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Old 13th December 2008, 01:38 PM   #1
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Default What Power Supply for 12AX7 Preamp ?

Hi there,

I am very new to the world of valves but I want to give a valve preamp a go to sweeten the sound of my 100w gainclone.

I'd like to give this 12AX7 preamp a go but I have the feeling that its switching power supply with PMW chip is not a good quality solution (I might be wrong)

So, I was thinking something the lines of a shunt regulator or something.

Can someone propose a good quality, not too expensive PSU for this 12AX7 preamp?

It seems it needs +260V and +12V but don't know at how much current.

Thanks
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Old 13th December 2008, 04:35 PM   #2
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Default Re: What Power Supply for 12AX7 Preamp ?

Quote:
Originally posted by chatziva
I'd like to give this 12AX7 preamp a go but I have the feeling that its switching power supply with PMW chip is not a good quality solution (I might be wrong)
If you want to add a tube pre to a gsinclone, then consider something else than that design. A simple 6SN7, ECC99, 6H30, 6H6N anode loaded will give a better result than two 12AX7 stages with a ton of gNFB. As chipamps typically have a lot of gain, adding more will only mean you will throw it away in the volume control.

As for power supply for any tube amp, go to duncanamps.com, download PSUD2 and put in some values for Hammond transformers. If you want to do a regulator, search for Maida. Each stage of the 12AX7 design will draw about 1mA, so about 4 total and the heaters about 175mA/bottle in 12.6V config.
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Old 14th December 2008, 01:48 PM   #3
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Default Re: Re: What Power Supply for 12AX7 Preamp ?

Quote:
Originally posted by Brett

If you want to add a tube pre to a gsinclone, then consider something else than that design. A simple 6SN7, ECC99, 6H30, 6H6N anode loaded will give a better result than two 12AX7 stages with a ton of gNFB. As chipamps typically have a lot of gain, adding more will only mean you will throw it away in the volume control.
I totally agree with you. But, as I said I am very new to the tube world... so I need to start from somewhere.

I think what you are saying is to use valves for a buffer stage, with unity gain right? Can you propose a design?

Thanks again
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Old 22nd December 2008, 04:40 PM   #4
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Please note that a valve based preamplifier will not automatically add 'sweetness'. It's not as simple as that, despite what the audio-fools may write in their expensive magazines.

There seems to be little merit in the 12ax7 preamplifier found in the provided link. The 12ax7 has far more gain than you can use and they apply feedback to reduce it, then provide a complex power supply, and then a volume control at the output (i.e. a variable output impedance). Best to leave it on the page.

I suggest you first try a passive potentiometer in front of your amplifier. If you find that you need gain then look to the Aikido kit from Tubecad, which comes fully documented with a suitable power supply.
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Old 23rd December 2008, 05:41 PM   #5
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Quote:
Originally posted by Gordy
Please note that a valve based preamplifier will not automatically add 'sweetness'. It's not as simple as that, despite what the audio-fools may write in their expensive magazines.

There seems to be little merit in the 12ax7 preamplifier found in the provided link. The 12ax7 has far more gain than you can use and they apply feedback to reduce it, then provide a complex power supply, and then a volume control at the output (i.e. a variable output impedance). Best to leave it on the page.

I suggest you first try a passive potentiometer in front of your amplifier. If you find that you need gain then look to the Aikido kit from Tubecad, which comes fully documented with a suitable power supply.

Good points about the 12AX7 preamp. Thanks for mentioning the Aikido designs, they are quite nice.

In terms of power supply I think I now have some choice, like the Aikido kits or Salas's HV shunt regulator.

In terms of the preamp stage itself I think I am better off with AlexW88's topology with 6922 or 12AU7 - Aikido's kit are slightly limiting the choice of tubes
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