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The Lead Channel

A project log for Designing a Modern Tubeamp (Matthews 14)

Ground up design of a guitar tube amp using old and new technology.

collin-matthewsCollin Matthews 01/01/2024 at 03:010 Comments

The lead channel went through multiple renditions, a lot of things did not sound good, or just resulted in adding noise. Eventually I came to the conclusion that more gain stages does not make it sound better, and the multi gain stage amps you see actually have a lot of tone shaping and phase delay happening, hence the need for the stages, not just for extreme gain.

I ended up using a single 12AX7 with two sets of tone shaping. The tube is setup as a warm / cold clipper.

  1. The 1st stage has a low cut to reduce the bass signal. This helps keep a more even distortion across the spectrum instead of power cords that are over driven and mushy, while the high end is still sterile sounding. This is implemented via C17 shown below, the 0.68uF capacitor equates to ~120Hz high pass mathematically. It also has a warm bias, so you can start overdriving the tube with not a lot of signal.
  2. The 2nd stage has a bass boost to recover some of the bass you want with a power cord. This is implemented in a less then ideal way, it has an RC network (R12 C16) in parallel with R32, this reduces gain at this stage as the frequency goes up. This stage also adds distortion from running near its cutoff region, it has a bit of non linear amplification as the input voltage swings low.

In order to help create a quieter lead channel I also reduced the series input resistance to each tube stage. Typically you see 68k grid resistors (R26 R35) on amplifiers going into each tube, this works with ~150pF of internal tube capacitance to make a low pass filter ~10Khz to stop the tube from amplifying non audible signals. By adding additional capacitance in parallel with the tube (C5 C18) you can shrink the grid resistors, and reduce the noise they add. The buzz you hear on a high gain amp is mostly coming from the grid resistors, and 60Hz from the mains transformer (which I do not have).

Shown below is the approximate circuit used for the lead channel, the simulation shows this gain stage in isolation, so all the tone shape you see is from just what is visual.

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