Filament Voltage
 

Why was the voltage 6.3 volts chosen for standard filament voltage? Why not 6.0 volts? A 5U4 has a 5.0 volt filament.

Curious minds want to know.....

6.3 volts was chosen back in the days radios were powered by batteries. 6.3 volts was the voltage of a lead acid battery, which was typically used for an "A" battery.

One of the main reasons 6.3V was chosen as a filament voltage was car radios. Back before they figured out how to convert utility voltage to something tubes used 1.5V, 2.5V and 5V signal tubes were common in battery powered home radios. Car batteries were standardized at 6.3 volts from the beginning until the mid 1950s when 12V became the norm. Car radios started to become possible and popular around 1930 and tubes started getting standard RMA part numbers around then so the industry standardized around tubes that would be good in car radios. The lower heater voltages persisted in battery powered stand alone portable radios like Zenith transoceanic. In the 30s there were also higher voltage heaters developed for cheaper radios to meet the demands of great depression America...1,2,3,4,5 and 6 tube heater series strings would divide the full 117VAC from the wall ...the 5-Tube AA5 (all American 5) became the standard of cheap enough and good enough for nearly all places and sold best. Single 117V tubes were common in one tube wonder phonographs, and were sometimes used in battery/AC portable radios... Sometimes as just a rectifier, but some 117V tubes had both a rectifier and power penthode that would be used in lieu of the battery output tube for more audio power when running from AC.

Not sure why rectifiers favored 5V (they may have adopted that as a standard before car radios), but there were exceptions like the 6X5 and oddballs like the late 50s introduced 3DG4.

The filament voltage question is a good one.

In the early twenties, 5.0v @ 1A was standard for such tubes as the UV-201. In 1922 thorium filaments provided better emission facilitating lower filment current. The UV-201A in 1922 filament ran at 5.0v @ 0.25 amp. Note 5.0v was chosen to allow a 6 volt lead-acid battery terminal voltage to drop with the difference taken up with a series rheostat. In 1922 Carbon-Zinc dry cell tubes became available such as the WD-11 1.1v @0.25A and UV-199 3.0v @ 60mA.

In 1925, the UX201A was introduced. Characteristics the same as the UV-201-A only a different base. In 1925 a power output tube to drive bigger loudspeakers became available: the UX210 and in 1927 the UX250. Both filaments were 7.5v @ 1.25A.

1927 is when US manufacturers introduced the first of the indirectly heated tubes to run on AC. In the US 2.5 volts @ 1.75A. The UX226 in 1927 was directly heated with a center tap resistor providing the emitter return. It's filament ran at 1.5v @ 1.05A. The UX226 must have about the shortage use in design of any US tube as it was only used for just over a year before becoming obsolete!

So it looked like 2.5V was to become the US standard. In the late 20's and early 1930's a wide range of pentodes, tetrodes and triodes were introduced with 2.5 volt filaments. In 1931-32 however a few 6.3v indirectly heated tubes began to appear intended solely for car radios. Note that each lead acid cell is 2.0v and a car battery has three in series. This means the battery yields 6.0v and the extra 0.3v was to account for the slightly higher terminal voltage as the car battery is constantly being charged by the car generator.

Manufacturers in 1933-34 began moving away from 2.5v filaments in preference to 6.3v for household sets. I think this was to reduce proliferation of tube types which was beginning to be a problem. Also it reduced the amount of copper required in the filament wiring of a set! The current drop from 1.0A to 1.75A for the 2.5 volt tubes to only 0.3A for the 6.3 volt tubes was a benefit in saved wiring cost. (2.5v was originally chosen as less hum induced into circuits by the lower filament voltage.)

In the UK and other countries there was a different story. The UK for example used 4.0v AC from the late 1920's until about 1939 when the UK proceeded with US tubes at the beginning of WW2.

Early transmitters used 5V filaments as well. The 6166 tube used in the TT25BL transmitter was 5V. It was used as a power amplifier driver. It was a rather bad idea as the filament contacts would burn up over a period of time due to the high current. On a cold morning you might have to reach in there and twist the tube in the socket to make contact before you could sign on.

I had a Majestic TRF that used four '26's, three for RFs and one 1st audio. Didn't know till now that the tube had only a one year run.

Yeah I think the short run of the 26 was pretty amazing. Tube development was running pretty fast in the late twenties and early thirties. By 1934-35 most of the development which would last decades until the end of the vacuum tube had completed. There were minor updates like octal bases on 1935, the glass button base 7 pin tube and 1947 the 9 pin. But the miniature tubes simply replicated the larger ones in a smaller size.

The 26 I believe was introduced because of manufacturing problems with the early UY227 or 27. The earliest 27's used a twin bored ceramic core to allow the filament heater wires. The ceramic was fit into the cathode sleeve. The earliest UY227 was a notoriously slow warming tube which took a couple of minutes. I suppose in your Majestic where a 27 I believe was needed for the detector the slow warm up was less obvious. (The 26 could not be used for the detector because of the AC filament hum problem).

I have an RCA Radiola 62 from 1928 with a row of seven 27's a UX-171A output and UX280 rectifier. The 27's I believe are all original. I have tested the emission and transconductance of the Radiola 62 UY-227's on my AVO Mark III tester and when warm they are still fine after 97 years. But it is hilarious how long it takes after switching the Radiola 62 on before it makes any sound.

The early 24's (UY-224) have the same slow warm up. The UY224A came out I think around mid 1929 and the A denotes a new heater cathode assembly using a ceramic paint on the helix wound filament wire in the cathode sleeve. It warms in about 40 to 50 seconds which was a considerable improvement.

From 1929 through 1933, the improvements were phenomenal and by 1934 the standard warm up of about 30 to 40 seconds had been achieved.

There were sets that used both the 26 and 27. My 1928 Atwater Kent model 40 (oldest radio I own that I've made work) uses 3 26 TRF stages a 27 as a grid leak detector, and IIRC a 26 1st audio and a 71 output.
I'm using the later RMA standard numbers for those types to avoid repeatedly typing UX- on my phones tiny keyboard.

Continuing this, anyone know why Philco chose the "odd" filament voltages for their SF CRTs? Always been curious about that

Could you provide more information? What is an SF CRT? I assume you mean the odd lower filament voltage on some Philco CRTs? I suspect they were variants to be used in series string sets? If you check it is likely the filament current matches or fits in with the rest of the series filament string.

Interesting. Some of the ultra-reliable Bendix "Red Bank" series of tubes went with a similar construction, with an alumina ceramic heater core with tungsten wire threaded through holes, then inserted into the cathode sleeve. This provided excellent vibration resistance and heater to cathode breakdown voltage capability, at the expense of warmup time.

SF = "semi=-flat". The 17", 2.68 volt CRTs were used with a power transformer that had a tap on the filament secondary. the 21", 2.35 volt were used in series string sets.

I believe the lower filament voltage (and lower filament power) was a result of the tiny electron gun used.

So lessee.. semi flat referred to the new 110 degree deflection scheme, which required a narrow yoke barrel to put the windings physically closer to the electron beam. The smaller neck required a smaller gun structure and smaller cathode of lower heater wattage, which led to cathode emission going kaput very early. Dunno if that's right, just hillbilly musings. :o

110 had been around a couple years already but with longer necks and conventional guns. Philco repurposed a heater/cathode/grid from a miniature 7/9 pin receiving tube to make the neck much shorter.

https://live.staticflickr.com/65535/...78448baa_c.jpg

Thanks guys. It is interesting looking at the gun setup in say an 21EAP4 vs a 21FDP4. The latter looks "conventional" while in the EAP you can see the smaller tube style setup. And it's interesting as far as longevity the few EAPs I have all have decent emissions and cutoff. The one 21FDP4 I have is strange. It has super emissions but pretty much no cutoff. Personally I've never seen that before. Can't wait to test it and see how the picture actually looks