of this thread, mistakes do get through. Please let me know if you find any! Thanks! Version history 1.0 - 05/12/2006 - Original thread set up A quick note regarding nomenclature: ‘6SN7’ is not an actual tube designation – the earliest designation was ‘6SN7GT’. In this thread, ‘6SN7’ is loosely used as a ‘master term’ to refer to 6SN7GTs, 6SN7GTAs, 6SN7GTBs, 6SN7Ws, etc – in other words, all the available permutations of the 6SN7GT tube. It is interesting to note though that European tube manufacturers sometimes just labeled tubes ‘6SN7’. This thread is divided into 6 sections: 0. Introduction and Preamble (this post) 1. VT-231s 2. US-made NOS 6SN7s 3. 5692s 4. European NOS 6SN7s 5. Other non-US NOS 6SN7s And so it begins… This thread archives the physical construction of popular NOS 6SN7 tubes, with the aim of helping tube buyers make informed purchases. Tube relabeling was indeed common ‘back in the day’, but a tube’s construction will *always* betray its true origins. More disturbingly, as the demand for NOS 6SN7s rises, so will the number of unscrupulous sellers counterfeiting tubes with intent to make a quick buck. The information contained here is the best weapon tube lovers can use against such counterfeiters. The 6SN7GT tube type was formally registered in 1941 by RCA and Sylvania. The 6SN7GT is an octal base medium-mu (20) [i.e. medium amplification] double-triode tube. It runs on a heater current of 0.6A (@ 6.3V). The 6SN7GT type is essentially two 6J5-type single-triodes within a single envelope. The 6SN7GT is the direct descendant of the straight sided 6F8GT. This in turn was a straight-sided glass envelope which descended from the ‘coke-bottle’ shaped 6F8G. [Tube nomenclature 101: having a lone ‘G’ designation after the tube type indicated that the tube utilized a ‘shoulder-tube #12’ type glass envelope; the ‘coke-bottle’ shape. A ‘GT’ designation in turn meant that the tube had a straight-sided glass envelope.] The 6F8G was a first attempt at placing two 6J5G triodes in a single envelope, but was unpopular due to its unwieldy shape and top grid cap, which necessitated extra connectors. 6SN7 equivalents There are a few direct electrical equivalents of the 6SN7GT, and they are as follows: 7N7 – short lived loktal base version registered by Sylvania/Raytheon in 1940 CV1988 – British designation; Civilian Valve #1988. Made by Brimar. 6N8S – Soviet designation, still in use today B65 – metal based tube produced by the Marconi Group of companies 13D2 – Brimar made ‘industrial’ 6SN7GT (many GE manuals wrongly characterize this tube as having a 12.6V heater.) It is useful to note that the following types, namely: ECC32 ECC33 5692* (see note below) …are, contrary to popular belief, not perfect electrical equivalents of the 6SN7 type. Substituting these tubes in place of 6SN7s in your equipment may result in damage. *: 5692 types can be run at 6SN7 specifications and function identically even though they were technically not designed to do so (more on that later). More commonly, unique nonlinearities in amplification caused by deviation from 6SN7 specifications lend characteristic ‘sounds’ to these tube types. These unique ‘sounds’ in turn are often misconstrued as being ‘better’ – a notable example of this would be how the ECC32 type is often perceived to have ‘better sound’ due to it’s higher gain. The human mind gives preferential attention to louder sounds, and the result of substituting ECC32s in place of 6SN7s is more often than not subjectively perceived to be ‘better’. 6SN7 variants The 6SN7A and 6SN7W were both short-lived designations for 6SN7GTs made to military specifications. Their construction included an additional ‘free-standing’ support post between the micas for greater vibration tolerance. It is likely that there were little, if any, additional selection criteria imposed upon A/W designations that were not already fulfilled by GT specifications. These designations used from (approximately) 1941 to 1945. The VT-231 (‘Vacuum Tube #231’) designation was used throughout WWII by the US Army Signal Corps. It should be noted that the term ‘VT-231’ was simply an inventory control number and thus lends no special significance to the tube so labeled. There is a full section devoted to VT-231 and JAN tubes later in this post. The 6SN7GTA and GTB versions were introduced in 1950 and 1954 respectively. They have ‘upgraded’ ratings, which are as follows. GTA: maximum anode dissipation upgraded to 5W (the GT is rated for 2.5W) GTB: identical to the GTA, but with a controlled heater warm up time for use in equipment with 600mA heater strings This is significant because it means that a circuit designed around the 6SN7GTA/GTB cannot have 6SN7GTs substituted into it. Substituting a GT in place of a GTA/GTB in your equipment will precipitate its rapid destruction and possibly damage your equipment. Tubes labeled 6SN7GTC were made in the late 60s and early 70s. They are electrically identical to 6SN7GTBs but have their glass envelopes replaced by black colored metal envelopes. These tubes were mostly manufactured in South America. VT-231s and ‘JAN’ rated 6SN7s The VT-231 designation was a military inventory control number, and should be taken as nothing more than that. Assuming identical construction, there is zero difference between a tube labeled VT-231 and a 6SN7GT of the same vintage. A lot of mystique has been built up around VT-231 labeled tubes by tube sellers, and many people mistakenly believe ‘JAN’ stands for ‘Joint Army-Navy’, and was meant to indicate that the tubes labeled as such were ‘passed’ for military use. However, no additional criteria that were not already fulfilled by the 6SN7GT specification were known to have been imposed. ‘JAN’ labeled tubes are identical to non-JAN tubes of the same vintage and construction. The ‘JAN’ naming convention was always written as follows: JAN-<manufacturer>-<tube type> Thus examples of JAN ratings would include: JAN-CTL-6SN7GT, JAN-CHS-5U4G, JAN-CRC-6AS7G, etc. JAN manufacturer abbreviations: CHS – Sylvania CHY – Hytron or CBS-Hytron CKR – KenRad CL – GE CRC – RCA CRP – Raytheon 5692 types The 5692 was designed by RCA in response to a particular problem involving radar-controlled artillery pieces. Bouts of intense vibration (while firing) were rapidly destroying 6SN7GT types in such equipment. The 5692 was designed to withstand such stress, and has extra rugged construction with features such as an extra top mica, 5 additional freestanding support posts and a low profile. The 5692 was patented in 1948 but not produced till 1951. 5692 types were advertised as a 6SN7GT substitute that had a vastly extended lifespan – 10,000hours. This outlandish claim, however, falls apart when you examine the recommended operating specifications of the 5692. The 5692’s recommended operating parameters are much lower than those of the 6SN7GT; any 6SN7GT would last far longer (maybe even 10,000hours?) when run at 5692 ratings. There are 2 ‘general types’ of 5692s – red bases and brown bases. The red bases in particular may have their color vary from a bright cherry-red to a dark, deep red easily confused with brown. Who actually produced 5692s in the USA is a controversial issue amongst tube lovers; here are some of the competing theories: -All red base 5692s were made by RCA. -All red base 5692s were actually made by GE, but for RCA (explains the high incidence of GE labeled red bases). -All brown base 5692s were made by CBS-Hytron. -Raytheon made a brown base 5692. -Sylvania made a brown base 5692. -Westinghouse made a brown base 5692. [My opinion, based on personal experience, is that GE made red-based 5692s for RCA, and that CBS-Hytron (earlier) and Sylvania (later) made brown-based 5692s.] There is only one known European equivalent of the 5692 type: a Swedish-made brown base version designated either 33S30A or 33S30B. In the late 1960s, 6SN7GTBs were often relabeled as 5692s. Remember – if the construction doesn’t include the special features unique to 5692s (mentioned earlier), it isn’t a ‘real’ 5692. (Very) rarely, 5692s have been known to be labeled as 6SN7GTBs. A note on glass colour (‘smoked’ glass tubes) Tubes with black or grey glass were only made en-masse till the early 1950s, after that they become vanishingly rare. What happened? The black/grey coat was a carbon/graphite coat, designed to prevent electrons from massing on the glass and exerting an undue influence on tube operation (massed negative charges). Whether its eventual disappearance was the result of cost-cutting measures, or the introduction of new glass that was somehow resistant to this electron massing is unknown. ** Descriptive terms used in this guide ** This guide uses descriptive terms liberally. To prevent confusion, they are defined as follows: Base: Bases were almost always made of plastic (Bakelite?). Brown bases are usually made of micanol, a ‘low loss’ material – 6SN7s with micanol bases are sometimes designated 6SN7GTY. Rarely, some bases are ‘metal’ – a metal (nickel alloy) strip was wrapped around the glass base of a tube and glued on. These metal strips will corrode if improperly stored and often fall off due to old age. Some tubes produced in the late 1960s to 1980s come with ‘button’ bases – the plastic base is no longer a large cylinder but looks more like a plastic button affixed to the bottom of the tube instead. Glass: Almost all 6SN7s have straight sided glass envelopes. Glass may either be ‘smoked’ (blackened or greyed) or clear. Glass envelopes often vary slightly in height even amongst examples of identical construction and vintage, but generally fall into 2 broad categories – ‘tall bottle’ and ‘short bottle’. An extreme example of a tall bottle 6SN7 would be the Sylvania VT-231 while the opposite would be any 5692 tube. Getter: More accurately known as ‘getter flashing’. A getter is the silvery coat of pure barium metal deposited on the inside of a tube to ‘get’ (react with) free gas molecules within the tube, which would otherwise interfere with tube operation. A ‘top getter’ is located at the top of the tube, a ‘bottom getter’ is located at the bottom of the tube, and so on and so forth. When referring to the getter ‘shape’ we are talking about the ‘getter holder’. The getter holder is a structure within the tube that held the barium metal before it was ‘flashed’ onto the inside surface of the tube. Common getter shapes in 6SN7s include ‘ring’ (also known as ‘O’), ‘D-shaped’, ‘foil’ (looks like a piece of aluminum foil), ‘square’ (a square ring), rectangular (a rectangular ring) or ‘cup’ (looks like an inverted cup). Mica: Mica is a naturally obtained mineral, and is used extensively in tube construction due to its ability to withstand high temperatures whilst maintaining its electrically insulative properties. Micas are located near the top and bottom of tubes, and are described in terms of their location and shape. An example would be ‘round top mica with rectangular bottom mica’. Descriptors such as ‘round’, ‘rectangular’, ‘square’ and ‘racetrack’ are self-explanatory. A ‘scalloped’ mica is a mica that looks as though a creature has taken a small nibble out of the edge at regular intervals. A ‘spiked’ mica is one that has little spikes extending out. Scalloped and spiked micas can occur with any mica shape. Mica spacers: Mica spacers are pieces of mica attached anywhere within the tube that serve to brace the internal construction against the glass to improve its vibration resistance. Plate: Plates are more correctly known as ‘anode plates’, and are the large metallic structures that extend between the top and bottom micas. There are always 2 separate plates in 6SN7 tubes. They are described in terms of color and shape. Color is self-explantory – plates are usually either grey or black. Shape is a little more tricky though. ‘T-plates’ (also known as ‘box plates’) are plates that have a ‘T’ shaped cross section. ‘Triangular plates’ are plates that have a triangular cross section. ‘Flat-plates’ are not actually perfectly flat, but have thin and long rectangular cross sections. ‘Round-plates’ or ‘oval-plates’ are plates that have a circular or oval cross section. ‘Ridged’ (or ‘embossed’) refers to the raised shapes stamped onto the plates. These often have a pattern that looks somewhat like a ladder. ‘Smooth’ plates are plates that have no ridging/embossing on them – in other words, perfectly smooth. ‘Angled plates’ are plates that are at strange angles to each other – in other words, not directly mirroring each other in placement and orientation. ‘Staggered plates’ are flat-plates that are parallel to each other, yet at a ‘funny’ angle when viewed from above, like this: / / or \ \ (as opposed to | | or =) Examples of plate descriptions would include ‘black T-plates’, ‘grey staggered flat plates with ladder ridging’, etc. Support posts: support posts are additional metal posts that extend between the top and bottom micas and serve to brace the tube internals against vibration.