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Old 2nd November 2006, 05:39 PM   #1
Bengali is offline Bengali  United States
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Default Aikido Preamp

Hi,

Has anyone tried the 12ax7/12au7 tube combo for a preamp?

The akido pcb is on it's way. I would appreciate your help to get

get me started.

thanks!
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Old 2nd November 2006, 06:02 PM   #2
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People around here are always willing to help, based on basic knowledge until expert knowledge. Thing is, Aikido has been discussed a lot already and you will probably learn lots from reading old threads, and off course tubecad.com

IN the case of a 12AX7 as the first tube... you will get a gain of about 50 from this preamp, this is way to much for preamp duties. Normal CD players have an output of 2VRMS. The aikido using 12AX7 could theoretically amplify this to 100VRMs, while the power amp may need maybe 1VRMs. Do you understand this?

Erik
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Old 2nd November 2006, 06:12 PM   #3
Bengali is offline Bengali  United States
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Hi,

yes, thanks for your reply. I did search and have been reading up on it.

I understand the 12ax7 has a high gain. I have these tubes. My Jolida 502B uses the 12ax7 and 12at7 for the preamp stage.
The prima luna preamp also uses the 12ax7. Are they using a lower B voltage to decrease the gain?
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Old 2nd November 2006, 06:30 PM   #4
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A 12AX7 is indeed a high gain tube, and is used as such to drive power tubes, as in the amps you mention. Example: an EL34 requires something like 30VRMS, so the driver tube must amplify the 2VRMs coming from the CD player to 30VRMs needed through the EL34. This is about 15x gain, less than the 12AX7 develops. No problem actually, because other factors, as Negative feedback which will reduce the gain of the 12AX7 tube.

What I say is not completely right, other factors also influence it and so on, but it is just to give you an idea. If you are really interested in learning 'why', and not just 'how' I recommend you to buy Morgan Jones 3rd edition of Valve amplifiers, available at Amazon.

The use of 12AX7's in line preamps is common practice in comercial amps, but you will not see this much among DIY'rs. For line level duties there are much better tubes around, most having lower gain and other characteristics that make them suitable to drive long cables (the only use of a preamp, IMHO).

Erik
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Old 2nd November 2006, 06:34 PM   #5
Bengali is offline Bengali  United States
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thanks for the suggested reading. might just get that book.
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Old 2nd November 2006, 10:37 PM   #6
jayme is offline jayme  United States
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Definitely buy the Morgan Jones book. Then buy his other book, Building Valve Amplifiers, as it focuses on the actual build process once you have a circuit.

I would have been completely lost without them.
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Old 3rd November 2006, 03:29 AM   #7
2wo is offline 2wo  United States
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Agreed the 12AX7 probably has more gain than you need. 12AU7-12AU7 might work well. You mentioned 12AT7, it might work as a second tube but I have not looked into it…John
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Old 4th November 2006, 03:00 AM   #8
Bengali is offline Bengali  United States
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I just ordered both books. I will stick with both tubes as 12au7's to start.

I'm looking at the chart for the 12au7. The output gain is about 1db difference between 100V and 300V for the B voltage.
What's the advantage of using a higher B voltage?

is anyone using a soft start circuit? I'd like to have no thumps on the speaker when powering on/off.

also, the cap between R15/R16 can be from 1-10uF. How does changing this value affect the sound?

there's no place for input coupling caps, are they left out for a reason?

thanks.
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Old 4th November 2006, 09:18 AM   #9
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Hi Bengali

Do you already have the 12AU7 in hand? Otherwise I would go for the 12BH7A or the 6CG7, or still other noval with similar gain...all this because the 12AU7 has higher distortion figures... the more experienced people around here do not like them

But if you have them, go for it. You can always change for another tube later. Higher B+, within limits, means lower distortion, and as you have seen the gain is not influenced much. I am sure John Broskie got values for B+ and resistors to use together with the 12AU7!

Are you using SS amps or valve amps? If SS you are right to worry about thumps, but just when you turn both amps on together. You are safe when you turn the SS amp on after half a minute after turning on the aikido. You can also include a relay connected between the output signal and the ground of the Aikido...this relay should be NC and powered by a delay circuit, with a 555 timer, for example. You can use the filament voltage for this circuit, given you are using DC on filaments - or rectify the AC and smooth with a big cap, I would not worry much about regulation. Or the soft start circuit, but that makes the PS more complexe.

I thins the cap between R15 and R16 must be the output cap. It works as a high pass filter together with the input impedance of the power amplifier. I am sure Broskie explains this...but for SS power amp I would stick with at least 4,7uF - but you are free to experiment.

An input coupling cap is not needed, as the gird of the first tube is kept at gorund level through the gridleak resistor.

Good luck with it

Erik
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Old 5th November 2006, 03:16 AM   #10
Bengali is offline Bengali  United States
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thanks Eric for the good input. I really value that you mentioned the 12au7 have higher distortion. I'll stick with the 6CG7 then.

Yes, I plan to use with a SS amp. thx.
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