2. amps-preamps
  3. 1218975014

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
guygus OP
22 posts
 
Blindjim
No, only the phono stage tubes in the pre get affected
No, I have not had to raise the volume level more
It was an electronics repair shop, not the manufacturer that checked it
swampwalker's avatar
swampwalker
5,138 posts
 
You will hear noise in the phono stage tubes at lower level than in line stage due to high gain. That's why some tube sellers "grade" their tubes as driver grade, line grade and phono grade (decreasing noise). IF (no idea) your pre runs the tubes hard (some are famous for that) and if the tubes are a little noisy to begin with, then its easy to see that you could go through lots of tubes. You may ease the cost burden by cycling them from the phono to the line stage when they start to get noisy and see if they can be used in that portion of the circuit.
atmasphere's avatar
atmasphere
11,577 posts
 
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).
onemug
784 posts
 
Atmasphere,

Do you feel the 6dj8 family is equally bad for phono and line stage preamps or not so good for line stage and worse for phono?
atmasphere's avatar
atmasphere
11,577 posts
 
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.
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