Marshall JCM 900 2501 50w Hi Gain Master Volume MkIII

NO Schematic

Model/Circuit Number: JCM900 2501
Years of Production:
1990 – 1999
Configuration: Combo
Controls: Gold aluminum front-facing, Controls: 1-10
Knobs: Gold Knobs with Black Pointer

  • Front: Input, Pre-Amp, Gain, Master (Channel A), Master (Channel B), Treble, Middle, Bass, Presence, Footswitch, Standby, Power
  • Rear: Effects (Loop Level, Send, Return), Line Output (Recording Compensated, Direct), Impedance Select (16, 8), 2 x Loudspeaker Jack, Mode Switch (High, Low), 2 x Output Valve Fuse, Mains Input (Power), Fuse


  • Dimensions: 29.8″ x 12.25″ x 8.6″ (759 x 310 x 220 mm)
  • Hardware:
  • Handle: Black plastic strap handle with black plastic caps
  • Feet: Rubber with plastic mount
  • Corners: Black plastic with brass rivets

Covering Material

  • Tolex/Tweed: Marshall Black Vinyl Elephant Skin
  • Grill Cloth: Marshall Black Grillcloth

Logo: Cabinet mounted, Large white plastic script Marshall
Weight: 41 lbs.

  • Size: 1 x 12
  • Impedance: 8 ohms
  • Model: Celestion G12T-75

Watts: 50 watts

  • Pre amp: 2 x 12AX7
  • Phase Inverter: 1 x 12AX7
  • Power: 2 x EL34

Bias: Fixed Bias
Rectifier: Solid-state


One Response to “Marshall JCM 900 2501 50w Hi Gain Master Volume MkIII”

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  1. Comment by Scott OlsonApril 19, 2016 at 8:56 am   Reply

    My 50W Marshall JCM900 Combo seems to be having bias issues. I’ve replaced the power tubes once with Russian made Electroharmonix and again with Groove Tubes I bought the amp second hand with. All tubes including the preamp tubes have been replaced recently with no improvement in the clean sound. The dirty, crunch sound is fine. I have only recently heard the term “Bias” in apparent reference to some circuit modification that needs to be made when replacing tubes.

    What to do?

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