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
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
11,793 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.
onemug
784 posts
 
Atmasphere, Thanks.
lamphear_electronics
9 posts
 
Sounds allot like either a filament or high voltage supply design flaw or defect due to a blown part. It would not be difficult to modify or repair.
guygus OP
22 posts
 
Hi Guys,

Thanks for all the responses!
The last time I listened to vinyl I was plagued by the same problem. That is,on turn on I would listen to vinyl at low volume and everything was OK for the first fifteen minutes. Then, a low level distorted sound that got intrusive very quickly would appear. On FM everything sounded fine.

Today, I replaced the phono section tubes with a set from the line stage and replaced those with a known set of good ones. Upon turn on there was distortion right away on phono but not on FM.

I believe that there has to be something up in the circuit like Lamphear says.

One other thing. Phono sounded good before with a Shelter 501. Now I have a Shelter 9000 and it is a LOT better ( when the phono stage is not acting up).
I use a step up transformer to the MM section of the pre. The sound has a lot of the audiophile attributes we look for. But, how come with a cheap Kenwood tuner from the seventies the sound is more 'pleasing'?
It sounds fuller, richer. Not deeper bass or higher highs and air all around,
just a more relaxed sound. And this is I presume with CD's broadcast from across town. I have not had this sound on vinyl. Would loading and capacitance have something to do with this? As I don't have any options in this regard.

Chears
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