Wednesday, 3 August 2016

We have a preamp stage

Progress in the designing phase - we have a preamp stage.

The topology is simple: Grounded Cathode initial gain stage using half of a 12AX7, giving approx. 50x gain, AC coupled to a split-load (or Cathodyne) phase splitter using the other half of the 12AX7. This section has unity gain.

From there, each phase of the signal is AC coupled to half of a 12AU7 to give the necessary voltage gain to drive the KT88s I plan to use on the output.

The 12AU7 driver stage should give around 10x gain.

This is the preamp design:

Click the image for a readable size

I could have done this the easy way and just stolen a circuit off the internet. But my objective was to learn something, not just assemble a jigsaw puzzle. So I have spent many hours in the last week reading about bias points, gain, grid resistors, different phase splitter topologies, etc...then I spent a long time modeling in TubeCad and then double-checking with the calculator. This means I know what's going on in this circuit (well, that's the theory anyway)

So we should have enough gain here to power the KT88s to full power from a 100mV input signal at the grid of the first valve.

It will be very interesting to build this and see how far my measurements differ from the theoreticals predicted by TubeCad and my calculations.

These are the bias points I've chosen

Bias Point for 12AX7 Initial Gain stage

 Bias Point for 12AU7 Driver stage

Next up - a peer-review of my preamp stage, then on to designing the output. Stay Tuned...

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