Been seeing photos of fluorescent tubes with their wattage marked as "65-80w" and was wondering how the multi-wattage aspect works.

Have you tried to look for their datasheets?

I see there two possibilities:
First is they would be "energy saver" lamps, so lamps designed to replace 80W lamps, but have just 65W real power input, but designed to operate on an "80W" ballast in an "80W" fixture.

Other possibility is, the lamp may work at two current (so power) levels, depending on the exact connection or the style how the ballast feeds the arc current. If you feed the arc via just a single pin on each end (like in a preheat circuit,...), all the current has to pass through the single filament from the terminal to the point where the arc root is, hence the e.g. 65W power.
But if the ballast feeds the tube the way the arc current is fed from both filament terminals in parallel, through each filament section flows just about half of the current, so causing less dissipation there, so for the same filament heating power the arc may get higher total current (about 1.4x for the same total resistive heating power, in reality a bit less because the cathode fall dissipation goes always from all the current), hence the 80W rating.
But this second way would be possible only on lamps, where the operating current is dictated by the electrode loading and not by e.g. the overall lamp thermal balance.

But the details and conditions will be described in the datasheet (so e.g. the compatibility with two ballast types, one feeding 65W and the other 80W).

I think that European 65W T12 lamps, don't have this 65-80W printing, but only British lamps.

The UK 5 foot T12s were designed to work on the abundance of mercury ballasts, hence where the 80 watts rating comes from.

It's also why they originally had BC caps, as it was easier to use what was available rather than set up a new production line for bi-pin caps during and just after WW2.

The first 5 foot tube was the British bayonet-capped version introduced in 1939.  It operated at 865mA on a standard 223Ω high pressure mercury ballast, with a tube voltage of just over 100V.  It was a poorly optimised design - introduced only for emergency reasons at the start of WW2 and using standard caps and ballasts.  Due to the high power loading for such a length, it was rather inefficient and its life was short.  But even that was considerably better than the alternative of incandescent or other discharge lamps, so it still sold in vast volumes and quickly occupied most of the applications that could be converted to fluorescent.  This made it challenging to replace it by something better.

It was not until 1964 that an improved 5ft tube was developed - rated 65 Watts and for use on a new 240Ω ballast with somewhat higher 110V across the tube, and reduced current of 670mA.  Due to the lower loading its efficacy increased considerably and this became the favoured tube for new installations, especially in Europe where no 5-foot tube had existed previously.  Gradually over time the 65W rating also became popular in Britain.  But for Britain and its colonies there was a problem in that a very large quantity of 5' 80W installations still existed.  Therefore from 1964 the tubes sold in those territories were dual rated 65/80W, and suitable for use at either 670mA or 870mA.  When used at 80W the voltage of this new universal tube decreased to 99V, close enough to the original 80W model to be a suitable replacement.

In Europe there was no legacy of the 5ft 80W tubes, and it was possible to make a different and slightly more optimised lamp rated exclusively for 65W at 670mA on the new 240Ω ballasts.  Such lamps were quickly burned out if accidentally used on the British 80W circuits.  Its different cathodes were rated for preheating at 1000mA whereas the 80W and 65/80W tubes were for preheating at 1300mA.

As if this wasn't complicated enough, it is important to realise that in addition to the different discharge power ratings, the 5ft T12 also existed with two quite different types of cathodes.  The original 80W and 65/80W tubes were produced with high resistance 8.0V cathodes (the actual resistance being 11Ω at 65W and 12Ω at 80W when tested under reference conditions at 8.0V, but possibly slightly different on actual ballasts depending on their construction).  The 65W tube only was also offered with low resistance triple coil 3.6V 6Ω cathodes, such as had first been invented in the USA for its Rapid Start lamps in 1941/42 as a way of reducing the energy losses in continually heated cathodes as were common in USA.  These were intended for starterless operation (types RS, IRS), and would not deliver satisfactory performance in switch-start luminaires.

Needless to say, the average customer had no idea of these design differences which were not even widely noted in manufacturers catalogues of the time.  New installations were of course made with the correct lamps to match their luminaires but replacement lamps were invariably of the wrong type.  This situation became especially severe with the ban on regular T8 tubes and the stop of production of types having the formerly standard 8V high resistance cathodes - only the RS and IRS types having metal stripe and 3.6V low resistance cathodes continued in production, intended for their respective special applications.  Naturally many customers simply substituted the metal stripe lamps for the plain originals and then noted a huge reduction in life.  That probably helped further accelerate the demise of T12 types in Europe.  Only a few specialised lighting contractors knew about the differences and realised this would not work - or other experts who study the electrical specifications of their tubes as detailed in the IEC 60081 standard.

Fascinating, you learn something new every day 

Thanks for all the replies. I find it really cool how the manufacturers came up with a way to make the newer lamps work with both the old and new gear. 

Great info regarding the various types of 5ft fluorescent lamps. I didn't know that Europe got the rapid start optimized cathodes too.

I've also been curious about that as the preheat only and rapid start only 4ft T12 lamps here were quickly changed into a type rated for preheat and rapid start which seems to give satisfactory performance on either type of ballast.

I remember doing my Level 3 C&G Electrical course last year and one of the powerpoint units talking about fluorescent lighting mentioned the 80W tube being based off the 80W MBF choke, as well as the 8ft 125W tubes being based off the 125W MBF chokes also. Of course as shortages and rationing slowly weened off over the decades after WW2 as well as advancements in their performance, they went down from 5ft 80W to 65W, and 125W 8ft to 100W; though they could still be ran off the old mercury chokes fairly well.

  Well first of all 125 watt tubes cant run directly off 125 watt choke because these tubes have high reignition peak with out capacitor in series.  Even if u add it on 125 watt choke it would
 Send the tube into over drive.   What they done is used a 80 watt mercury choke and 7.2 uF cap in series.  The 100 watt choke is not much different as it about same current as US
 Mercury ballast  HX ballast.  By adding a cap to it would work and run that 125 watt tube no problem.  But using a 125 watt mercury gear with a cap will send 125 watt tube into over drive.
And it wont run directly off tube without a cap as it will just blink away like it EOL.

The first 5 foot tube was the British bayonet-capped version introduced in 1939.  It operated at 865mA on a standard 223Ω high pressure mercury ballast, with a tube voltage of just over 100V.  It was a poorly optimised design - introduced only for emergency reasons at the start of WW2 and using standard caps and ballasts.  Due to the high power loading for such a length, it was rather inefficient and its life was short.  But even that was considerably better than the alternative of incandescent or other discharge lamps, so it still sold in vast volumes and quickly occupied most of the applications that could be converted to fluorescent.  This made it challenging to replace it by something better.

It was not until 1964 that an improved 5ft tube was developed - rated 65 Watts and for use on a new 240Ω ballast with somewhat higher 110V across the tube, and reduced current of 670mA.  Due to the lower loading its efficacy increased considerably and this became the favoured tube for new installations, especially in Europe where no 5-foot tube had existed previously.  Gradually over time the 65W rating also became popular in Britain.  But for Britain and its colonies there was a problem in that a very large quantity of 5' 80W installations still existed.  Therefore from 1964 the tubes sold in those territories were dual rated 65/80W, and suitable for use at either 670mA or 870mA.  When used at 80W the voltage of this new universal tube decreased to 99V, close enough to the original 80W model to be a suitable replacement.

In Europe there was no legacy of the 5ft 80W tubes, and it was possible to make a different and slightly more optimised lamp rated exclusively for 65W at 670mA on the new 240Ω ballasts.  Such lamps were quickly burned out if accidentally used on the British 80W circuits.  Its different cathodes were rated for preheating at 1000mA whereas the 80W and 65/80W tubes were for preheating at 1300mA.

As if this wasn't complicated enough, it is important to realise that in addition to the different discharge power ratings, the 5ft T12 also existed with two quite different types of cathodes.  The original 80W and 65/80W tubes were produced with high resistance 8.0V cathodes (the actual resistance being 11Ω at 65W and 12Ω at 80W when tested under reference conditions at 8.0V, but possibly slightly different on actual ballasts depending on their construction).  The 65W tube only was also offered with low resistance triple coil 3.6V 6Ω cathodes, such as had first been invented in the USA for its Rapid Start lamps in 1941/42 as a way of reducing the energy losses in continually heated cathodes as were common in USA.  These were intended for starterless operation (types RS, IRS), and would not deliver satisfactory performance in switch-start luminaires.

Needless to say, the average customer had no idea of these design differences which were not even widely noted in manufacturers catalogues of the time.  New installations were of course made with the correct lamps to match their luminaires but replacement lamps were invariably of the wrong type.  This situation became especially severe with the ban on regular T8 tubes and the stop of production of types having the formerly standard 8V high resistance cathodes - only the RS and IRS types having metal stripe and 3.6V low resistance cathodes continued in production, intended for their respective special applications.  Naturally many customers simply substituted the metal stripe lamps for the plain originals and then noted a huge reduction in life.  That probably helped further accelerate the demise of T12 types in Europe.  Only a few specialised lighting contractors knew about the differences and realised this would not work - or other experts who study the electrical specifications of their tubes as detailed in the IEC 60081 standard.

I think the most amusing example I have seen of people not realising the difference between 65W Rapid Start 3.6V cathode tubes and 65W/80W 9V Preheat/Quickstart cathode tubes

is the Modern 65W RS Tubes, Sylvania made with *Bayonet* end caps for the MoD, keep mind EVERY Bayonet cap fixture out there is going to be an 80W Fixture! safe to say the one Andy used in his own 80W BC tube fixture did not last long at all!


I am curious James, why did Sylvania not make a Quickstart, MCFA/TL-A grounded starter stripe 9V tube, surely that would have been exempt from the bans on the same grounds that the TL-M style starter stripe tubes where?

(it is worth noting that Quickstart circuits also require 9V cathode tubes also, and will quickly munch their way through a 3.6V tube!)

The Sylvania-branded bayonet-capped 65W tubes were not made by Sylvania.  It was a British aftermarket company who added the bayonet caps and created that impressively unreliable combination!

The market did not ask for a metal striped 8.5V 5ft tube and so such a type was never developed.  I suppose manufacturers were not so interested in educating customers on that, and were happy when the same customers kept coming back to buy more new expensive lamps with at least 5 times the usual frequency!

To make a metal stripe T12 tube of either cathode type was indeed legal under the earlier regulations 244 and 245 of 2009 as amended by Regulation 1194 of 2012, but that loophole was closed by the latest Regulation 2019 of 2020.

80 watt fluorescent lamps were also used in Finland. They were probably quite rare. https://www.lighting-gallery.net/gallery/displayimage.php?pos=-214386
65/80 watt lamps were also on sale. https://www.lighting-gallery.net/gallery/displayimage.php?pos=-125952

The Sylvania-branded bayonet-capped 65W tubes were not made by Sylvania.  It was a British aftermarket company who added the bayonet caps and created that impressively unreliable combination!

The market did not ask for a metal striped 8.5V 5ft tube and so such a type was never developed.  I suppose manufacturers were not so interested in educating customers on that, and were happy when the same customers kept coming back to buy more new expensive lamps with at least 5 times the usual frequency!

To make a metal stripe T12 tube of either cathode type was indeed legal under the earlier regulations 244 and 245 of 2009 as amended by Regulation 1194 of 2012, but that loophole was closed by the latest Regulation 2019 of 2020.

are you specifically referring to/thinking of a hypothetical TLM type tube but with 8.5V cathodes rather then 3.6V cathodes (ie 8.5V cathodes but the starter striped is tied to tube potential via resistor like a TLM tube rather then grounded like a MCFA/TLA tube)


as you state a metal striped 8.5V type was never developed, but there is the MCFA/TLA type tube type! which is the very first striped tube, and all types had high voltage cathodes

heres a lovely 80W BC example that Andy has

https://www.lighting-gallery.net/gallery/displayimage.php?pos=-135398

plenty of manufactures made them alongside the more modern TLM type tube, so I am just a bit confused by that statement, or are you meaning that Sylvania never made an MCFCA type? as thinking about it cannot recall ever seeing a Sylvania MCFA tube!


another example of a 9V Cathode MCFA starter striped tube is this 2ft 40W example

https://www.lighting-gallery.net/gallery/displayimage.php?pos=-221987

compared to a TLM type Thorn tube which is marked IRS For Instant Rapid start

https://www.lighting-gallery.net/gallery/displayimage.php?pos=-50746



although a side note I noticed the tube right along from that seems to marked "IRS/HR" I gotta wonder does that HR stand for High resistance cathodes? never noticed that before

https://www.lighting-gallery.net/gallery/displayimage.php?album=1488&pos=218&pid=50747


ill leave a comment to see if Micheal can check the cathode voltage