Listening for tube degradation

I 'work', when I actually do, out of my home. I turn my 'casual' system on at about 7AM and turn it off at about 4PM every weekday. So, on this system, I change my power tubes in my amp at least once a year.

I have a second 'system' (different electronics - same speakers) I use for 'serious' listening sessions, those when I'm not multi-tasking - usually at night or on weekends. In that system I've maxed out power tube performance and average about 1500 hours on the power tubes. I get to change these about every three years.

I guess I'm more than a bit wierd but I just love music and don't watch TV in my spare time. FWIW.

I've considered moving towards a SS amp as a result of this or investing in a tube tester. I get paranoid as well because I'm never sure if I'm getting optimum performance. I love the sound of tubes but I'm not sure if it's worth the maintenance effort. The best thing to do is to probably keep a stock pile of tubes on hand just in case.

Any recommended tube testers?

It always seems that I find tube failure being preceded by increased noise, such as when a tube in my phono stage started kicking off big thumps through my speakers and subwoofer. It is often difficult to hear when tubes have aged because they slowly deteriorate over time, whereas when you change them you get all of the performance back at once. Often you will just get a dulled performance over time.

The easiest, and most reliable approach is to always have at least one new set of replacement tubes in reserve. Every year or so, replace the tubes with the new set and see if the sound has changed markedly for the better; if so, it is time to replace the tubes. Do this separately for small signal and power tubes because small signal tubes usually last MUCH longer.

Tube testers are generally NOT reliable for this. Most testers are good for spotting gross problems but not subtle deterioration. Most testers are not terribly reliable because they need to be calibrated every so often and who really sends them away for that kind of maintenance?

One of the best testers out there is one that is in current production and is quite expensive. It is the Amplitrex tester. It is pretty much idiot proof (a screen tells you exactly what to do step by step), accurate, free of need for recalibration and can test for a wide range of factors. You can even hook it up to a computer to trace curves. One nice feature is that it will give real, meaningful readouts (e.g., transconductance) and not some proprietary "value," provide the tubes specifications to compare with the test results, as well as provide gross evaluations ("excellent," "good," "weak" and "bad."). I have this tester and like it a lot. The only big weakness is the limited number of tubes that are programmed into the tester (you can do your own programming if you have the proper data or you can have the factory program a CD ROM to update your machine).

The problem with any tester is that, depending on the application, a tube may test extremely bad and still be perfectly fine for the particular use in the amplifier. I have a number of extremely weak 6sn7 tubes that sound WAY better than any current production tube in my amplifier. I doubt this would be the case if the tube were pushed much, but, in this amplifier the tube's capabilities are taxed very much. Even the Amplitrex is best used for testing to see if a tube is generally good, and the extent to which the vendor stretched the truth as to its "age," and not for making any kind of objective evaluation of the sound of the tube. My amplitrex has already proven itself by showing that a "new" rectifier should not be used (no voltage drop in EITHER direction, i.e., shorted out).

Hitman and Larry:

As I stated earlier, I'm not an expert in electronics but I do believe an inexpensive tube tester can be a great value to most hobbyists with any tube gear. B&K and Hickock both made a number of good testers that can be found in the used market. I got a B&K 707 which, with the added Instruction Manual (including simple calibration instructions) and tube charts, cost me about $25.

The key is mutual conductance testing. I don't simply rely on "Replace" readings for tests but I record the Gm reading directly on new tubes (with a Sharpie), then note changes after a period of use. In general, I've been confident with readings within 15% of the new test.

For anyone playing their system on a regular schedule like Newbee, estimating hours of play can be relatively easy to indicate time for replacement. My listening is much more erratic so estimating hours of play for me would be a real WAG.

If you have a simple circuit with only one or two tubes it may be easier to simply replace periodically or have a dealer test them for you. Otherwise, for a sense of involvement with your system, find an inexpensive tester and learn to do it yourself.