• 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.

Small Tube/SS hybrid amplifier design 9V

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porkchop61

porkchop61

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Joined 2004
webpages.charter.net/porkchop/
#1
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:http://www.beavisaudio.com/Projects/ValveCaster/MatsuminValveCaster.gif 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.
Glenn
 
#6
porkchop61 said:
Thanks, I'll give them a look.
Maybe this is better:
http://diyaudioprojects.com/Solid/1...damp/12AU7-IRF510-Headphone-Amp-Schematic.png
It's for headphones, so maybe I'll just couple it to the LM386 for the power amp section.

Glenn
Click to expand...

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.🙂

Regards
M. Gregg
 
#7
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!
 
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#8
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.
 
#9
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" 😀 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.
Glenn
 
#10
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.
 
#11
I've already agreed that the schematic in the first post is not good for what I'm looking to do. I was now focusing on the second schematic in post #4. Since this is a headphone amp, I would think it would have to be halfway decent to listen to. I'm just thinking of coupling that to some sort of small chip amp for a compact stereo amp that has a nice tube sounding front end to it. This approach also has the added benefit of being able to use it with a pair of headphones and bypassing the power amp section altogether.

Glenn
 
#12
But it still has far too low a supply voltage, so the only "tube sound" you will get is distortion. If you want to use a valve then you need 30-40V minimum; even then you will get some distortion. Valves don't magically wipe away "SS sound" - like any other component they either add/subtract very little (when used correctly) or add their own distortions (when used badly).
 
#13
DF96 said:
But it still has far too low a supply voltage, so the only "tube sound" you will get is distortion. If you want to use a valve then you need 30-40V minimum; even then you will get some distortion. Valves don't magically wipe away "SS sound" - like any other component they either add/subtract very little (when used correctly) or add their own distortions (when used badly).
Click to expand...

I know I thought the same thing so I built it! How can a tube run on such a low voltage. However in the link the guy refers to the tube being used in low voltage applications in the battery circuits in the military! Not with an inverter!

So I built the circuit out of curiosity and I have to say it sounds quite good even when amplified at party levels! OK it's not hifi, however it gives mini systems a run for their money! You can hear people breath as they sing and the wood on acoustic guitar!

Regards
M. Gregg
 
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#14
The sound reminds me of an old HMV record player with better high frequency response!

I thought about ECC83 ECC81 etc however this seems only true for the 82. I have an old RCA 82 playing kate bush as I type this!
 
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#17
It was done as a fun project!

Just for fun I built mine in a cigar box!
LOL couple of hours after work!

🙂

DF96 I have now put an LED under the tube, because it annoys people!
 

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