Fender 400 PS Bass

Schematic/Layout:

400ps.pdf

Model/Circuit Number: 400PS
Years of Production: 1970 - 1975
Era:
Configuration: Head
Controls: Black forward facing with white labels
Knobs: Chrome skirted -10 Knobs
Faceplate

Front: Power Sw, Standby Sw, Pilot Lamp - Bass: In, In, Deep Sw, Vol, Treb, Bass - Normal: In, In, Bright Sw, Vol, Treb, Mid, Bass, Reverb, Speed, Intensity - Master Volume
Rear:

Cabinet

Dimensions: 11½'' x 28'' x 12¾''
Hardware:
Handle: Black Strap (x3)
Feet: Casters
Corners: Chrome Corner with Lip

Covering Material

Tolex/Tweed: Black Tolex
Grill Cloth: Blue Grill Cloth or Black Grill with White Trim

Logo: Grill Mounted, Raised, Chrome and Black Script Fender with tail
Weight: Cabinet: 126 lbs ; Head: 84 lbs
Speaker

Size: Fender Bass Bottom Ext Cab 1 x 18 with folded-horn design
Impedance: 4 ohms
Model: (For more info, check out the Jensen Replacement Speakers)

Effects: Tremolo, Reverb
~Watts: 435 watts
Tubes

Pre amp: 2 x 7075 ; 12AX7, 6L6GC
Power: 6 x 6550

Bias: Fixed
Rectifier: Solid State

Comments: The 400PS was the first large amp made by CBS, and the largest ever made by Fender. The chassis alone weighs as much as a Twin Reverb.
The output of the 400PS was divided into three 145 Watt power sections so it took three cabs to get the entire 435 Watts.
Keith "The Arkansas Tube King" notes:
1.The correct tube for position V6 is the 12AU7 - NOT the 12AX7 as shown on Fender schematic.
2. As well, there is a 12AT7.
I realize the schematic shows a 12AX7 in the V6 position, but anyone familiar with the 400PS design knows that this is actually an error on the schematic as originally issued by Fender/CBS If one places a 12AX7 in V6, the amp will function, but the 12AX7 will not be able to drive the 6L6 properly, resulting in reduced output power.
Thus the 400 PS has several unique circuit innovations, as compared to the typical Fender design.
1. A single ended triode Class A driver stage (!)
2. No "long tail phase splitter" as is seen in most Fender class AB1 amps.
3. An interstage transformer for phase splitting (a concept shared by a very few other Fender amps.)
4. 6550 Power tubes operating in CLASS AB2 mode to achieve higher power output as compared to other brands' 6550 equipped designs.