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all tube 12v amps
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Monday, 22 May 2006

This page contains informations about my latest projet, building a low voltage low volume ALL tube amp.

I have already played with the idea of making an hybrid amp (tube preamp into a solid state power section). The result was nice (see the ruby_tuby) but, it still was a bit of a pain to work with and didn't have the sound I was looking for.

Enter low voltage tubes. There was an entire series of tubes that were ment to operate with a max b+ voltage of 30V! This was exactly what I was looking for. Originally, those tubes were intended to be used in car radios using the 12V battery to run them. This makes the initial design very easy, b+ and heater power = 12V. Therefore, we could get by using a single power supply. This was possible, I just needed to ensure that the volume was adequate at such a low voltage. While investigating using low voltage tubes (also refered to "space charged" tubes), I selected a few possible candidates:

  • 12U7 for the preamp. It's a dual triode very similar to a 12AU7 but with an gain factor of 20
  • 12K5 for a tetrode power tube. This is can supply roughly 1/4 watt on 12V.
  • 12CX6 for a pentode power tube. This is supposed to supply roughly 1/2 watt on 12V but I couldn't get it to be as loud as the 12K5...

Sopht amp using a 12K5 as a power tube

Last version Sopht amp using a 12K5 as a power tube - updated 10/12/2004

Ok, I thought that I had finished playing with the 12k5 amp, but, I guess not! After doing a lot of reading and some comments about the lack of bass in the last incarnation of the amp, I went back to the drawing board. I basically read all sorts of stuff about frequency and tone of each of the preamp stages and how caps affect it. My first experiment was to mimic the frequency response of a champ amp. Since the 12k5 is a 2 tube amp without a tone stack, I thought it would be perfect to try and capture the tone of the champ amp. Since I don't have a Champ amp (yet), my only point of reference was a POD emulation of the amp. I think that it has the same character as a champ but not the same sound. Here is the sound clip for the champ version. And here is the schematic:


Luap has built a Sopht 12K5 and has a very nice site about the Sopht amp and his build. He also developed spice models for the 12U7 and the 12K5. Please click here to go to his site.

Scoping the Sopht 12k5

Well, after what seems like an eternity, I finally got around to putting a scope on the Sopht amp. Nothing like a little xmas vacation to get the time and incentive to do it! Please note that the pictures suck but, we can see the info that we need. I used the simple CD player as a signal generator trick. Bascially, you generate a 1k sinwave WAV file. Burn this WAV to an audio disk and use the output of a small CD player as an input for your amp! Here is a picture of the input signal (.1v peak to peak)

Here is a picture after the first stage, no clipping seems to occur at this stage. Even when the input signal increased, both sides were symetrical. The voltage divider on the first stage was .2v. We seem to be getting a gain of about 4 on the first stage.

Here is a picture after the second stage. We can see some clipping at the second stage. The sides aren't symetrical and there is a bit of compression on the top part. Voltage divider was 1v so, again about 4 as a voltage gain.

Here is a picture taken at the output transformer. We can see where the distortion comes from, yup, the power tube! We can see some clipping happening and it does increase as the input goes up. The voltage divider for the output stage is .5v. Breaking out the formula to calculte power of an amp (V * V / speaker impedance), we have .5 * .5 / 8 = 31mW, yes, milliwatts. That is inline with the "power" expected from these tubes at 12v.

Sopht amp using a 12CX6 as a power tube

Sopht amp using a 12AL8 as a power tube

The is the latest version as of 08-JAN-2005

I have hooked this up to a speaker and it doesn't sound good. As a pedal, it sounds very nice but not as an "amp". The version 2 is still the best choice so far. This is the latest version of the 12AL8. It will probably move some more, but it is a very nice version. It's basically a 12k5 preamp with a 12AL8 section. There is preamp and power amp distortion all of which comes from the 12AL8. You can listen to a clip of it here. The clip was recorded using a direct in box. This would make a nice tube pedal as well! Please note that the pots should be log or audio pots. Here is the current schematic:

older versions

This was all nice, but, I still wanted more gain. Further investigation revealed a marvelous tube, the 12AL8. This tube is a power tetrode with a medium gain triode in the same tube! This gave me the luxury of adding a third gain stage. I rebiased evertyhing because I think that I was getting more preamp distortion than power distortion. Anyways, I ended up making the desing a bit simpler (go figure!). Here is a short sound clip of the Sopht amp with a 12AL8.

Sopht amp using a 12AL8 as a power tube - Version 2

After playing a bit with the design of the Sopht amp using a 12AL8, I played a bit with a few resistors. The first was to put a resistor in series with the output transformer. The datasheet specifies a load of 800 ohm, so, I added 200 ohm in series with the 600 ohm transformer. This "cleaned" up the sound a bit, but it was a very good cleanup. Without the resistor (r9), the distortion was very POW. With the resistor, the distortion is alot more natural. I also played with the resistors R6 and R8 to bring them to datasheet spec (2.2Meg specified in the datasheet). This brought in some nice low end! Another very nice mod! Finally, I backed down the R7 resistor as I felt I still had a bit too much gain (can I say that :) ). I backed it down to 10k and I know have a nice hard rock amp!


Rich Chernock has built a 12AL8 and sent me a few pictures of his creation. The only modifications that he did was to add a volumne control between the first two gain stages. As you can see from the pictures, it is excellent worksmanship!

Last Updated ( Friday, 23 February 2007 )