12AU7 or 12AT7 instead of 12AX7 ? - diyAudio
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Old 23rd June 2004, 10:41 AM   #1
Gubby is offline Gubby  Belgium
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Lightbulb 12AU7 or 12AT7 instead of 12AX7 ?

Hi,

Currently I'm using a Vincent KVH111 headphone amp as preamp (with a Nelson Pass DIY Zen amp).
This amp is based on a 12AX7 double triode (mu~=100).
I red somewhere on the web that it could be a good idea to replace a 12AX7 tube with a 12AT7 or 12 AU7 to have a little less amplification factor so less distorsion and more precision on the volume controle.
What do you think about this ? Can I try this change without any risk for my precious amp/preamp ?

I think it's more easy to find 12AU7 or 12AT7 NOS at decent prices than NOS 12AX7. Am I wrong ?

Many thanks.
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Old 23rd June 2004, 12:19 PM   #2
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Hi Gubby,
The tubes have different operating conditions apart from different 'mu' . So IMO puttting in a different tube into that socket might not give you the best result.
You could however change the resistors to suit the tube you are going to use. If you look at the operating curves of the tubes you will understand that they can't be swapped that easily. I guess that also depends on the circuitry.
The easiest method might be to use two (or one ) resistor at the output of your 'pre-amp' to drop the voltage gain of the system. Easier and no problems I would think. Your 'pre-amp ' should hopefully have enough output range to ensure it would not clip .
You could post the gain you have in each circuit and Z in of the power amp. It would be easier to give an exact answer.
Cheers.
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Old 25th June 2004, 04:38 PM   #3
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Default 12AU7

I am considering doing the same to my DIY 6N1P based Headphone amp. There is just too much gain with a CD player having 1.5-3 volts output.

You can change the input tube but you need to know the values of the B+ voltage, plate resistor, cathode voltage and resistor.

Look up the operating data for the 12AU7 at Duncanamps.com and adjust those components as necessary.

for the 12ax7: http://www.pentalaboratories.com/pdfs/ECC82.pdf
for the 12au7: http://www.retrovox.com.au/12AX7AWV.pdf

The 12au7 variety are great tubes and NOS specimens are reasonably priced. I just replaced a stock Ruby (uck) 12AU7 with a Mullard in my tubed output SACD player with wonderful sonic improvement.

Dr. Peels
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Old 25th June 2004, 11:38 PM   #4
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Hi,

Instead of swapping tubes around why not use a voltage divider network, either at the source output or at the preamp's input?

At least you won't have to be recalculating anode resistors and bias resistors to find the correct values for a tube type other than the one the circuit was designed for.

Cheers,
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Old 26th June 2004, 12:29 AM   #5
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Although not an expert by any means, resistors for me, have not controlled the gain from CD player to tube preamp or tube headphone amp or to tube power amp and preserved the soundstage, imaging, or the various other reasons we love these glowing things.

Respectfully,
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Old 26th June 2004, 12:31 AM   #6
karma is offline karma  Canada
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hmm i had the same problem with my preamp i just maid

fdegrove's idea work's i was thinking i did something wrong

sound's great now

not sure it i did it right i used 2 330k resister's input to ground.
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