Marshall JCM 900 2501 50w Hi Gain Master Volume MkIII
Model/Circuit Number: JCM900 2501
Years of Production: 1990 – 1999
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)
- Handle: Black plastic strap handle with black plastic caps
- Feet: Rubber with plastic mount
- Corners: Black plastic with brass rivets
- 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
2 thoughts on “Marshall JCM 900 2501 50w Hi Gain Master Volume MkIII”
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?
Check values of bias resistors, screen and dropping resistors in the power supply.