Marshall JCM 900 2501 50w Hi Gain Master Volume MkIII
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
Faceplate:
- 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
Cabinet
- 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.
Speaker
- Size: 1 x 12
- Impedance: 8 ohms
- Model: Celestion G12T-75
Effects:
Watts: 50 watts
Tubes
- Pre amp: 2 x 12AX7
- Phase Inverter: 1 x 12AX7
- Power: 2 x EL34
Bias: Fixed Bias
Rectifier: Solid-state
Comments:
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?