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porkchop61 24th September 2010 11:28 AM

Small Tube/SS hybrid amplifier design 9V
I've been thinking about making a small amplifier using a 12AU7 preamp, and an LM386 power amp. This is not Hi-Fi, but something I would like to use at work to power a pair of small speakers, either from the PC or MP3 player.
I was intrigued by this design for the front end using a 12AU7 running on 9VDC from a wall wart: I was surprised you can run a 12AU7 with 9V on the plates.

Now this is intended for a guitar, so the gain might be too much for an MP3 player. Can anyone suggest ways to reduce the gain if this is the case? Can I eliminate the gain pot altogether since I will have a volume pot on the front of the LM386 amp? I'm planning on just using the typical LM386 circuit from the data sheet with a gain of 20.

Maybe there are better chip amps than the LM386 (higher fidelity) that also run off from 9VDC? I'm open to suggestions.
I'd like to build this into a small Hammond die-cast box, probably a "BB" size.

Any thoughts from the tube gurus? I'm open to suggestions.

Thanks in advance.

SY 24th September 2010 12:23 PM

Enormous distortion, severely restricted bandwidth. It's for guitar effects, not music reproduction.

Ian444 24th September 2010 12:33 PM

There is a couple of low voltage tube preamp kits from that might interest you, probably a better alternative. Just for ideas.

porkchop61 24th September 2010 12:45 PM

Thanks, I'll give them a look.
Maybe this is better:
It's for headphones, so maybe I'll just couple it to the LM386 for the power amp section.


firechief 24th September 2010 04:26 PM

PorkChop - reading your first post made me ask for you to define your goal. Guitar, mP3 player, what? Also more about the speakers. Please.

M Gregg 24th September 2010 04:37 PM


Originally Posted by porkchop61 (
Thanks, I'll give them a look.
Maybe this is better:
It's for headphones, so maybe I'll just couple it to the LM386 for the power amp section.


I used this into a small vellman amp 15W+15W. It works really well if you use bypass caps on the supply! If you use a wall wart it will hum badly. I used a laptop power supply and it sounds quite good for a mini system with Ipod.:)

M. Gregg

M Gregg 24th September 2010 04:54 PM

It is component dependant. I used metalized paper caps for the input! Also you will need to have an output mute at switch on / off, because it thumps on power off! Time power off so output off then power off timer! Power on then output on! Or have a separate switch for the power and pre sections! I used an "I tube" before this, the Itube is in the cupboard since switch on!

boyofwax 24th September 2010 05:39 PM

I'd build the amp only with the chip, because it doesn't need the tube as a voltage amplifier.

If you want to expiriment with the tube and simple circuitry you'd better take the schematics (sorry it's hv) from the tube's datasheet and use it as a voltage amplifier for an SE class A MOSFET or BJT.

porkchop61 24th September 2010 06:22 PM

It's definitely NOT for guitar, I want to just power some small JBL bookshelf speakers. That's why I was afraid that the first schematic I posted just wouldn't do. It's really meant for overdrive/clipping for a guitar, and probably voiced for it as well.

I may build the headphone amp then and couple that to the LM386, thanks for the reply. Good to know about the caps and the mute switch also. I did say wall wart, but I know better than that. I have a nice clean 9V power supply that I built for my guitar pedals.

I really wanted this project to have a tube front end for the sound. Just an LM386 amp is too "normal" :D I've built enough Hi-voltage tube amps for both guitar and Hi-Fi use, I just wanted a nice little mini amp with a tube front end. Again, my goal isn't super HiFi, just something more as a conversation piece that I can use with my MP3 player at work.

Is there a better alternative to an LM386? An LM3886 is nice, but it requires 28v, and it's got a lot more power than I need.

Thanks everyone.

DF96 24th September 2010 07:05 PM

Running a 12AU7 at 9V won't even be normal HiFi, let alone "super HiFi". As SY said, it is an effects unit. As you have now said, it is a conversation piece i.e. a piece of artwork, not engineering. Why not just use a diode/resistor network if you want distortion, or mount a non-functioning valve with LED underlights if you want pretty.

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