tube burnout


I have owned a tube pre (Grant 100) for a few years and have been satisfied with the sound generally. My concern is that it eats up tubes. It takes six 6922's
with two of them in the mm phono and the others in the linestage. I have gone through about two dozen tubes in the last few years, and it does not matter what brand tube it is either.
This almost always happens to the phono tubes where they start out fresh, only to discover a month or two later these tubes start to get noisy. When replaced,
I get a month or two again before the noise starts up again and they must be replaced. While this is going on, the other four tubes in the linestage are not affected and play fine on any source other than vinyl.
I have tried 6922 from most manufacturers including new and NOS.
I had the pre checked and they said it was fine, just bad tubes to be replaced.
Which I did, and did, and did. All to no avail.
Still the problem persists. I can't believe that all the tubes I keep installing
are 'lemons'.

Does anyone have any suggestions?
guygus

Showing 3 responses by atmasphere

Years ago Audio Research had a similar problem with the 6DJ8s in the SP-10 (when you tried to use a low output MC cartridge). The tubes would only last about 2 weeks. Higher output cartridges allowed you to run the tubes *a lot* longer.

If the 6DJ8/6922 is not set up right, there are a number of design parameters that can cause it to go noisy prematurely:

* the tube requires a thorough warmup without B+, so you need a time delay circuit
* its possible to 'starve' the plate, and while you get good gain, the grid will eventually get contaminated, so
* its better to run the tube hot and get less gain.

It was the experience with the SP-10 that made us realize that when we did our preamp that it would probably be a good move to avoid 6DJ8 family tubes. They are not intended for audio, so low noise and low microphonics are challenging (although they are quite linear).
My experience is that its very difficult to come up with 6DJ8/6922 tubes that do not exhibit some sort of microphonic signature, much more so than tubes that are actually designed for audio, like a 12AT7, 6SN7 or the like. It seems that 6DJ8s are far more likely to offer a signature, the louder you play the system the more you hear it.

We never considered them for our phono circuitry after a variety of experiences we had with them in the 80's, the SP-10 being among them. We did use them in our amps up until about 1990; finding tubes that didn't ring was challenging enough that we moved away from the tube after that.
If your cartridge is not properly loaded, you are missing a bet with LPs. Proper loading can take out a lot of harshness and improve detail.

If the cartridge is a few years old, the cantilever suspension can perish (like bad shocks in a car) and the cartridge can get a lot harsher and not track as well. Of course, proper arm setup is *crucial*!