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Old 18th May 2003, 10:32 PM   #1
karma is offline karma  Canada
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Default going to build a 12au7 pre what do you think?

what do you think good bad. your thoughts


http://www.bonavolta.ch/hobby/en/audio/12au7lin.htm
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Old 18th May 2003, 10:52 PM   #2
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The high impedance active load produced by a mu-follower allows the lower valve to produce its lowest distortion. Unfortunately, ECC82 will produce 13dB more distortion than 7N7 or 6SN7GT in this configuration because it's a horrible little valve designed for field scan in televisions (I have tested this). 7N7 is much cheaper than 6SN7GT, and you can get PTFE sockets. Don't use the high impedance output, it will increase distortion. Go for the circuit, just use a 7N7.
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Old 19th May 2003, 05:46 AM   #3
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Default Re: going to build a 12au7 pre what do you think?

Quote:
Originally posted by karma
what do you think good bad. your thoughts


http://www.bonavolta.ch/hobby/en/audio/12au7lin.htm

It's one of the circuits in my mind for my next (and the first tube) project. I've read good reputations from other forum. If I go with this one, I would use well-filtered DC supply for heater (6.3V for parallel connections). Power supply section could be a little bit different from the circuit. I may go with tube rectifier and chokes.
Anyway, if you decide to go with this circuit, please let me know. We can go along together. This would be fun. :-)

BTW, have you thought about using 7N7? Is it pin-to-pin compatible to 12au7?
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Old 19th May 2003, 05:59 AM   #4
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Isn't 7N7 a locktal 6SN7?

I don't see what everyone's problem is with the 12AU7.. go for it. Maybe it's one of those minority type things... you know.. like... it's obviously a very popular tube for something since everyone has thousands of them, so naturally you just can't use them in DIY hi-fi since they're a commoner's tube...

Circuit looks fine but personally I'd just ignore the hi-imp. output. I mean, what's the point? The low impedance output gives just as much voltage and gain (well probably a little less) but having a lower Zo it has higher frequency response, especially when driving cables.

Tim
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Old 19th May 2003, 10:35 AM   #5
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I don't mind mu followers, but I wouldn't build that. In my opinion/experience the 12au7 is crap. Try a 6SN7, 6FQ7, 6H30 etc as the lower element and a pentode, or better yet a CCS like one of Gary Pimm's, as the upper one. The mu of the lower tube will set the gain for the amp.

Good article on the mu follower by Allen Kimmel can be found here.
http://laplaza.taos.nm.us/~fvuotto/mustage.html

I'm also not a big fan of doubler supplies. I bet there's a Hammond with the correct secondary (+ filaments) to do the job with a simple fullwave rectifier available cheaply.
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Old 19th May 2003, 03:15 PM   #6
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Default Re: going to build a 12au7 pre what do you think?

Quote:
Originally posted by karma
what do you think good bad. your thoughts


http://www.bonavolta.ch/hobby/en/audio/12au7lin.htm
do you really need the gain????

whats your source voltage and your amps input sensativity???

dave
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Old 19th May 2003, 04:22 PM   #7
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This circuit looks like Bruce Rozenblit's first preamp, a lot of people have built it and like it. Even better is his Grounded Grid preamp from his book Audio Reality. This uses the same tube. A lot of people _really_ like this preamp, don't worry about the tube - the preamp sounds great.

Clive.
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Old 19th May 2003, 04:42 PM   #8
karma is offline karma  Canada
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i ended up buying 8 12au7 off e-bay i got em cheap. so when thay
get here ill make a few changes and share it with you as im building it
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Old 19th May 2003, 05:20 PM   #9
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Quote:
Originally posted by karma
i ended up buying 8 12au7 off e-bay i got em cheap. so when thay
get here ill make a few changes and share it with you as im building it
Are you going to use the power supply as drwan in the schematics? What transformer will you use? Hammond? I'll order one from Angela pretty soon.
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Old 19th May 2003, 08:12 PM   #10
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Quote:
Originally posted by Brett
... the 12au7 is crap. Try a 6SN7, 6FQ7, 6H30 etc...
I always find it funny when people say this. They have the exact same curves, +/- a few mA.

Tim
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