Tubes at idle

Excellent post by Larry, as always.

Regarding the original question, I'm pretty certain that Wolf is correct, and that there will be insignificant, if any, difference in preamp tube longevity as a function of whether or not the preamp is processing a music signal while it is powered up.

The current that heats the filament will not be affected by the audio. And since nearly all tube preamps operate their tubes with class A bias, the power dissipated (consumed) by the tube, and the current passing through it (averaged over each cycle of whatever audio frequencies may be present), will be essentially the same whether a signal is present or not.

Regards,
-- Al

Koestner,

I want to say upfront that I have no formal training in electronics and my comments are based solely from my personal experience with one tubed preamp, a VTL 2.5 that I bought new in the Spring of 2006. I had an Aragon 4004 amp at the time, paired with an Adcom GFP-565 solid state preamp, and I was looking for a more dimensional and 'in the room' type of presentation. I had borrowed a local dealer's demo unit for a few months, with the stock Russian or Chinese tubes, verifying that this preamp gave me the presentation I wanted before ordering a new unit along with a pair each of 12au7 and 12at7 NOS Mullard tubes from Upscale Audio.

I received the new unit in mid May 2006 and immediately replaced the stock tubes with the Mullards. I was completely new to tubed preamps at the time and had read many forums debating whether to leave tube preamps on 24/7 or to turn them off after each use. At the time I wanted my $160 worth of tubes to last as long as possible, so I mistakenly reasoned that less total 'on' time would allow the tubes to last longer. Therefore, I initially turned the preamp off after each listening.

However, I quickly and clearly noticed that the tubes sounded significantly better after they had been on for awhile; the longer they played (both during each listening session and in total hours played) the better they performed. I also was weary of waiting for the tubes to warm up and sound their best from a cold startup. About this time, I read several forum posts from tube preamp users claiming that switching vacuum tubes on/off was more detrimental to tube life than leaving them on 24/7 and avoiding the repeated surges of electricity caused by powering on. This theory struck me as being counterintuitive but, at the same time, very intriguing and worthy of testing.

So, about the end of May, I began leaving my preamp on 24/7. This was very convenient and my music sounded great from the first track through the last. The only possible downside was the anticipated shortening of tube life. But my system was sounding so good I just chalked this up to the cost of having a very good sounding system.

To make a long story a bit shorter, I'll fast forward 6 1/2 years to January, 2013: my system still sounded good except I noticed a bit of thinness, lack of dimension and that 'in the room' quality to the sound that I enjoyed so much earlier. There was also an odd sameness to the acoustic perspective no matter what track was played; the sonic illusion being centered on the stage and about 20-30 feet back. So, I replaced the 4 NOS Mullards with a fresh set ($200 total cost, just $40 more than my originals that were $160 total almost 7 years earlier) and everything lacking was restored and the sonic illusion presented was unique again on each recording.

I related my experience because I thought it's relevant to your stated concern and demonstrates why I would suggest that 12au7 tubes lose less life at idle than when playing music and would recommend leaving the tubes on 24/7. I think 6 1/2 year tube life would be considered more than acceptable by most owners.

However, I think it's important to realize this is just one person's experience with one preamp and one brand of tubes. It is likely a bit risky to generalize from such a limited sample size. Results may vary not only by preamp and tube brand, but by more specific variables such as tube type and between manufacturing dates and sites and even between same date batches of the same brand and tube type produced at the same sites/plants.

No matter which method you choose, tubes do have a finite life and will eventually require replacement. If you don't have an expensive tube tester or access to one, small preamp tubes degrade so gradually that you'll need to either remain acutely aware of subtle sound degradation and changes as the tubes age or just wait until you notice them eventually to determine when the tubes need to be replaced.

I now realize that a thinning of the sound, lessening of 3d imaging and a similar sonic perspective on various tracks can all be considered signs of tube aging and result in a degradation in performance. I would suggest it may be simpler to just change tubes after a given amount of time or hours of usage rather than trying to discern the point in time that you notice these somewhat subtle sonic changes. This method also allows you the freedom to relax and just enjoy your music.

Hope this helped,
Tim

Noble and others, thank you for the responses. I am leaning towards leaving them on. Since this is a hobby I feel that a yearly, or bi-yearly tube purchase is not unreasonable. After all I would probably be itching to roll something else anyway. My only other concern is the fire danger, but since this issue is about 50/50 there must not be a lot of instances of houses burning down. The tubes I use, 5814A aren't as costly as Mullard CV4003 that I was previously using, so leaving them on is what I have decided to do. Thanks again.

I don't think actual fire is a common event, but, then again how much is even a small risk? If replacement tubes are cheap, the only other major cost to consider is the amount of electricity the component burns at idle (some manufacturers give buys that figure). You have to decide for yourself whether the environmental issues matter to you. If the amp remains reasonably cool, then heat degradation of other components will not be that big an issue.

The only real advantage to keeping the preamp on all the time is the issue of warmup--that can be a big deal if you don't have time to wait or really dislike the sound of your system while it is warming up. I would at least listen to my system to decide whether warmup is that big a deal or not; I would not rely on consensus opinion on the internet.

For my own gear, I must turn it off to conserve the tubes because none of them are that cheap. My linestage runs four 310s and two 311s, plus a rectifier. My amp runs four 348s and four 349s (even more expensive than the linestage tubes). My phonostage runs two 300Bs as rectifiers, two tubes that I cannot recall the type (reasonably cheap) and two ECC803S tubes (quite expensive).

Koestner,

The risk of a fire from a blown or faulty tube is much lower with a tubed preamp than with a tubed power amp. However, that doesn't mean there is no risk, so it would be a good idea to turn the preamp off/unplug it from the wall ac if your going to be out of the house for several days and want to be extra cautious.

Tubes have been used in preamps since the beginning of home audio (supposedly, archaeologists have unearthed early examples in prehistoric caves in ancient Europe)and are still being used today, decades later. It seems a bit odd and disappointing that the issue, of whether it is best to turn tubed preamps off after each use or leave them heated and 'idling', has never been identified, tested and definitively resolved in all those years. Perhaps electricity and tube costs were so high that they never even considered leaving things powered.

Just some thoughts,
Tim