Matching power amp and speaker impedance

I'll keep it as simple as possible.

Q#1 Continue to use the 4ohm taps. If you get bored compare the sound to the 8ohm taps you might like it better. It won't hurt the amp.

Q#2 Tube life will not be affected by your choice of impedence output tap. Whether 8 months is too long or too short is totally dependent on hourly use. In your case that would be 1200 hours and thats is a tad on the short side, but some tubes die early. I assume you probably just had one premature failure. Also, you said you used the amp 5 hours a day - do I assume that you mean it was turned off the other 19 hours? If not, calculate the hours of 'on' time, not 'playing ' time. Idle time burns up the tubes just about as fast as playing time.

Typically tube life depends on the amp design (does it drive the tubes to their maximum output or is the amp operate its tubes conservatively, well within the tube power envelope which extends tube life). An EL34 typically can put out 25wts per tube max so an amp with 2 tubes per channel that puts out 35wts is conservative, or 4 tubes per channel that puts out 70 watts would be conservative. An amp which used 4 tubes per channel and puts out 100 watts per channel would be using them close to max.

1500 to 3000 hours 'on' time would be typical of power tubes in most amps.

Q#3 Yes, I believe so. The power output in triode is usually halved so the tube is less stressed and runs cooler (as will the amp itself). Cooler running usually equals longer life. Don't know by how much though. I expect quite a bit.

Hope that helps a bit.

Thank you Newbee. That's very helpful.

What's the sound of the 4 ohms output tap compared to 8 ohms and 16 ohms?

The amp has 4 EL-34s per channel. According to you, the amp power output is on the conservative side. In terms of efficiency, does it mean the 16sohms is the "easiest" to drive and sound more powerful than the others?

5 hours is the playing time. Total on-time may be 7 hours a day. I have tried different brands for the passed 10 years. I settled with "C-Wing". They are more stable than others in my experience.

Typically you would expect that the 4ohm tap would match 4 ohm speakers, principally because the 4 ohm taps can deliver the power required to drive the speaker in that part of its bass response, that is at or below 4 ohms. You would/could run out of power if you were running off the 8ohm tap.

However, if you have a high power amp but only listen at low levels it might not make a meaningful difference to you. In that case, you might find that the 8ohm taps will give you a smoother mid/high frequency response, assuming that is a problem for you. As I suggested, listen to both taps and pick the one that sounds best to your ears, period. The selection issue really has no other pratical effect. BTW, the 16ohm tap is only there to accomodate speakers with a nominal 16 ohm impedence speakers, which are very hard to find. I suspect if you used it at any reasonable volume your bass response would take a vacation.

SED EL34's are my tubes of choice. I use(d) them in several amps. The are very rugged and have a good life span. I've never lost on via major shorts or premature death.