Instructables

Step 9: The Preamp Schematic

Picture of The Preamp Schematic
I intended to include a section here on using loadlines to design a preamp stage. I think I'll hold off, and keep it general.

There first preamp stage uses very typical guitar-amp values for the plate resistor and cathode resistor. Much of the tone is formed here.

The second preamp stage is "goosed" quite a bit. More conservative values might still be inserted here. This amp will feedback at some settings, and squeezes a fairly aggressive tone from only two preamp stages. Since it's controllable, I'm OK with that, for now.

For both preamp stages, smaller cathode bypass caps will shape the tone in a brighter direction.

One, or both of the coupling caps could be changed to 0.01 (vs. 0.02) for more treble.

--On repeated playing, this amp is a little "bottom heavy"; yet it never gets muddy. It just doesn't have stinging highs, except from distortion. It's more a Marshall-ish rather than a Fender-ish sound (actually, it's really more Supro-ish or Magnatone-ish than Marshall...)

But note the input section. This is pretty typical wiring to create a input variation with only three resistors. The bottom input, which is a ground reference followed by a grid stopper resistor, has a more "bassy" sound. The top input jack uses the 56K resistor together with the other two to form a voltage divider, attenuating the signal. This is a a standard Fender-style "pad" input scheme. The divider input looses some "omph," but seems to retain a bit more high-end.

I rather prefer the Fender tone. At some time in the future, I might change the coupling and a couple of the bypass caps to enhance it....

Or I might just leave it alone--jangly, harsh, high-treble amps are a dime-a-dozen. Might be fun to keep it as-is.

Other Possible Mods

--Rather than change any values on the the second preamp stage, a negative feedback loop could be added. This would tone down the "ballsy-ness" a bit.

Negative feedback could also be adjustable. A second "presence" control, if you will...

(NOTE: I tried it, but the amp became "farty", so I ripped out the NFB loop right away.)
 
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