About the amp:
So starting at the input jack:
On the GT120s the input impedance is ~220k or so because they put a 220k resistor from the input jack to ground, where as we've used a 1M (standard for many amps) . This is a small change since that is not a huge decrease in input impedance and can still be easily driven via your guitar or any pedal.
We've removed the original Drive control which is a combination of a Bright switch on/off and a feedback based attenuation circuit. When the Drive is in the Low position (Bright cap on or off) there is a negative feedback loop around the first gain stage which effectively reduces the stages gain significantly (slight changes in frequency response as well).
Depth is also called Bass Boost on some GT models, sometimes also called Depth (OrMats and some Matamps) they tended to change around control names, we have expanded it to 12 positions with sequential caps rather than the values used on the original.
We have removed the option of a bright cap all together.
Gain pot (known as Volume on the originals) is now 500k instead of 1M because I added a 470k resistor from the Depth to the gain pot so that there is atleast a 1/2 reduction in the signal from stage 1 to stage 2. in the original there is no fixed reduction in signal from stage 1 to stage 2 (when the drive control is set to Hi) and this can result in blocking distortion when you crank the volume know up to a certain level.
Although this will reduce gain by a smidge it will prevent gross sounding distortion and its beneficial in the long run.
The EQ is the same.
instead of the original drive control I have put a pre-phase inverter master volume after the EQ to act as the Drive control (note the bottom lug of this pot a 10k resistor should be placed to ground so that the pot will not kill the sound 100% hence preserving the "Drive" aspect of the name).
We have removed the negative feedback circuit from the amp (often done in 18W for extra distortion).
We have changed the LC-C (aka as the mid lift circuit) from the phase inverter driver and instead have put a 50uF cap there for max gain across all frequencies.
including the Drive control in the design will allow you to control the signal throughout the preamp so making this stage have max gain will help recover a bit lost due to 470k resistor on the gain pot.
Phase inverter design is the same.
all of the supply voltages are much lower in this compared to the original GT.
Pics:
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