Note: there are many versions of the schematic for this amp, proceed with caution!! VERIFY which version of the board you have by matching part numbers with the schematics posted here. Part numbers in this mods list match this power amp schematic and this preamp schematic, which are the most common that I’ve seen.

Preamp mods (4100 only):

• Cut down the “fizz” by strapping a 68pf capacitor across R60. This dumps treble after the tone stack, getting rid of some of the “bee swarm” sound from the distortion.
• Reduce the channel B diode clipping. You have two options here — either remove BR2 entirely, or replace D10 with a red LED. If you remove BR2 altogether, channel B will have about as much gain as channel A, but with a different voicing (more midrange, more growl). If you replace D10 with a red LED, it raises the diode clipping threshold and provides a softer clipping characteristic, for a more natural and open overdrive sound — with more gain than channel A, but less than stock channel B. Be sure to observe correct polarity.

Power amp section mods (applies to 2100 and 4100 models):

• First make sure that your bias circuit is properly configured for the type of tubes you’re using. Many 900s converted from 5881s to EL34s had the bias circuit incorrectly configured. The earliest schematics have a bias voltage divider made of a 15k (R29) and 47k (R28) resistor, with a trim pot of 22k connected to ground. Many 900s end up with a 56k resistor in R28 instead of a 47k, which puts the bias voltage too high for most EL34s, running them far too cold. (Don’t change these resistors unless you have a bias probe and are able to bias the amp afterward.) 56k is better suited to 5881s, but there ARE schematics floating around with EL34s and a 56k value on them. Use a value that gets you the bias voltage you need.
• If you want, change your screen resistors (R31-34) from 2.2k/4W to 1k or 1.5k, 5W. These are more standard Marshall values. Keep the leads long-ish so that they can radiate heat up away from the board. In the picture below you can see the heat damage caused by the original screen resistors being mounted close to the board.
• Replace C15 (47nF “class X” capacitor which feeds D1/the bias supply) with an equal value cap with a higher voltage rating. This cap is underrated in the early 900s, at around 250VAC (later schems call for 630V), and fails all the time. When it goes, goodbye power tubes. I usually replace it with a 1000V/47n film cap (picture below is an absolute overkill 3000V part).