Building a non-murderous tube amp (part 1)

I always wanted to build my own tube amp since I heard about them because they are quite expensive. The problem is that most run on high voltages (around 300V) so it's not safe to poke around in. Now I'm gonna make one that runs on 12V.

Certain tubes function fine when the B+ voltage (high voltage supply for the tubes) is way lower than 100V. One example is the 12AU7 / ECC82. This one is almost the same as the 12AX7 / ECC83 used frequently in the preamp for guitar amps. The difference is that it has 5x less gain but works fine at 9v instead of dangerous voltages.

With a very low B+ voltage it becomes impossible to get a lot of power out of the tubes so the design (so far) is a hybrid amp. The preamp is a fully tube driven circuit and the power amp will be a solid state hifi chip amp. This will give me 5 watts of output power. 

The design goals

This is what I want to accomplish with my tube amp design:

  • Modular, each tube get its own PCB so I can easily add or remove gain stages or swap out different designs.
  • Doesn't kill me. The amp won't contain any high voltage components.
  • Ridiculously configurable. Expose as many resistor choises as pots on the frontpanel.
  • Modern power supply. No big transformer and bridge recrifier but a switch mode powersupply (external, from laptop) and internally filtered/stabilized with linear power regulator chips.

In the above mockup renders you can see the general idea behind the modularity. The case contains a power supply board and has space for several amp module boards. In the second image it has a input jack module, 12au7 gain stage module (2 gain stages because it is a dual triode), a tonestack module, 2 more gain stage modules and then a master output module (containing a solid state power amp.

On the gain stage module you can see all the configuration parameters for a single tube:

  • Bypass switch (whoo! true bypass)
  • Grid supressor 1 (basically the gain)
  • Grid supressor 2
  • Bias 1
  • Bias 2
  • Output volume

By putting these potentiometers on the frontpanel you can change the complete sound of the amp without opening it up, creating a hardware variant of something like the Positivegrid Bias Amp software.

The development board

I can't just put some schematic together, order a PCB and try if it works. That will take a long time and cost a lot of money for every design change. The first stage of the design is the breadboarding stage. 

Above is the photos of creating the development board and the first circuit. The board is a cheap ebay breadboard with adhesive back and two ceramic tube holders. 

The circuit is the final circuit from the Master Thesis of Eric Nolan Sporer. That circuit is designed for the 12AX7 preamp tube, with the 12AU7 tubes the design has a lot less gain and becomes a very simple clean preamp. It delivers enough power to drive my headset from the output and gets some tube-ey distortion when pushed with a clean boost. The next step is expand the circuit with 2 more gain stages from the second 12AU7 tube.

This will be covered in the next post about this amp.