• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    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

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Joined 2003
#1
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
 
#7
"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:
 
#9
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 🙄
 
#11
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? 😀
 
#12
JojoD818 said:
Any circuit you have for 6N1P tubes you wanna share? 😀
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" 😀

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.
 
#13
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!
 
#18
I also saw that you have a mic preamp with a compressor (separate schematic) and wowow! Have you actually tried using it? Can you describe how it sounds? Seems that all the parts are locally available here except for the LED/LDR part. Maybe I can just build a discrete LDR/LED for such use?`
Click to expand...

perhaps the WHAT-compressor?
 
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