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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12AX7 sound ... going with the trend

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TobWen

Member
Joined 2003
Hi there,

I have read a lot of interested things in this forum.
Normally, I'm working with OPAs to get amplification (preamps etc.), but now I want to go with the trend and build a combination!

The problem is: In this forum, you're mainly discussing about power- or phono-amps ... I need something with LINE-level without amplification...

In detail:
Behringer (ugly, I know) has two models of solid-state based mic-preamps with tube-stages:

http://www.behringer.com/MIC100/index.cfm?lang=ENG
http://www.behringer.com/MIC200/index.cfm?lang=ENG

It's build on NJM4580-amplification with a 12AX7.
AFAIK the 12AX7 doesn't do any amplification - it only makes the tube-sound and distortion.
So you cannot call it a tube-amplifier, like Behringer does!

Does anybody have some schematics of the tube-effect only?
The input should be a line-signal and the output should be the same.

Is that the correct way to go?
http://www.dogstar.dantimax.dk/tubestuf/mchowit.htm

Best regards,
Tobias
 
JojoD818

JojoD818

Staff Emeritus
Joined 2003
geocities.com/jojod818/index.html
Optical said:
im interested in this stuff too, but nobody who is really into tubes has much to do with it unfortunatly

check this website out also

http://www.paia.com/tubestuf.htm

a tube compressor/limiter without the use of complicated hard to find transformers/led-ldr combos is what i would really like to see!
Click to expand...


Very interesting indeed!
 
Geek

Geek

Member
Joined 2004
"Starved plate" is a great way for fuzz, but also adding noise. Would you run a transistor thingy in your stereo line with 1 volt on the Vcc? 45Va is basically the same thing.

IMO, if you need line-level amplification, dump the 12AX7 and think 12AU7, 12AT7, 6DJ8 or my favorite, the Russian 6N1P ;) :2c:
 
Geek

Geek

Member
Joined 2004
12AX7 is a very high gain tube. The noise voltage induced could cause problems.

I found in my own experiments that the noise level is 10 to 20 dB higher with a Va below 60 or so volts and going to about 40dB higher at 25 volts.

I cannot comment on the Behringer because I have not listened to it, nor have I seen circuit configuration or chassis layout.

I do know for a fact though, that improperly dressed wiring or poor chassis layout can make the starved-plate noise moot, as I have seen countless times in tube guitar/bass pedals :rolleyes:
 
JojoD818

JojoD818

Staff Emeritus
Joined 2003
geocities.com/jojod818/index.html
Geek said:
"Starved plate" is a great way for fuzz, but also adding noise. Would you run a transistor thingy in your stereo line with 1 volt on the Vcc? 45Va is basically the same thing.

IMO, if you need line-level amplification, dump the 12AX7 and think 12AU7, 12AT7, 6DJ8 or my favorite, the Russian 6N1P ;) :2c:
Click to expand...


I agree with your comments, I am just explaining why the guys who designed that circuit used +45V, I mean they intentionally did that to starve the tubes.

I also love the russian 6N1P, have lot's of them coz they come in relatively cheap here locally. :)

Any circuit you have for 6N1P tubes you wanna share? :D
 
Geek

Geek

Member
Joined 2004
JojoD818 said:
Any circuit you have for 6N1P tubes you wanna share? :D
Click to expand...

I use them mainly in White-CF or mu stages, as they seem to be ideally suited to that :)

Here's a linedriver mu-stage I made for my Akai AT-2400 tuner:
http://www3.telus.net/small_business/Geek/TunerOut.png

My microphone preamp contains the same output, but in a White-CF:
http://www3.telus.net/small_business/Geek/TubeMicPreampFINAL.png


Can you tell me / show me the circuit of a solution, based on 12AU7, 12AT7, 6DJ8?

Watch the link of the MIC100/MIC200 I've posted at the start of this threat. As you can see, the chassis is VERY small.
Click to expand...

Y'know, I re-read your original post and Fred's McTube is a good little box. Pop an 'AT7 in it and she goes from "warm" to "crystal" :D

You can also try a mu stage based on the 6DJ8. Check the link I posted for Jojo, reduce the Va from 250 to 120 and you'll be OK.
 
Geek

Geek

Member
Joined 2004
Oh, forgot to mention, the 12AT7 is also a good White-CF:

http://www3.telus.net/small_business/Geek/12AT7-White-CF.png

Slightly less "grumba" since the cathode of the 'AT7 is teeny-tiny compared to 6N1P. But it has a beautiful, mellow quality that is unique. If I were building a linedriver for a person who loved Blues/Jazz, I'd make them a 12AT7 CF or mu.

One caveat with an 'AT7 - they loooooove to motorboat. Decouple the power supply line good!
 
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