I was at a friends house and I noticed the two fluorescent light fixtures in the kitchen had this slight flickering. One with 4 tubes looks like it had rings running down one side and the other side were just slightly flickering. The other fixture with 2 tubes just had this slight fixture. So I pull the cover off to get a closer look and saw that all the tubes had some small black specks on the inside of the tube. The tube ends however were not dark at all. The tubes were F34T12 but on the ballast it says for F40T12. I'm not sure if the 34 watt lamps are compatible. The fixture and the tubes look like they are at least 10-15 years old so I imagine these tubes have been in there for a while. Would this be possibly be a bad ballast or just needs new tubes? Thanks.

Although the ballast may not be designed for 34 watt tubes if they are not listed on it as such, the problem of specks and swirling around or moving rings in a 34 watt tube are quite common. This is especially true if the lamps are not running at optimum temperature. Upon turning on a fixture with these tubes, they can flicker and swirl for quite some time before stabilizing, even after break-in period is complete.

If the problem persists beyond about an hour of running at room temperature (with no cold air ducts throwing cold air at the tube) it can mean a ballast with partially shorted windings.

Would it then very noticably overheat ?

Any ballast running lower wattage lamps, even if it is listed as such, will run it warmer than larger lamps. The danger in running 34 watt lamps in a ballast not rated for them is constantly running it at slightly above design temperature. Other than swirling and rings in the tubes at startup, 34 watt lamps will not usually harm rapid-start ballasts in the short term. In the long term, if it is not designed for those lamps, it will fail earlier than it would running full wattage (40 watt) lamps. Since ballast life in part depends on operating temperature, it will also shorten somewhat the life of a ballast rated for 34 watt lamps as well.

One quick way of telling if the ballast is bad is to try full-wattage lamps in the fitting. Replace both lamps with 40 watt tubes. I believe the colours available are daylight deluxe and cool white deluxe. Standard cool white is virtually impossible to find in 40 watt tubes nowadays, unless you have a NOS source. You should run the lamps for at least one hundred hours to give the new tubes time to break in, and then if the flickering continues, you know you have a bad ballast. Hope this helps.

Thanks for the input. Actually these fixtures are located in the kitchen so its fairly warm location. I didn't leave them on long enough but I'll see if the flickering and swirling goes away if I leave them on longer.

I didn't have any extra 40 watt tubes to try out since my other fixtures are F32T8. I think I'll just replace the tubes since they are quite old and somewhat dim. Actually sometimes the hardware stores have the F40T12 on sale for like $1 a tube but haven't seen that in a while. For the kitchen I would prefer warm white, regular cool white doesn't have good color rendering so food doesn't look so great.

Sorry, I mixed up topics, answer on wrong place and didn't find, how to delete it back...

The F34T12 can be effective on energy saving, if a triphosphors F34T12 replaces a deluxe or halophosphors F40T12.
As the triphosphors have higher brightness than halophosphors and deluxe phosphors, a triphosphor F34T12 can replace a halo/deluxe F40T12 with much less losses of light output, but with much more reduced wattage.
In such a way the european T8 energy saving works: Replace a halophosphor 36W T8 lamp with an energy saving 32W T8 lamp, and you will get the same amount of light as the halophosphor 36W T8 lamp, but at reduced wattage, as a triphosphors 36W T8 is brighter and may cause overlighting.

The f34 tubes can't be used on preheat or tulamp since they will draw more current and overheat it as well.