As with all projects, there comes a point when you find you've exited your comfort zone, and this mysterious world of endless yet seemingly unachievable possibilities lies before you, leaving you with the choice of trudging forward magnificently or collapsing into a weeping puddle of ineptitude (panel 8/9). I'm currently doing a little of both.

Here's a picture of the schematic for the BH5 which I found here, with thanks to Craig Barnett for creating it.

I've started building this design stock (hope to have it functional within a month or two), but in looking at the project, I've found my first major design hurdle (one which should have occurred to me long ago, but oh well). Notice that tone stack? Notice how it's after the first valve? From my limited understanding, the first valve increases the input signal, then the tone stack absorbs much of the power which the second valve replaces before feeding the signal to the output tube. Amplifying the signal before the EQ helps with the signal-to-noise ratio. Now think about replacing those tone stack pots with digital ones. Yeah, that's the problem. 

The high voltages in the tone stack might be too high for the vast majority of digital pots, not to mention some other nasty situations might occur during start-up or shutdown of the amp when various caps charge/discharge. I've found some digital pots, like the Analog Devices AD5290, which can handle 30 volts, but I'm not sure if that's enough depending on the situation. Digital pots sound very bad when they clip, so even a little extra voltage above the limits, while it might not damage the chip, isn't acceptable. And even though I might be able to get away with digital pots on the 5W Spuntone design, the 200W design might require higher voltages in the preamp stage (although it might not... after all, it's the preamp stage).

There are alternatives to digital pots, however. JFET (second link) or MOSFET transistors can be used to create voltage controlled resistors rather simply. While more complicated, op-amp VCR's (page 12) are also an option to consider. Is it possible to move the tone stack to before the first tube? Maybe, although it would need to be redesigned for the drastically lower voltages and far less optimized signal-to-noise ratio (this would effectively be the same as placing an EQ in your effects chain and replacing the tone stack with fixed resistors or removing it altogether). 

Either way, it's time to hit the workbench and the simulator, not to mention the phone to call my buddies who got good grades in college for some pointers. Now that my home and work lives have finally calmed down, I've actually got some time to devote to my hobbies again; but still, the end-of-2014 deadline is starting to look reeeeeeal shaky.