SWISSONOR  45 SE

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A bit of Tube History

"Just Enough Power"

VTV MAGAZINE / ERIC BARBOUR  -  Issue # 15  -  SUMMER / FALL 2000

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Introduction
At Westinghouse Electric and Manufacturing Company's tube laboratory in Elmira, NY, E.D. Wilson's engineering team created the UX245 in June 1928. There was an obvious need for a triode that put out more power than the UX112A or UX171A on a typical radio plate supply, and without the high plate voltage of the UX210 or the UX250. Using the latter tubes in SE mode meant heavy filtering of the filament supply, because their 7.5v voltage requirement increased hum in the output.

 

So, what was needed was a directly-heated triode that used a relatively low filament voltage. Engineers working on tubes for battery radios had discovered that if the filament was AC operated, keeping the filament voltage low minimized the magnetic component of the AC hum, which interacted with  magnetic metal parts in the tube. 2.5 volts was chosen as a good compromise, suited to low hum yet giving adequate temperature and emission for a power triode.

 

Although the low distortion of the tube was well-known to engineers of the day, only in the last 20 years did users fully recognize the audio virtue of this device simply known by its later type-number, 45. Ironically, such recognition came long after manufacture of the 45 ceased.
 

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The Design
It is a minimal structure ... a single W-shaped filament in a single crimped-nickel anode. The oxide coating of the filament gave enough emission for about half the plate resistance of a UX210, considering the small anode. This allowed the use of a less costly output transformer (meaning easier to wind). The 45's lower mu allowed its use in no-feedback circuits, similar to those used with the UX171A, while producing more power than the UX171A.

 

Dynamic loudspeakers came into wide use in the late 1920s, because they gave 
far better frequency response and lower audible distortion than previous magnetic speakers. Unfortunately, dynamic drivers needed more power than 
the 171A could deliver. There was a performance gap between the 12A/71A triodes and the 50, a gap which the 45 filled perfectly. Loose tolerances and 201-like construction made the most of existing tooling. Since the oxide filament ran cooler than the UX210's thoriated filament, the grid also ran cooler, allowing the use of molybdenum grid wire, without gold plating to prevent secondary emission. The UX-245 was very roughly similar to the earlier (and cruder) 
Western Electric 205D. WE never made a tube that was close in ratings to the 45.

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The UX245 was made in many forms, by a variety of factories. Arcturus (type 145, in blue glass), Ken Rad, National Union, Raytheon (in their famous "4-Pillar" form), RCA, Sparton (they called it a 182), Speed, Sylvania, Triad, Tung-Sol, and others claimed internal production of their own variations of the UX245 and/or the 45 before WWII. Some manufacturers had their own unique numbers for their versions. Cunningham labeled their RCA UX245 as CX-345, while DeForest called it a 445. The bubble or globe envelope ceased production in 1932; thereafter, all conventional 45s were in ST-14 envelopes. Like the 12AX7, so many variations existed that a complete catalogue (at this late date, anyway) would be nearly impossible to document fully.

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Not many premium versions of the 45 were made. The most notorious was the VT-52, made by Tung-Sol primarily for military equipment. The VT-52 has a 7.5v filament, so it was obviously intended as a 10 replacement, while making it unusable as a 45 replacement. Otherwise, a special "ruggedized" version apparently did not exist; designers preferred indirectly-heated tubes or 2A3 variations for such jobs. The Speed 295 was a type 27 and 45 in a single envelope originally designed for Loftin-White amplifier circuits in 1930s cheap table radios. No RETMA-series versions of the 45 were made after the war, as designers regarded the 45 as obsolete by then. A 45A version was made by Sylvania, using a larger ST-19 envelope, to allow cooler operation. This version was apparently sold mostly by Sears, under the Silvertone label and under the Colonial Radio brand. There was also a GT bottle 45, believed to have been made by a smaller tube company and very rarely seen today. The most amusing fact about the 45 is that it was considered very, very obsolete by 1938.

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Until recently, when fringe SE tube audiophiles noticed the high linearity of the 45, its only demand came from radio collectors needing replacements. Millions of 45s and variants were sold, starting from 1929 until Sylvania production ceased in 1959. It is possible that some 45s were made offshore after 1959, under contract, though we have not been able to verify this. The RCA tube manual RC-19 of 1959 shows it as a "DISCONTINUED type, listed for reference only."

 

Old stocks were plentiful thereafter due to declining demand, so it stayed available for several more years. Yet it was such a pedestrian tube, that it was often misused and even abused. We're told that some World War II military equipment used 45s as AC line fuses--the filament makes a very nice 3-amp fuse, with just the right characteristics. And Norm Braithwaite reports that his 1937 Wards Airline model 62-197 table radio uses a 45 as the output-tube bias rectifier. 

 

Closing Comments - 2A3 vs 45

In general, 45s and single-plate 2A3s enjoy a better reputation for clean sound at low power than double-plate 2A3s. This fact has given rise to the common notion that single-plate construction has some kind of magical property. We at VTV feel

that construction quality and tolerancing have more to do with linearity than the number of plates. It's simply easier to get a linear tube if there is only one grid (in one plate structure).

 

Double-plate 2A3s came about because of the cost and difficulty of aligning the complex "harp" filament in the early single-plate 2A3. In fact, the double-plate 2A3 strongly resembles two 45s in parallel. This problem also applied (and still does today) to the 300B. The 45 was just the right size and just the right scale for a single-plate tube. Easy to make, low cost, and reliable.

It is striking that the 45s did about as well as the 2A3s, even though the 2A3s are rated for much more power output. All other things being equal, one would expect distortion of the 45s to be higher. Their plate resistance is about twice that of the 2A3s. Even so, with the same 3200-ohm load, they came out about the same. 

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