A few years ago I decided to try a tube preamp to pair with a SS power amp. That choice became a McIntosh C220 which now fronts a Mac MC-402 ss conncected through a Mac MEN 220 unit (room correction).
My only ongoing disappointment about the C-220 is that the tubes in the phono-stage (2-12AX7) seem to be extremely vulnerable to wear/burnout, even (for want of a better word) blowout. I've been told more than once that I can expect "up to" 10K hours of life out of these tubes (Mullards) but I've never been close to that. I put new tubes in and they're dead quiet for awhile, even with the gain all the way up, then after awhile--a couple or few months--I start getting noise in them. To be sure there have been some extenuating circumstances more than once. The first time there was a power outage and when the power came back on the tubes blew--my recollection is that it was at least the linestage, if not both line and phono. So I invested a couple hundred bucks in a Brickwall unit.
That seems to have saved me from "total" blowouts but I'm still getting what must be abnormal tube wear. I detected background noise twice in the last year+ and replaced tubes (the first time all of them, the second just the phono stage). Everything was fine, or seemed fine, until the other day when we had a micro-second flicker in the house power which was not enough to reset the various digital clocks in the house. At that time, and I had music playing, the MEN 220 shut down but the amp and preamp did not. (I should add that the amp and preamp are ganged together with a little cord of a sort that allows both units to power on when one "on" button is pushed. The MEN220 is not so ganged at this point.)
When I powered the MEN220 back up I flipped over to phono (no turntable running) and ran the gain up and there was noise. It wasn't so loud as to noticeably intrude on music at a reasonable listening level (and so I cannot be sure it wasn't there to begin with before the flicker) but it was, and is, there. So the question is, aside from the power anomolies described, will tubes wear out in such a way as they are still amplifying but are making progressively more noise? It's rather irritating and it's costing money. Should I contact McIntosh? Should I use a power conditioner instead of a Brickwall unit? Help!
That's the way it is. Tubes are best when they are new and they are slowly wearing as they accumulate hours. Some brands of tubes can be hardier than others in situations where they are driven hard, but which brands will hold up better in your preamp may be a matter of trial and error. However, you may find a tube that lasts longer, but there is no guarantee you will like the sound as well as the tubes you are using now. Mullard CV4004 (12AX7) are about as good as it gets, but supplies are running out, so don't wait.
Did you speak to the manufacturer about this problem? It clearly spells the design flow of using maximum bias to the tube for the maximum gain. That's why I tend to like more solid state phononstages. If you like sound of your Mac preamp, you can get an outboard SS phonostage(Pass X-Ono or similar), you'll still be able to benefit from valve preamplification.
Personally, I would have called Mcintosh with the second incident. Mullard is a quality tube and my experience is that they do last beyond 10,000 hours. You might also get some good feedback by redirecting this post to Audiogoners that own a MAC C220. Good Luck!!
A line disruption should not affect the tubes at all.
OTOH, it is normal for signal tubes to start out quiet and slowly built up noise as they age. In the old days there were some tubes that were rated for 10,000 hours but no-one does that today. So you may just have to replace tubes every 2000 hours or so.
If you run a high output cartridge the tubes will seem to last much longer! 10,000 hours might be quite reasonable then.
If you leave the preamp on 24/7 you are wearing out the tubes needlessly. Its best to start up the preamp about an hour or so before listening.
Did this also happen prior to your acquisiton of the MEN220? I'd take the MEN220 out of the loop and see what happens over a period of time. How old is the MEN220? If fairly new (within last 6-9 months or thereabouts), the issue has been resolved.
The early MEN220's caused MC501's to run extremely hot on one side of the heat sinks because of a design issue. There were reports of this design issue also affecting other McIntosh amps like the MK1.2's (IIRC) & not sure if the MC402 was also included since it's a stereo amp vs. a mono.
Tubes should last along with the guidelines you expect & certainly longer than you've experienced.
You folks have been great and I do appreciate the time you've all taken. In response to one query, the MEN220 is new, and I had had three of these tube replacement occasions before I installed the unit between the C220 and the MC402.
One other thing came up this afternoon. As I'd mentioned, I had had a "trigger" cable connecting the C220 to the MC402 but not one connecting the MEN220 to the two other components. When I had previously powered up the C220, the MC402 powered up without incident. Now I have added another cable to get the MEN220 into that trigger chain (and have ascertained that the connection sequence is proper) but NOW there is an audible pop in the speakers within three seconds of powering up. There might be no connection between the two issues but I bring it up in case it provides any sort of usable evidence/information.
It should first be established if there is in fact a problem with the preamp. We do not know what quality tubes are being used and if the OP is buying the tubes from a reputable dealer. I had a similar problem with a Quicksilver Full Function preamp. The input tube is run very hard and the preamp was eating Golden Dragon tubes for lunch. Looking for a more reliable tube I tried an Amperex tube and never had another problem.
Did you buy the preamp new or used? If used, I would be getting this into an authorized repair center. I bought an Art Audio PX-25 amp used and when it arrived it didn't sound to what my expectation was. I sent it back to Art Audio and they found someone had jacked with it and changed out a number of resistors and caps. They restored it to factory spec and all was good.
I agree with Atmasphere. It may just be time after 2000 or so hours to replace the tubes. However, you can also take the unit to an authorized McIntosh labs service technician for evaluation. But first, do a little math. How many hours do you feel you have on the tubes? if close to or more than 2000 hours, they need to go.
I had a couple of brand new Sovtek 12ax7lps tubes fail in my preamp after less than 9 months of use. So I googled "what would make sovtek 12ax7lps fail" and got this: http://music-electronics-forum.com/t9514/ I don't know what design your preamp employs, but this may help explain. BTW, I have switched to NOS by RCA, Sylvania, Telefunken because I have been so disappointed by modern tubes. Also, you really should have your gear checked by a pro on a bench, you're just postponing the inevitable here.