Fender 400 PS Bass

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

Schematics

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.

6 thoughts on “Fender 400 PS Bass

  • July 9, 2012 at 1:38 am
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    The correct output tubes are 6550A, different tolerances then the 6550.

    Reply
  • December 9, 2016 at 10:21 am
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    I am looking for a rear grill (protects tubes) for my 400PS amp.

    Reply
    • March 3, 2019 at 8:16 am
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      I needed a rear grill for mine also I stopped in Ace Hardware and they were able to order the exact type of material that the original Fender Grill was made of. It cost $18 for the sheet and it’s very easy to fabricate into a replacement Grill.

      Reply
      • September 29, 2020 at 11:02 pm
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        Do you still have rear grill? For sale?

        Reply
  • August 6, 2020 at 4:42 pm
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    Does anybody know of a tech interested in working on these amps, and familiar with the setup routine for this amp? As I understand it, the circuit design is not typical, and the setup routine as prescribed by Ed Jahns at Fender required some unorthodox techniques. The only guy I ever ran into who seemed to really know much about these amps was Rich Koerner at Time Electronics, and I was going to send mine to him, but he passed away a few years ago. I bought mine around 1978, and played a good bit until the late 80’s, and it has been sitting in a closet since then, but I’d love to have it look over and brought up to spec, assuming it may have some age related issues.

    Reply

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