loverocker wrote:
I'm sure I'll have other questions when I fire up and debug the amp, but for now my main question is whether you guys have any favourite values for the cap and resistor between the 2nd and 3rd stage? I'm planning to start with .0022uF and 68K. Thoughts?
As you know, the cap and resistor between the 2nd and 3rd stage have other functions besides the cap blocking the 2nd stages plate voltage off of the grid of the 3rd stage, and the resistor referencing the grid of the 3rd stage to ground. The resistor is also in parallel with the plate resistor of the 2nd stage when you calculate the gain of the 2nd stage. Normally that resistor is a 1meg, so it doesn't have much effect on the gain of the 2nd stage. But using a 56k to 68k resistor there will significantly affect the gain of the 2nd stage. And, I believe, that is why Mr Fisher used such a small value there. It is a way to lower the gain of this preamp without resorting to some tone sucking voltage dividers, etc.
That cap and resistor also form a high pass filter. When the resistor is a normal 1meg, this is no problem. The 3db down point is below the range of a guitar. But when you lower the resistor's value to 68k, the 3db down point will go much higher. Well into the range of a guitar depending upon the value of the cap.
Bottom line is you can't go too high on the value of that resistor or the preamp gain will be wrong. Once you get the preamp gain where you want it(by adjusting the resistor value), you can adjust the value of the cap for tone.
Sometimes builders will comment on how bright their Trainwreck clones sound. I wonder if it is excess brightness, or just a lack of low end from the effect of this high pass filter. Perhaps both?
There are people here more experienced with Trainwrecks than me, but my feeling is that these values will need to be determined by trial and error for each individual amp. The values you have chosen are certainly a good starting point. The preamp tubes, guitar, speakers, etc. you use will also play into this, so try and stick with the same ones at least until you find your first sweet spot.
I could maybe hook up a temporary pot to help find the sweet spot, though...
This is the most complex circuit I've done yet, and I'm hoping the oscillation gremlins and SNAFU pixies stay away.