Would you cook a ballast rated for f34 tubes of you are using both an f34 and f40 tube on the same two lamp rapid start ballast?

It depends on the total losses within the ballast and the consequent operating temperature.
In the last post I neglected one important "detail" (because living in a 230V area, I'm not that much used to think about HPF lead style transformer ballasts): Not all losses are related to the lamp current only, some are related to the line current (so with HPF ballast that becomes related to the real power transferred to the lamps).

So it really depends on the ballast concept. NPF ballast is quite easy to analyze: There all currents are related to the lamp arc current, so higher voltage lamp means current reduction, so reduction of the power dissipation, so lowering the temperature.

With HPF the secondary winding will see lower current with higher arc voltage lamp, but as the real power is higher, the primary will see higher current and that means higher dissipation there.

So except NPF or a series choke, it does not have to be really straight forward, so the only reliable method is to have two sets of lamps, one set with the rated lamps and the second with the lamps you want to use. Then run each of them for about a hour or so (until it thermally settles) and measure the ballast temperature (with heavy magnetic ballasts, the case temperature is sufficient for the compare, but you may try diferent spots on the ballast case). If the intended lamps do not cause higher temperature, they will be most likely fine even long term on that ballast.

I experimenting with the heat-up of ballasts (on 240V). What i found :

I used athermal sensor with LCD display, the type that is used in some "gaming" computer cases to display the temperature of a component of choice (the LCD is in the front panel, the thermal senssor is on a wire so you can stick it in the GPU heatsink etc)

I found out that the sensor sees the highest temperature (so the one that depends most on the ballast and not directly on ambient air) when the ballast is laying on a layer of cardboard, and i stick the sensor between the cardboard and the ballast, under the center of it

Earthing the ballast body is a must. With Switch Start, when the ballast make a kick, some of it is capacitively coupled from the coil to the core metal, and some of that to the thermal sensor (that ias in very thin isolation and probably enough is coupled). I normally dont earth a ballast when its just for experiment, i put it on an isolating surface and thats it. When i tried with the thermal sensor, the LCD went blank the moment the ballast made a kick, i had to cycle the power to the LCD module to make it work again. I can imagine that it can get damaged from a kick like that

Running one 40w and one 34w isn't as bad as two 34w on a 40w-only ballast from what I understand but I still wouldn't advise it.  (On series rapid start that is; I think on a tulamp preheat ballast it's bad either way but I'm not as familiar with those). If the ballast is rated for F30T12 30w 3 foot lamps too you should be OK though, at least from what I understand.

Another common energy-saver lamp here in North America is the 60w F96T12 8 foot single pin instant start slimline lamp.  It's supposed to replace 75w slimlines, but here again can sometimes cook old ballasts, though to a lesser extent than their 4' rapid start counterparts since often the slimline ballasts are also rated for F72T12 6 foot 55w lamps, and electrically 60w energy saver 8 foot lamps fall in between.  

Another common issue with the krypton-filled energy saver fluorescents in general is that they tend to be very sensitive to cold temperatures, especially the 60w 8 foot T12s.  They'll be dimmer, and exhibit "rings" of "moving light" moving up and down the tube, sometimes from each end and moving in the middle.  This disappears when they warm up properly but often below about 50F that never happens, especially with the 8 foot 60w lamps, the 34w 4 foot DO eventually seem to warm up in near-freezing temps.

Thanks. I could've sworn I saw 59w f96 tubes at Menards though.

The thing is, just don't run f34 tubes on preheat. Tulamp ballasts are a lead lag circuit with an autotransformer and then the ballast coils. That's about all I know about tulamp ballasts, except that these things are bricks. Maybe Medved or someone else more knowledgable could explain it better.

The "tulamp" is an autotransformer with two separate secondaries, one connected in series with a capacitor.
It is one of the ballast types nearly impossible to predict - as the reactive power gets canceled between the secondaries (those are related to the arc current), so the primary handles just the real power (so whatever increases the real power, it will warm up more). So that you would have to measure via the temperature...

Medved delivers. Thanks again.

I believe that tulamp ballasts will tolerate an F34 lamp on the lag side only. The lead side almost always results in the series cap failing. If you're not sure, avoid them altogether.

If you see f34 tubes in a fixture: when in doubt, take them out!

You're right sol, plus the old ballasts are already delicate. I don't think I've seen starters for f34 tubes.

FS2s WILL work for 34w, as will FS4s. 

Single-lamp LPF 30/40w preheat ballasts should be absolutely fine with a 34 watt lamp.  I didn't know the lag side thing about the HPF Tulamp units but when in doubt don't use energy-saver lamps.  What side/wire color is the "lag" side, anyway? (In case I ever find myself having to put a 34w lamp in a Tulamp preheater).

The 59w F96s...were they T8s? Those are somewhat-common lamps.

The lag side of the tulamp ballast is the one with a standard starter loop. The lead section has a starting compensator in the ballast casing so it goes from one end of the lead lamp to the compensator in the ballast housing to the starter to the other end of the lamp. This can be readily seen in the ballast wiring diagram. Unfortunately I don't remember the colours of the leads for the lead and the lag lamp. The starter loops are yellow, though.

I found a diagram: http://www.lighting-gallery.net/gallery/displayimage.php?album=197&pos=100&pid=6284

Here we only have limited choices of retrofit tubes in Hong Kong

Tubes currently available here (T8, T10 and T12): (retrofit tubes are marked with "#")
F10T8 (330mm, 0.23A)
F15T8 (~440mm, 0.31A)
F18T8 #
F20T10
F20T12
F30T8 # (900mm,0.36A)
F30T10
F36T8 #
F40T10
F40T12
F58T8 # (F65T12 is long gone)
All type of T12 Slimline tubes (interior lighting for public buses with Duple Metsec DM5000 bodywork only, no one uses Slimline tubes for general lighting in their home )

Tubes NOT available in Hong Kong:
All standard G13 tubes longer than 5ft
All HO tubes, excluding T5 HO
All VHO tubes