Power Tube Life Span (theory + practical) question

[jordan86]

Was thinking about power tube life span in relation to biasing method. I've picked up some assumptions (hearsay or heresy?) along the way. Search function didn't necessarily provide answers so wanted to ask.

1) I've read that operating tubes at a lower plate voltage increases tube longevity. I have no reason to believe this untrue, but wanted to seek affirmation on that. Makes sense.
2) I've also read biasing tubes "hotter" shortens their life ("burn them up faster"). This would imply that a lower idle current would increase their longevity, yes?
3) So would a pair of EL84's at 300V in a cathode biased amp then last longer if they were biased at 80% instead of 90-100% of their max plate dissipation?

Does biasing method have any effect? Would a fixed bias vs cathode bias affect tube life span?  Is one harder on the tube? Say cathode biased at 100% vs fixed at 70%? Assuming our voltages were similar.

Just curious...

[SILVERGUN]

Think of a tube as an electrical appliance.
If you use the appliance harder it will wear out faster.
Voltage itself will not kill a tube (under our normal operating conditions), but current certainly will (if dissipation is ignored).

Was thinking about power tube life span in relation to biasing method. I've picked up some assumptions (hearsay or heresy?) along the way. Search function didn't necessarily provide answers so wanted to ask.

1) I've read that operating tubes at a lower plate voltage increases tube longevity. I have no reason to believe this untrue, but wanted to seek affirmation on that. Makes sense. You can kill a tube just as fast with low voltage + high current as you can with high voltage and inappropriate current, becuase power is an equation.
Amps = Current  and  Wattage (Power)= Volts X Amps

2) I've also read biasing tubes "hotter" shortens their life ("burn them up faster"). This would imply that a lower idle current would increase their longevity, yes? Yes


3) So would a pair of EL84's at 300V in a cathode biased amp then last longer if they were biased at 80% instead of 90-100% of their max plate dissipation? Yes

Does biasing method have any effect? Would a fixed bias vs cathode bias affect tube life span?  Is one harder on the tube? Say cathode biased at 100% vs fixed at 70%? Assuming our voltages were similar. Cathode biased at 100% will burn up faster

Just curious...

In the realm of TONE none of this matters because a tube is a consumable item. We have the freedom to run them how we want them and deal with the consequences. Ignoring data sheets can come with consequences, and there are lots of designs where it appears someone was ignoring the data sheet.
Matchless didn't care how may EL34s you had to buy.
They sure sound good while they last.
Don't get emotionally attached to electrical appliances.

[pdf64]

Simply put, lower dissipation (especially lower sustained peak dissipation) = longer operational life.
At typical operating points \ conduction angles, 100% of design centre limit in cathode bias is probably easier than 70% in fixed bias.
Because with the former, when overdriven, peak dissipation self limits.

[shooter]

In the commercial world tubes were changed by hours used/ON.  so the PA tubes were changed at 17khrs.  some engineer got paid good to calculate mean time to fail, MTF, subtracted a fudge factor and blessed it so.


so if you gig a hot biased amp once a week for 3hrs and you buddy gigs once a day for 5 days a week.......
as SG stated, they're a disposable part used to make a guitarist happy.
I knew one touring guitarist that had his amp-tech ONLY put in nearly expired PA tubes in his twinRV because; "There is nothing like the sound of a twin, dimed, with dyeing tubes!" 

[PRR]

Are tube costs really destroying your budget?

The nominal limits in the manual (not always observed in g-amp practice) are an excellent balance between running cost and "output". Yes, you could run a pair of 6L6 at 200V and get 2 Watts "forever", but if you actually want 30 Watts then this is expensive (15 pair). For most of history, e-guitar players favored the MOST power today, with little concern for long life. (This whole geetar gig is a fad and might blow-over soon.)

[jordan86]

Thanks all. I have no budget concerns. And I embrace the tonal and financial repercussions of running tubes hotter. Just been experimenting with a Princeton lately at different voltages and bias points (50,60,70% plate dis) and enjoying a cooler bias, so these things are on my mind.

Got me curious though on fixed bias vs cathode. The self regulating (or maybe preserving) nature of cathode bias made me think they may last longer, but wasn’t sure. I also see adjustable fixed bias easier to manipulate though and set at more moderate levels.

Just a curious mind. Thanks for indulging me with your insights.