/diy/ - Do It Yourself

In certain guitar amps you find valve pre amps with a so called shared cathode configuration. Basically two triode halves "share" their cathode resistor and cathode bypass cap (pic related left) The signal gets amplified by each half (in parallel) and the there is a slight difference in how they signals get filtered. One half has capacitors on the pots and mixer resistor so that the signal has way more treble at any time. The other half does not.

Now I wonder: Why would you need *both* triodes? They amplify the same signal before it gets filtered. Therefore should one triode not be enough? Pic related on the right.

Spice seems to agree with my theory, the difference in my simulation is really, really small. Amplitude and gain is basically the same, or actually even higher. Only frequency response differs very slightly at 5k and above.

So why is this done? Can anybody school me on this?