Recently I borrowed a bunch of colored Chinese T4 tubes in 12 and 16 watts and wonder what the working parameters are. Seems to be quite obscure, like this is some Asian pseudo-standard, and lamps are usually sold combined with their own custom luminaires having custom built-in ballasts.

Surprisingly, there is Sylvania document that lists voltage-current for their 7-10-18W series of T4 tubes. Current seems to be in 195-228mA range, so just slightly above of ~170 mA one expect for no-HO T5 tubes. Not sure how it corresponds to Asian lamps though.

My tubes seems to light up OK when hooked to a regular 6-13W Osram T5 ballast.

I am getting about 14W of system power at ballast input with 16W tube and about 11W with 12W tube, a bit underdriving considering ballast losses, so Sylvania's rating of around ~200-230mA may be true for Asian lamps, too.

I have two 6W T4 tubes, I have been trying to find their specs as well. I would assume it would be less than T5, but maybe different electrode design IDK. I haven't seen datasheets for any of them.

I used 8W T4 on a 155mA ballast and it run at about 10W (on the tube itself), so my guess is the correct current would be in the 120-ish mA ballpark.

So we can suspect different kinds of Asian T4 lamps are made to different specs. My ballast consumes ~14-14.3W when connected to a genuine Italian 13W T5 tube, so power measurements seems OK.

Also, my T4 tubes have notably lower electrode resistances than 170mA T5's. I see about 16-17 Ohms cold on Italian T5, ~12-13 Ohms on older Soviet T5's, ~8 Ohms on 16W T4 tube and ~3.5 Ohms on 12W T4.


 

I would not take the filament resistance as a guide, because these tubes were practically used only with electronic ballasts, where it is beneficial to use lower ohmic filaments - for startup these ballasts drive currents way above the normal operation and that I mean more than 2x..3x higher (vs ~1.5x for the typical preheat ballast), so there way more room to feed even higher current filament and ensure it gets heated correctly for the start. For normal operation lower ohmic filament ensures lower voltage drop across its length, so less variation on the exact cathode root position and mainly lower losses in the filaments (the heat is useful only around the cathode root where it contributes to easier electron emission, but when radiated outside of the cathode root are practically unnecessary losses; so thicker filament has lower ohmic losses and relies more to the ion heating of directly the cathode root itself).
So I won't be surprised to see 0.12A lamp with filaments requiring 0.5A for preheating (while standard T5 are designed for ~0.16A discharge and ~0.24A preheat currents).

And may be not very well known, but T4 tubes, despite having smaller sockets of course, have the same pin spacing as T5 tubes, so fit just well in T5 lampholders!

Lampholders yes, but not fixtures, they are of different length.


But interesting, when I got a Burgess Safari lantern normally designed to run F8T5, the tube in it was somewhat thinner, my guess would be like 13 or 14mm (instead of the standard 16mm), made by GE and still labeled as F8T5. Dunno what that was about...

Interesting, so about 120mA-ish. I really wish these tubes were standardized, that would make life so much easier.

The tubes I have are certainly not 120mA. But mine are higher than 10W of power. 368mm end-to-end for 12W and and 470mm for 16W one.

But interesting, when I got a Burgess Safari lantern normally designed to run F8T5, the tube in it was somewhat thinner, my guess would be like 13 or 14mm (instead of the standard 16mm), made by GE and still labeled as F8T5. Dunno what that was about...

Seems to be a recent 'economy' trend. I have a couple of very recent (2024 vintage) Chinese Osram 11W PLs, that have visibly thinner tubes than you usually expect on 11W lamp. May be an effect of thinner glass walls, as the lamps feel very lightweight.



 

@thinner F8T5: I don't think so. Batteries for the Safari lamp are not made for many decades anymore, while the lantern was quite corroded from leaky batteries, so I doubt it was used that much from the mains, So I would doubt the tube is any newer than somewhere from the 70's or so...

So 'not so recent' economy trend, someone decided the lamp certainly won't be used in a flashlight at its full design power and for its full life, so why not to cheapen a bit with thinner tube and less phosphor (and probably 3'rd world manufacturing also, like Hong Kong or Taiwan at older times)

So I won't be surprised to see 0.12A lamp with filaments requiring 0.5A for preheating (while standard T5 are designed for ~0.16A discharge and ~0.24A preheat currents).

It will not be possible to achieve in a simple 2-transistor series resonant circuit typically used for such tubes. Such inverter displays inherent current-limiting behavior by itself. Commutating ferrite ring will saturate more quickly when load current increases, increasing inverter frequency and so backing off the current. So preheat current may be in range or just slightly above nominal tube current.

One can do it with a more complicated circuit typically used in non-integrated HF ballasts, involving auxiliary windings on the ballast choke.

Commutating ferrite ring will saturate more quickly when load current increases...

The ring saturates, but because the frequency and mainly the current slope gets higher and amplitude increase the base drive (the voltage across the ring gets way higher, increasing the base drive way beyond what the transistor needs for the currents, so causes the Ts to go way higher), the quite long storage time in the transistors makes the current to go way larger. And this effect causes the current to go way beyond the normal operating current.
It is not uncommon to see currents 3x higher than normal, but more like 5x...
In reality the opposite is more of a problem: To keep the current from soaring really high, saturating the main inductor and frying the transistors, mainly when the lamp ignition fails (normally the high current rise is stopped by the discharge clamping the voltage across the resonant capacitor).

Also without this much of current boost, it won't be able to generate sufficient voltage for discharge ignition.
Taking an example with a 11W, 0.15A lamp, series inductor about 3mH, resonant capacitor 2.2nF, neglect the series coupling capacitor for the simplicity (it makes the things even worse, as it efectively reduces the inductance), fed from rectified 230V so about 300VDC so about 150VAC at the halfbridge.
Normal operating frequency would be about 36..40kHz, yielding the 0.15Arms (L/R time constant fed by a square wave).
A starting frequency around 140kHz would yield about 0.45A and barely 220Vrms across the capacitor (Vstart = Ires * (1/(2*Pi*Freq); 150V/Ires = 2*Pi*Freq*L - 1/(2*Pi*Freq*C); assume operating on the inductive resonance side). And that is already quite low, even when speaking about 140kHz.
The 140kHz means 3.5us long pulses, only the second half of it is carried by the transistor itself (the first comes from the parallel diode, because it flows in the opposite direction) so about 1.75us while the Ts of the transistors use to be in the 1.5us ballpark. So the ring core saturation needs to happen about 250ns after the current zero cross, which corresponds to about 0.15A saturation current it would have for normal operation.