heat cycle rate versus duration in vacuum tube longevity?

[soapfoot]

Has there ever been any controlled experimentation on heat cycling vacuum tubes versus leaving them on long periods?

Specifically:

If a vacuum tube were in use for five hours per day and idle for five hours (alternating one hour on, one hour off) is it better for the vacuum tube to:

• Leave it on for the full ten hours (only one heat cycle, but ten hours in operation)
• Switch it on and off only when in use (five heat cycles, five hours in operation)

I've heard plenty of speculation and debate, but would be very interested if any experimentation has been done and data gathered

 

[rock soderstrom]

I have no reliable scientific data on this, just an opinion!

I have the impression that tubes, like incandescent lamps, are more likely to be stressed when switched on.
Continuous operation does not seem to affect them much, I have seen radios and studio equipment that have been in continuous operation for decades...

If in doubt, I would rather keep the tubes in operation than switch them on and off too often.

 

[soapfoot]

That’s my sense as well—I’d just love to see it backed up with something!

An obvious difference with respect to incandescent light bulbs is that a tube is more than just a resistive element… how is the emitting potential of the cathode impacted by long stints of operation versus repeated thermal cycling? Etc.

 

[rock soderstrom]

That’s my sense as well—I’d just love to see it backed up with something!

An obvious difference with respect to incandescent light bulbs is that a tube is more than just a resistive element… how is the emitting potential of the cathode impacted by long stints of operation versus repeated thermal cycling? Etc.

Yes, these are indeed interesting questions and it would be nice to have robust data on this.

I think the type of tube and the circuit also play a role. A small signal tube in a microphone with an anode current of less than 1mA will behave differently than a big power tube in a power amp. Class A vs AB or B also play into this.

Similarly interesting is the question of whether standby operation damages the tubes? Keyword cathode "poisoning"

 

[Tubetec]

Ramp up time on the ht line , limiting inrush current to the heaters at start up ,as well as reducing thermal cycling helps tubes live longer ,
I dont have any scientific studies of the phenomenon to offer , but I have years of experience with tube gear , especially guitar amps ,
Ive left amps for a few days on standby , with no problems , even though its not reccomended .

There is another school of thought that says reducing the heater volts on standby is a good thing , thats something I havent tried myself ,so I dont know ,Morgan Jones who is a proponent of that system knows his stuff though .