12AT7 tube life - diyAudio
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Old 19th August 2010, 12:56 AM   #1
w5jag is offline w5jag  United States
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Default 12AT7 tube life

What type of tube life are you guys seeing for the 12AT7 in your Simple SE's?

My first one, a GE 6201 went a bit more than 4000 hours before it got crackly. It was used to start with, so I didn't think much about it.

A JJ 12AT7 went less than a thousand hours before it got crackly .

I just got through tossing a Mullard (branded Amperex) 12AT7 in the trash can because one section is now testing at minimum on my TV-7D. It had more than 4000, but less than 5000, hours on it and I am pretty sure it was unused before it went in the amp.

I was expecting around 10K hours or so for the 12AT7.

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Old 19th August 2010, 02:35 AM   #2
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Now that you mention it, I had a 12AT7 go staticy in my Simple SE. I pulled it and replaced it with another and didn't give the matter much thought.
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Old 19th August 2010, 03:26 AM   #3
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Is it possible the plate dissipation is high, or the heater voltage is out of normal range?
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Old 19th August 2010, 03:59 AM   #4
rknize is offline rknize  United States
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I have been keeping an eye on this very thing:

JJ ECC81/12AT7 Longevity

The JJ's are pretty lousy in terms of longevity. All of mine that have seen any real use (in the 100s of hours) are testing weak. The one in the SSE was particularly bad. The SSE runs them at fairly high current, but within ratings. A lot of the new stuff just isn't up to the task. I think the active plate load plays a part and is what makes them go crackly. It tries to drive them harder to keep the plate current up and so as they get weaker they are being "used up" faster. Eventually they are riding the edge of full conduction (what is left of it) and they crackle.
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Old 19th August 2010, 04:07 AM   #5
BudP is offline BudP  United States
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I have been using the Chinese 12AT7 from New Sensor in my AudioPrism PP amp for years. Superb sonics and about 5k hours on the ones currently installed.

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Old 19th August 2010, 12:12 PM   #6
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Quote:
Originally Posted by bob91343 View Post
Is it possible the plate dissipation is high, or the heater voltage is out of normal range?
Verifying correct heater voltages is always a good thing to do.

I believe the design of the Simple SE runs the driver tube in a sensible range. I took some actual measurements on mine when this topic was discussed previously. Here's my post:
Simple SE and 6BG6GA's

220 volts at 8.5 mA is about 1.9 watts. This is roughly 75% of the rated maximum dissipation limit as published in the data sheets. You might want to check your amp to confirm the plate and cathode voltages are reasonable.
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Old 19th August 2010, 03:05 PM   #7
w5jag is offline w5jag  United States
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Likely, my heater voltage is a bit high as I'm using an old hamfest transformer, although I don't know that it is so high as to be out of spec.

I think rknize's theory is very plausible, as the tube gets weaker, the ccs drives it harder and harder accelerating the ultimate tube failure.

I've got dozens of 12AT7's so it's not a big deal, I was just curious if anyone else had noticed this. A Toshiba 12AT7 that is unquestionably NOS has now replaced the Mullard.

I've ripped apart my Tubelab SE to put a Simple SE on that chassis optimized just for the little tubes, and I'm sorta considering doing a point to point layout just for fun and to see if I can do it without building a hum bucket.

If I try that, and that's a big if right now, I might split the 12AT7 into a couple of 6AB4's to spread the heat around a little. I've been looking at tube manuals for the last couple of evenings and can't seem to find any octals, either dual triode or single triode, that can replace a 12AT7. Other than 6AB4, it looks to be a unique tube.

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Old 19th August 2010, 03:20 PM   #8
jrenkin is offline jrenkin  United States
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I am just jealous of your 9k - 10k hours of casually reported listening time....24hrs a day for more than a year?
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Old 19th August 2010, 03:29 PM   #9
w5jag is offline w5jag  United States
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Originally Posted by jrenkin View Post
I am just jealous of your 9k - 10k hours of casually reported listening time....24hrs a day for more than a year?
I wish.

It's the PA in the entertainment system in our primary residence, so it works long hours. 15 Hours a day or more almost every day racks up hours fast.

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Old 19th August 2010, 03:59 PM   #10
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Default crackly- tubes?

I've replaced a couple of audio tubes over years because of pops and crackles and now I am having second thoughts. I replaced most of the paper caps and carbon resistors in my 1961 build PAS2 this spring, pop, pop, pop, pop. I did them two at a time so I didn't create too many problems between tests. Back installed the paper caps, still pop, pop, pop. The problem was not where I worked, that I washed pretty well, the problem was either rosin or dirt on the PCB's everywhere else. Turning the lights off at night helped me zero in on the arc points, not where I had worked, actually. As the PAS2 is sealed, probably not a lot of modern dirt. Clean, clean, scrub, scrub, pops and crackles all gone. Paper towel, water, finally a little dilute bathroom cleanser on the PCB's (the non-chlorine kind in a spray bottle, imitation scrubbing bubbles.) The PAS2 was assembled in 1961 by a minister, worked well enough when I bought it but like I say, couple of crackles over the years I replaced tubes on, could have actually been "repaired" by lower humidity the next day.
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