Tube plate voltage "High" vs. "Low"

[mikko]

I´m relatively new to tube world and hence I have a lot of (obvious) questions.
In the recording studio where I use to hang out and play with my friends, is one of those modern tube mic pre-amps. It has a plate voltage selector with two possible selections “Low” and “High”. It is usually set on “Low” setting. I know if changes the sound but how about the tube life time? Does it shorten the life time, or vice versa, if you use tube with lower plate voltage than it is designed for?

[Doc B.]

The setting might affect how easily the preamp goes into compressing the signal on overload, but there's not really enough info to offer a thoughtful opinion. It would probably useful to see what the manufacturer says about the setting change.

[mikko]

Pre-amp is ART Pro MPA II. I was told that the plate voltage is 60V or 150V depending on settings. Owner´s Manual doesn´t say anything about it though (http://artproaudio.com/downloads/owners_manuals/om_prompaii.pdf?v=1429131475)
Tubes are 12AX7A.

[Doc B.]

I would guess that the switch won't make that much difference in terms of lifetime. 150V is not that high for a 12AX7. But without more info about exactly where the tube is being operated in terms of plate current and heater voltage it's hard to say how long the tube might last, or which setting might wear the tube out faster.

[Paul Birkeland]

From the text in the manual, it is a reasonable guess that the switch does actually just change the available voltage for that particular stage.  If the limits of the tube's plate dissipation are reasonably observed, the difference in lifetime between these settings may be negligible.

-PB