The Pignose G40V and G60VR are two incredibly underrated amps that you can usually pick up for around $200 or less. Both were designed by the well-respected Dennis Kager whose resume includes working for Ampeg, founding Sundown Amplifiers, and designing amps for a multitude of major manufacturers.
Both G40 and G60 are very similiar to distortion channel from the Sundown Artist and Rebel series amps but with some of the nicer extra features removed. The G40 is the minimalist of the two, stripped to just the standard controls, while the G60 holds onto tube reverb, an effects loop, and V1 gain boost switch. Put in terms of well known amps, the circuit is similar in design to the 2204 JCM800 with some tweaks and more gain. Most mod information online tends to focus on changing the voicing in more of a JTM45 type direction and reducing gain. I really like the 2204 JCM 800 so my mods are mostly just personal preferences/tweaks and a bit of gain reduction since I'm not quite after the crunch. I highly reccomend playing with the Amp Books online calculators or Duncan's Amps tools to understand some of the changes better.
Mar 2021 Update: Corrected error in bias section and updated the schematic.
Dec 2020 Update: I've been tinkering alot lately and have updated this section to match the current state of my amp. I've also added some better explanations for changes.
Here’s a list of modifications I’ve settled on for my G40V and am quite happy with.
Circuit changes for noise.
Added a 33k grid resistor to V1A at the tube socket. (Helps with RF noise)
Replaced the input jack with a metal shorting jack (not really needed but never hurts)
Heater GND change: (This helps with noise in the preamp alot.)
Cut the preamp PCB GND plane to isolate the R7/R8 junction, the heater GND, from the preamp GND.
Solder a new GND wire from the R7/R8 junction to chassis GND by the transformer.
4. Circuit changes for “tone”:
V1a: Input Stage and Gain Control
Changed R11 to 1.5k (standard Fender bias)
Changed C8 to 1uF (Cut the gain for low Frequencys a little for a "tighter" high gain sound)
Changed C21 to 120pF ( shift gain pot bypass freq higher)
V1b: "Cold Clipper"
Removed C9 (cuts AC gain alot)
Changed C11 to 4.7nF (shifts highpass freq with R15 to ~225 Hz)
Changed R15 to 150k (gain cut due to load on V1b and alters high pass freq)
V2a/b: DC Coupled Cathode Follower
Changed R16 to 1.2k (closer to standard Vox bias)
Changed R17 to 75k ( Bit more compression to signal and lower Vkh for tube life)
Change C12 from 1nF to 470pF (adds a bit more mid scoop, JCM800 value)
Changed R19 from 33k to 56k 9 (slightly less lows, you could probably skip this)
Removed C13 (removes low pass filter, more highs)
Removed R36 (raises max drive to PI)
Changed C17 from 22nF to 100nF (Expands feedback freq, probably not needed)
Added adjustable fixed bias
Added 10R 1% cathode resistors to V4/5 for bias measurement
Changed screen resistors to 1k 5W.
Changed to JJ 6V6S power tubes.
Changed to a 10” Eminence Copperhead.
Replaced the grill cloth and knobs because the original ones looked lame.