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m145mcc 1st January 2013 12:17 PM

Hybrid amp design
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This is my first post so be gentle. For the last few weeks or so I have been wanting to build a hybrid guitar amp, this is my half-baked attempt. I mutilated Orange’s tiny terror amp design. I wanted to strip the power section and reduce the preamp section to just one duel triode tube (12ax7), while still maintaining the clipping characteristics of the amp (to a degree). To drive the tubes into saturation I used a high voltage op-amp (OPA-452). I then attenuated the output of the tubes so I could run the signal into a small chip amp, something similar to a gain clone. I’m posting this in hopes someone more experienced in this subject has any advice. I would also like to know if it’s a good idea a use a voltage divider as an attenuator, good sense tells me there is a better way of doing this. I can tell you right off the bat I know there are a few things wrong with my design; I am here seeking the infinite wisdom of the forum.

Mooly 1st January 2013 12:33 PM


I've deleted your other thread :)

m145mcc 1st January 2013 03:09 PM

I was also thinking about using 12 vac to power the tube heaters, bad idea?

DUG 1st January 2013 03:36 PM

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Based on the 150V supply and 100K plate resistor the load line is something like the red line in the jpg.

If you were designing (close to) symmetrical compression or clipping in that stage then I would think the bias point should be close to 1V.

That would make the cathode resistor 1V / 0.5mA = 2K

And that stage should be able to respond to about 1Vp before the onset of clipping.

Now, how much do you really need to overdrive this stage? 30Vp?

Just a question.

Someone more experienced in guitar amp design should answer that...I would like to know as well.

m145mcc 1st January 2013 03:48 PM

Thanks for the feed back. I'm going to look into that right now. May I ask where you learned so much? I need a shove in the right direction.

DUG 1st January 2013 03:48 PM

Looking at it again the 1K5 is OK...puts the bias point at about 0.9V.

Don't change it on my assumptions.


m145mcc 1st January 2013 03:53 PM

To be honest I literally know nothing about tubes, any info helps.

Robert Kesh 1st January 2013 05:18 PM

The attenuation is way too much. 300/100300 is an attenuation of 334 times. That load line can swing about +/-55 (less when you consider the AC load line, and even less when you look at what's going on at the cathode, but lets be conservative here) which is about 38V rms and typical line level is 1V rms, though you can set your chip amp up to accept a bit more.

So maybe a 100k and 3k3 would be better. Or maybe 220K and 6K8.

Did you raise the voltage under the second triode to increase clipping?

DUG 1st January 2013 05:39 PM


Originally Posted by m145mcc (
Thanks for the feed back. I'm going to look into that right now. May I ask where you learned so much? I need a shove in the right direction.

Look at schematics and read the data sheets...of everything you you can find.

There are many online "learn electronics" things on the internet.

Oh, and the basics are very, very important.

One phrase that keeps popping up is that it takes 10,000 hours to be really good at something.

DO NOT be discouraged by that large number. You can enjoy DIY with much less than that.


m145mcc 1st January 2013 07:05 PM

Nope, both triodes are at about 150 vdc. Now that I think about it your right, my attenuation is a bit low. Would I be better off using an audio transformer to to attenuate or just stick with the resistors? thanks for value recommendation.

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