Hello everyone!

I recently experimented with a 40W fluorescent tube, and because I didn't have a suitable ballast at hand, I used a 20W ballast.
Now I've realized that I like the slightly lower light output for the fixture in my living room.

Is it bad for the tube when it's permanently run on a ballast that is too small?

Some info:
- I'm using a 6V heating transformer parallel to the lamp.
- The ballast is for 18/20/23/24/26W TL/TLC lamps.
- When fully on, the heating voltage across the filament is at 3.2/3.4 volts.
- The ballast gets hand warm at best.
- The tube is a Sylvania 40W warm white black ender with normal electrodes.

Probably OK down to about 1/2 of the rated current even without extra cathode heating. Can be down to zero if some adequate cathode heating is constantly applied. Get yourself a dimmable electronic ballast to play. 1-10V versions can be controlled with just a simple 47-100K variable resistor across the input terminals, and most of DALI versions can be also controlled by applying 230V to DALI terminals through a pushbutton (check with a ballast datasheet).


Also, it is said to allow new lamp to run the first 100 hours at full power before attempting to dim, to complete electrode activation to form.

Okay thanks

I would also recommend using a different tube, that Sylvania tube you describe is extremely old and rare, Sylvania only used black end caps on T12 tubes until the mid 1950's by then they switched to aluminium end caps

so thats very much a tube that should be preserved and not used, I would love to see a picture of it on the gallery if your able to post one



also such an old tube will have 9V high resistance cathodes, so needs more voltage to keep the cathodes warm, your current system however would be much better suited for a Rapid-start specification tube, any tube with RS or Rapid start in its etch, for example a TL RS or TL-M RS

Ohh... I think I will switch to one of my Osram 40W/32 tubes then


Btw. I'm not worried about the electrodes, with 6V transformers they glow visibly red-orange.
The emitter just needs to be around 800°C/1470°F to be fully functional.

Ideally, extra cathode heating is adjusted with the reduction of lamp current, as too much temperature will unnecessarily increase cathode evaporation and too low will increase ion bombardment and so material sputtering. This is done by good electronic ballasts. The amount of cathode current adjustment when dimming also is tabulated in IEC 60901 for some newer tube types like G24q CFLs. But unfortunately, not for the older T8/T12 linear lamps.

 

You can attempt to use IEC 60901 published data for dimming 2G10/2G11 36W PL-L CFLs which are made to be electrically compatible with the older 36/40W T8-T12s. You see this is defined in a slightly weird way as a required sum of cathode terminals current squares as a function of longitudinal lamp current. But you can use a current clamp meter to easily verify if your circuit somewhat satisfies this - with a caveat this published formula is generally for HF operation on a reduced RMS lamp current!





   

As long as you keep the cathodes heated you can theoretically dim them as much as you want to. Older pre-electronic fluorescent fixtures had seperate heater transformers, which got powered by raw mains voltage. And then a large triac based dinner controlled the voltage going to the ballasts!

I regret not saving a few of those.

@tigerelectronics I know these FL dimmers. BBC had a very good model "Thyralux" for decades - the first version in the 1960s with a motorized potentiometer and two air cooled thyratron tubes. They also needed an ohmic load, mostly two incandescent reflector bulbs pointed at the blackboard. I have build my own FL dimmer with two antiparallel thyristors and permanent cathode heating in each fixture. The heating voltage depends on the transformer in the fixture, mostly it's 6 volts. The ballasts are part winding, that means there are two separated halves of a ballast in one core and the tube is connected in the middle, so the entire tube floats on half of the input voltage. This is required to safely start a fluorescent tube from zero.

There are still spare parts available on ebay. Maybe you can find one near you...?

Another option is one of these addons. They consist of a 4V heating transformer and a pulser circuit with phase angle detection. So you can connect your everyday ballast to it and supply it from a normal household dimmer and you're good to go. Works with T8 also.

@RRK an engineer at BBC made some long term tests in the seventies: two rapid start circuits, one with the transformer in parallel (reducing the heating voltage when fully on), and one with all time full heating.
The tubes with permanent cathode heating made an average of 19.000 hrs, while the ones with reduced cathode heating only made around 12.000 hrs.

I personally think, permanent heating with 6V should be fine. A friend helped me testing a few tubes and we found that most electrodes reach between 750 °C and 1150 °C on 6 volts. This is perfectly fine for the emitter.
However, we also found that heating the filaments without a discharge present blackens the ends much quicker. Some of my excess T12 tubes were tested on permanent heating for two days on the lab supply, they darkened pretty badly.
Whereas the tubes in my dimmer fixtures are like new, although having run a few hundred hours on 6V and reduced power.

Ohh... I think I will switch to one of my Osram 40W/32 tubes then


Btw. I'm not worried about the electrodes, with 6V transformers they glow visibly red-orange.
The emitter just needs to be around 800°C/1470°F to be fully functional.

thats a very nice Sylvania tube Canadian made too I was not expecting that! very much worth preserving rather then using

you mention that you put the cathode heating transformer on the output of the ballast, so that it drops when the lamp strikes, and thats what im concerned about "3.2/3.4V" wont be enough to keep the cathodes of that style of F40T12

T12 tubes come in basically 2 types, those with 9V Quickstart/High-Resistance cathodes and those with 3.6V Rapid-start/Low-resistance cathodes


its important that when heating the cathodes from a voltage source (rather then a current source) that you select the right voltage for the tube, or the right tube for the voltage

Ideally, extra cathode heating is adjusted with the reduction of lamp current, as too much temperature will unnecessarily increase cathode evaporation and too low will increase ion bombardment and so material sputtering. This is done by good electronic ballasts. The amount of cathode current adjustment when dimming also is tabulated in IEC 60901 for some newer tube types like G24q CFLs. But unfortunately, not for the older T8/T12 linear lamps.

Later versions of the IEC 60081 standard do include fully fleshed out dimming data for the popular T8 sizes and even the traditional T5 sizes, they even have dimming data for the F4T5! I was looking at that the other day and thinking, fun they figured that out, but I never actually seen a electronic dimming ballast for an F4T5! LOL)

here is the HF/Dimming section for the F36T8

I'm not concerned about the temperature of the cathodes because the 3.4V are applied when tube is running, so the ion impact adds to the filament heating.
In total, the filament receives enough thermal energy to keep the emitter up and running.

When not lit yet, the voltage is 6V which is good enough to heat the cathodes properly. I've attached a picture of my ATLAS 20W preheat tube on a 6V
heating transformer, you can see that the filament has enough temperature. The other tubes (40W, 65W) are in the same ball park.

The 9V electrodes are constant current type electrodes, they develop a drop of around 9V when run on a conventional ballast in preheat mode (short circuit
current). They are designed to reach emitting temperature when the preheat current is passed through them. The voltage across the filament depends on age
and state of the electrode. The voltage drop of 9V is valid for new filaments with the full emitter available, they tyically develop a voltage drop around
8 to 11 volts, that's why they are classified as '9V' electrodes. Wornout cathodes have a much higher voltage drop, I measured up to 14 volts on T12 and
up to 17 volts on completely blackened T8 lamps. CC electrodes can cross-ionize, drastically reducing their resistance, but that is not a problem when run
on a magnetic ballast.

Rapid start electrodes are constant voltage electrodes, they are designed to reach emitting temperature on 3.6-4 volts. The current through the filament
develops depending on age and state of the cathode. CV electrodes do never cross-ionize, as this had fatal consequences if the voltage source is not
current-limited. However, most heating transformers do not have the best magnetic coupling, so the output current is somehow limited.

If you put a rapid start tube into a preheat fixture, the short circuit current of the magnetic ballast is not high enough to heat the low resistance
filaments up to the correct temperature, so switch start fixtures would have a hard time igniting RS tubes.
Vice versa, if you put a preheat tube into a rapid start fixture, the preheat voltage is not high enough to bring the 9V electrodes up to temperature.

In both cases, the cathodes would run too cold, probably blackening the ends of the tube and paying with service life hours. Besides that, nothing else
would happen.

You see, IEC 60081 limits maximum aux heating voltage at 4.8V. Caveat - modern T8/36W tubes (high impedance) have in fact notably lower electrode resistance than older tubes say from 1970s.

As for an observation that applying a preheat for a long time when there is no discharge current causes blackening while when the discharge is also applied there is no notable blackening, I can not fit a satisfactory physical explanation. Rather contrary, electron bombardment in anode phase and ion bombardment in cathode phase will increase electrode temperature further, causing more evaporation. One idea may be when there is no discharge current, evaporated cathode material is landing on the glass in the shortest path, causing notable black ring around the electrode, but when the discharge is present, material evaporated is distributed by electron/ion flow at larger surface, making the blackening less visible.
 

I also noticed the blackening of heated cathodes without a discharge present. I was using this effect to degrade the electrodes for certain
experiments* with my own rapid start circuit. The attached picture shows a 40W T12 Tungsram lamp. It's electrodes were (separately) supplied
with 8 volts from a current limited lab power supply for like 5 hours each side. You can see that there is a lot of deposited material on
the inside of the tube. I do not know if this is tungsten only or if it also is part of the emitter - the tube still ignites quite well.

My thoughts: if this is the amount of filament deposit from only 5 hours, the amount when running the tube on permanent preheat should be
much larger. After a few 100 hrs of daily usage, the tube should be noticably grayish. However, I cannot see any such dark discoloration
on my dimmable lighting system, where the fluorescent tubes are permanently preheated with 6 volts and are driven on variable power levels,
depending on outside brightness, mood, etc. Only the normal darkening occurs due to wear and tear from daily use.
Therefore I must assume, that the blackening only happens, when there is no discharge that can drain the electrons from the emitter.

But I also cannot come up with a conclusive explanation, these are all thoughts based on observation and might be wrong or meaningless.


The thing with lower electrode resistance in modern tubes is, I noticed that the time it takes for the filament to heat up evenly has increased.
Observing filaments on UVC tubes had me find out, that parts of the filament glow very bright from the beginning, the rest of the filament
needs some extra time to even out until finally the whole electrode is heated uniformly. (Safety notice: UVC lamps are dangerous to skin and
eyes, so I do not ignite these lamps, not even for a few seconds.) This is why I prefer Aura electronic starters in T8 fixtures as they have
a preheat time of more that 2 seconds.