That's an even worse choice than a 6SN7. The tube will be biased into positive grid territory, which is not what you want in a preamp stage.
I agree with Terry that we have pretty much exhausted the discussion on different tubes for the TubeSuMo. Trying to do this would require pretty much a redesign of the input stage and that is probably a topic for at least another thread, if not a conversation in the tube forum.
Good luck!
I use Electro-Harmonix:
EHX.com | 12AU7 (also ECC82) - Preamp Vacuum Tube | Electro-Harmonix
with gold-plated pins:
https://www.tubedepot.com/products/electro-harmonix-12au7-eh-gold-pin-preamp-vacuum-tube
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In general how many hours does 12Au7 will work? Like 5000 hours or 8000 hours etc? or less?
I see in few links its stated as heater life is about 10,000 hours.
http://www.audiotubes.com/12au7.htm
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I'm puzzled about why you would spend so much time worrying about tube specs that have so little to do with how a circuit performs. With respect to tube life, tubes that are operated close to their maximum values tend to have shorter lifetimes than tubes that are operated more conservatively. But in either case, when tubes die or start to malfunction you just replace them and start a new cycle.
FWIW, the 12AU7 in the TubeSuMo is operated very conservatively, so there is no reason to expect that tube life would be a problem in that circuit.
As already mentioned in the previous posts by Ray and Jeff, lifetime very much depends on the tube type and operating conditions (same applies to solid state devices). 10 - 20 thousand hours is normal for small signal triodes. Some military-purpose production tubes (like 6922) are known to last up to 100 thousand hours (10 years non-stop).
12AU7 here is used in easy way, so I agree with Terry - you will have to take care of the electrolytics as a first priority.
Also, output transistors dissipate much more heat and warm up much longer for the best sonics. Some manufacturers specify warm-up time - for 12AU7 it's normally 11 seconds. Within, say, 1 minute - it's fully operational. Output stage - it takes 10-15 minutes to warm up the heatsink and reach the normal operating conditions for the best sonics.
Terry, you sound like a Naim enthusiast
I have never recapped. Is this really necessary?Most of the electrolytic caps get weak after 10k hours.Terry, you sound like a Naim enthusiast
As we already mentioned, 6SN7 is getting biased positively in this circuit.
You can see it, as the grid is connected to ground through R73, having roughly -0.7V at the cathode.
Positive grid current results in heating of the grid, for the same reason that the plate gets hot (i.e. due to the kinetic energy of the arriving electrons). Small tubes are not generally designed for this. Overheating of the grid is bad for two reasons. Firstly, since it is not designed to run at a high temperature, it does not have any way to retain its tension as the metal expands. This results in changing geometry and in the worst case melting or a short to the cathode, which is instant death. Secondly, the grid is generally contaminated by oxide from the cathode, and if it gets hot then it will start to emit electrons like the cathode, resulting in a substantial secondary current.
All these things lead to significant reduction of the tube's lifetime. It's just the wrong way of using the tube.
It will also not really work with 5.6V at the plates - even if there will be some current flowing through the tube, distortion will be huge.
Don't trust everything you see in the simulator - some things are just not reflected in the model. If you look at the Fourier and feedback loop AC analysis, you will most likely see some unwanted artifacts.
You can see it, as the grid is connected to ground through R73, having roughly -0.7V at the cathode.
Positive grid current results in heating of the grid, for the same reason that the plate gets hot (i.e. due to the kinetic energy of the arriving electrons). Small tubes are not generally designed for this. Overheating of the grid is bad for two reasons. Firstly, since it is not designed to run at a high temperature, it does not have any way to retain its tension as the metal expands. This results in changing geometry and in the worst case melting or a short to the cathode, which is instant death. Secondly, the grid is generally contaminated by oxide from the cathode, and if it gets hot then it will start to emit electrons like the cathode, resulting in a substantial secondary current.
All these things lead to significant reduction of the tube's lifetime. It's just the wrong way of using the tube.
It will also not really work with 5.6V at the plates - even if there will be some current flowing through the tube, distortion will be huge.
Don't trust everything you see in the simulator - some things are just not reflected in the model. If you look at the Fourier and feedback loop AC analysis, you will most likely see some unwanted artifacts.
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