andrewsrea
Well-known member
I built this Marble Drive in 2022, which is based on the 'Marvel Drive', to which I call mine 'Prodigy'. It was built to the schematic and bias values people cited, which is pretty much a Marshall model 1987 / 1959 (50w / 100w), except using J-201 FETS instead of tubes. It was really hard to bond with the pedal and I was having a hard time pinning down why, beside the fact that FETS don't quite respond like tubes.
Having recently reacquired a genuine vintage 1972 Marshall Super Lead, which I modded in 1991 (pictured below), I started comparing and adjusting. BTW, this is one of the finest sounding Marshalls on the planet, IMHO. I used a 1965 Fender Twin into a 4x12 Marshall 1960A cab loaded with Celestion 'Classic Lead 80's' for the comparison. I used a homemade guitar with a 12.7K Alnico 2 humbucker in the bridge, a P-90 in the middle and a Duncan SH- PG1N in the neck, with convincing coil splitting.
First, the FETs are way too efficient with the treble frequencies making the pedal extremely harsh, which was solved by unsoldering the Bright channel 470pF bright-cap C4.
Next was the right balance of 'tight & hard' vs. 'woolly, compressed & creamy'. FET bias trim had the most to do with getting this right. It could get constipated and spitty, all the way to mushy, depending on the bias settings. Using the 9v (50W) mode, I trimmed Q1 & Q2 Drain-Gate to around 6V, got the gain (not too little and not too much) and the clipping symmetry (slightly asymmetrical). Then I trimmed Q5 by ear using the bridge humbucker, starting with an overly starved setting and adding Drain voltage until it matched the real amp (using an A-B switch). The actual amp has a switch which goes from 50w to 100w, and the pedal pretty much reacted the same between the 9v & 18V settings.
Last was response / feel, which was basically finding the sweet spot on the Master Volume and Presence controls. This is all about output impedance and it ended up as pictured below. It is now a wholly different pedal, which sounds right in either mode and with single coils or humbuckers!
I'd be interested in hearing anyone else's experience / mods with the Marble Drive.
Having recently reacquired a genuine vintage 1972 Marshall Super Lead, which I modded in 1991 (pictured below), I started comparing and adjusting. BTW, this is one of the finest sounding Marshalls on the planet, IMHO. I used a 1965 Fender Twin into a 4x12 Marshall 1960A cab loaded with Celestion 'Classic Lead 80's' for the comparison. I used a homemade guitar with a 12.7K Alnico 2 humbucker in the bridge, a P-90 in the middle and a Duncan SH- PG1N in the neck, with convincing coil splitting.
First, the FETs are way too efficient with the treble frequencies making the pedal extremely harsh, which was solved by unsoldering the Bright channel 470pF bright-cap C4.
Next was the right balance of 'tight & hard' vs. 'woolly, compressed & creamy'. FET bias trim had the most to do with getting this right. It could get constipated and spitty, all the way to mushy, depending on the bias settings. Using the 9v (50W) mode, I trimmed Q1 & Q2 Drain-Gate to around 6V, got the gain (not too little and not too much) and the clipping symmetry (slightly asymmetrical). Then I trimmed Q5 by ear using the bridge humbucker, starting with an overly starved setting and adding Drain voltage until it matched the real amp (using an A-B switch). The actual amp has a switch which goes from 50w to 100w, and the pedal pretty much reacted the same between the 9v & 18V settings.
Last was response / feel, which was basically finding the sweet spot on the Master Volume and Presence controls. This is all about output impedance and it ended up as pictured below. It is now a wholly different pedal, which sounds right in either mode and with single coils or humbuckers!
I'd be interested in hearing anyone else's experience / mods with the Marble Drive.
