This is the digest version of all current circuit mods in Europa, my Marshall TSL. The post on Anchor States should be considered outdated now, but will provide useful background info about the process necessary to arrive at these changes.

I don’t consider this information to be any sort of “secret recipe” even though I do perform these mods for clients. The knowledge isn’t the primary value contained in the service that I offer — it’s the workmanship. This amp has been on the road for four years now, done over 400 shows in 20 countries and 200 rehearsals, recorded 2 albums and 3 splits, and has never once had a failure. It even recently took a severe beating from US customs but was working again within two days. I’m proud of that track record and think it speaks for itself. So if you’re local to me and interested in these mods, but are concerned about working on your amp yourself, get in touch.

Mods in italics have the highest impact.

Mainboard:

• R71 replaced with Mercury MC10H choke - stiffer supply, better bass on transients at high volume
• Rectifier diodes (D3-D10) replaced with 1kV FREDs - not much change, theoretically lowers hash/hiss/switching noise
• C18 changed to 470pf - DSL value instead of TSL value, bleeds more high frequencies off after initial distortion stage (V1AA) in crunch and lead channels
• TR2 removed - eliminates high-frequency boost @ ~7kHz (cathode bypass, V2AA) in crunch and lead channels
• TR3 removed - eliminates reverb muting on channel switch, introduces popping in some situations but makes reverb transition more natural
• C46 changed for same value, 3000V part - prevents it from shorting if hit with a flyback pulse, e.g. from an open load

Lead panel PCB:

• C9 changed to 100pf - bleeds less high frequencies past gain pot at lower settings (no effect when gain at max).
• R1 changed to 1k - increases current through optoisolators, speeds up switching time (very small effect, probably shouldn’t bother)
• C39 removed - eliminates lag/delay when switching from either drive channel back to the clean channel (listed as “C40” on lead PCB schematic, connected to RL1 and RL4 coils)
• 68pf mica cap added across VR2 (channel volume) - first-order lowpass filter bleeding off treble frequencies above ~4.5kHz
• Values of R25 and R26 reversed - increases available reverb mix on clean channel, decreases available reverb mix on both drive channels
• Replace C36/C37 with higher voltage parts, i.e. 500V or 630V - for reliability, these caps fail if a power tube shorts plate or screen to control grid, then bias gets mysteriously dragged down and the problem is extremely difficult to locate.

Crunch PCB:

• 68pf mica cap added across VR2 (channel volume) - first-order lowpass filter bleeding off treble frequencies above ~4.5kHz

Clean PCB:

• FET1, FET2 removed - speeds up switching at the cost of occasional pops (not noticeable at high volume)

Rear PCB:

• Jumper wire added across switching sleeve (ground) terminals of 16-ohm speaker jack - prevents open-load condition from oxidized ground contacts when using 4- or 8-ohm speaker jacks (from Jerry/FJA mods, see here)

Other:

• Mercury Magnetics MAR100-OM output transformer - bandwidth, headroom, reliability
• Reverb tank replaced with Accutronics Type-9, 9BB3C1D - longer, more complex reverb sound

Notes:

• Using high-speed optoisolators does NOT speed up switching time, and the more recent NSL-32 optos seem to be less reliable than the first generation. Leave the optos alone.
• Do NOT increase the size of interstage coupling caps if you downtune, it just makes the distortion farty.
• Avoid strapping a cap across R1 on the mainboard for “fizz-reduction,” since it will affect all 3 channels, potentially making the clean channel sound dull.
• FREDs really aren’t worth it. Try some UF4007s if you must change the diodes, they’re cheaper and nearly as fast as FREDs.
• If the footswitch goes “bad” it’s almost certainly just cracked solder joints. Open it up and touch up every solder joint in it. I’ve been kind to the cable on mine and never had a problem with it, despite its negative reputation.
• If you have an older JCM2000, check the grid stopper resistors on the power tubes (R7, R10, R66, R70). They should be 5.6k as called for in the schematic, but early production runs had 220k instead, making the amp sound muffled and “woofy”. You can just “piggyback” some 5.6k’s on the 220k’s without removing the board, if you need to make this change.
• The infamous bias-drift issue in these amps is caused by the mainboard PCB material becoming semi-conductive when heated up. The problem is more noticeable in the combo versions of the amp, because the tubes are underneath the chassis. The only solution for this is to replace the mainboard. My amp, made in 2005, has a later-issue mainboard (issue 6) with a new PCB material. The new material looks glossy and is a dark, cool green compared to the other PCBs. The old material is dull/matte and has a lighter, more yellow green color. The oldest boards also tend to be populated with carbon resistors (tan) that are more sensitive to heat; most later boards have metal film resistors (light blue) with smaller thermal coefficients (resistor type does not necessarily tell you which PCB material you have, some mainboards have metal film resistors but old PCB material).
• Schematics available here.