Can you do anything to make power tubes last longer ?

@inna For longest tube life:

1) keep the tubes clean and free of fingerprints.

2) keep them biased properly, if you can run them a little below spec. This works only if the sound isn't adversely affected.

3) use the Standby switch to warm up the tubes. If you don't have one on your amp consider getting one installed. A standby switch cuts off the B+ (DC high Voltage) for the tube. As its warming up, if the plate supply Voltage (B+) is present, you get a phenomena called 'cathode stripping' where the cathode coating gets eroded over time. So a standby switch can prevent this.

4) don't push the amp hard. In a class AB amp (most EL34 amps are class AB) the harder you push the amp the hotter the tube gets and heat causes it to wear faster. So make sure your speakers are easy enough to drive that the amp is loafing nearly all the time.

5) make sure you have adequate ventilation. Again, heat is the enemy of the tube.

6) Make sure your speaker is connected to the right output transformer taps. So a 4 Ohm speaker should never be on the 8 Ohm tap; that would cause the load on the power tubes to be much lower, causing some of the power they make to be dissipated in the tube rather than the speaker- so yes, they will run hotter, put out less power and make more distortion.

7) make sure your line Voltage isn't running high. 120V is normal now and the line is not supposed to exceed 125V for more than a second. But I've seen situations were 125V was normal and unsurprisingly the customer was going through power tubes a lot faster.

8) the controversial and last thing: Some amps push power tubes harder than others so go through tubes faster. You might want to take a thermal camera to see how hot the tubes are running and compare that to other EL34-based amps, or use the internet to see if there's a correlation with certain amps that tend to go through tubes faster.

Follow these suggestions and your tubes will last longer.

inna, you can bias amp’s output stage for 20% less current, and use 120<=>110V step down transformer. These two steps will increase tubes life time significantly, est. 30%+. 

Ralph, thank you.

It appears that I do everything right. There is no standby switch. Voltage is regulated by the regenerator, I hope it is. Ventilation is excellent from all sides. The amp seemingly drives the speakers with ease, no stress that I can perceive. The tubes are clean, no fingerprints. I might try to lower the bias a little and see what I hear.

atmasphere

... make sure your line Voltage isn’t running high. 120V is normal now and the line is not supposed to exceed 125V for more than a second. But I’ve seen situations were 125V was normal

If you’re talking about the US, you are mistaken @atmasphere. Nominal voltage in the US is 120VAC $$\pm 5\; percent,\; so\; anything\; between\; 114VAC\; and\; 126VAC\; is\; normal.\; That\; is\; established\; by\; ANSI\; C84.1\; standard.$$

$$Note\; that\; the\; spec\; is\; measured\; at\; the\; service\; entrance\; -\; it’s\; common\; to\; have\; some\; voltage\; drop\; at\; the\; AC\; receptacle.\; That’s\; something\; dedicated\; lines\; can\; help\; limit.$$

What is "normal" is not necessarily ideal for tube electronics, particularly vintage gear that was built for lower voltages that were prevalent when they were made.  As I mentioned above, my local dealer who only sells tube amplification often has customers measure their outlet voltage and then orders the right transformer for them to use on the outlets supplying their amp and other tube gear.  He prefers to run the gear at much lower voltages than is common.  This is usually the cure for premature tube failure.  He also has his customers bias amps on the conservative side.  This is consistent with Atmasphere's recommendation.  

My normal voltage has been, for the longest time very steady at 117 volts at the outlet, but has recently crept up to 119-120.  My amp is cathode biased so I cannot do adjustment, but typically, cathode biasing means conservative levels.  If my voltage rises more than this, I too will consider a transformer.