Fender Studio Bass Club Wiki Page


Jul 6, 2019
Fender Studio Bass Club Wiki Page
  • Welcome to The Talk Bass
    Fender Studio Bass Club Wiki Page

    The Fender Super Twin and Fender Super Twin Reverb share similar circuits and make great bass heads! This club covers the trio.


    Fender Studio Bass Club Threads on TalkBass.com(top)


    Manuals(top)

    Manual Addendum(top)

    Front Panel Functions(top)

    EQUALIZER CONTROLS
    If the footswitch isn't plugged in, the EQ section is on by default.

    Rear Panel Functions(top)

    OUTPUT LINE/RECORDING JACK
    Unbalanced line level Output. It comes right off the speaker jack but is padded down to line level. Includes the power amp Tubes in Output. [Source]

    Accessories(top)

    EQ / Distortion Footswitch
    with stomp buttons for punching the 5-band EQ in and out and for turning the Distortion on and off. The unit also had a slide switch that activated a volume boost in conjunction with the Distortion footswitch.
    [​IMG] [​IMG]

    Reverb Footswitch
    A separate stomp box for controlling the reverb on off.

    Tube Diagrams(top)


    Schematics(top)

    EQ / Distortion Footswitch
    Here is how the switch would be wired...
    Distortion switch to shield and ring of the TRS cable.
    Boost switch to the shield and tip of TRS cable.
    EQ switch to shield and tip of TS cable. [Source]

    The boost switch inserts a 39 ohm resistor in Parallel with the 100 ohm resistor in the negative feedback circuit. [Source] [Source]

    [​IMG]

    Reverb Footswitch
    Wired for TS cable [Source]

    Service (Technician) Notes(top)


    Mods(top)

    Stand Alone Head Conversion(top)

    A popular mod is to take the chassis out of the combo and make it a stand-alone bass amp head. Fender did not make any of these as a head only option. If you find one in the wild it's an aftermarket conversion.

    Examples:

    Headcases
    DIY Headcase
    (H) 8 3/4" x (W) 26" x (D) 9-1/4", Chassis Cut-Out 24-1/2"
    3/4 wood material
    Finger Jointed
    3M super 90 contact cement spray for the Tolex glue.
    Feet: 1/2" Thick Sorbothane pads
    Metal Plate. Goes on top of the case between case and chassis. [Source]
    Optional: Fan

    True biasing(top)

    Option 1: The amp as it comes does not have a true bias adjustment, just an "output Tubes matching" control. A true bias adjustment can be added by subbing a 400ohm or so pot wired as a rheostat (i.e. wiper shorted to one end) for the 220 ohm resistor that's right under the matching control on the schematic. That's how Ed Jahns (designer) wired the 160PS and the 300PS. [Source]

    Option 2: Biasing Mods Alternative by Glazenn [Source]

    Option 3: I remove the existing bias circuit and install a circuit with a separate adjustment potentiometer for each power tube, making the potentiometer shafts accessible from the underside of the chassis and adding measurement points at the back of the chassis (using a six-position switch to select the tube to measure). In order to allow for separate bias adjustment potentiometers, have to install a piggy-back board with additional bias bleeder resistors and capacitors. This is according to the Gospel of amp guru Kevin O'Connor at London Power (author of the amp mod book Series The Ultimate Tone - TUT). With this modification, you can use your tubes much longer even as they age in different paces. And, as mentioned above, you can also mix different types of tubes and experiment with some interesting combinations. [Source]

    better balancing the feed to the output tubes(top)

    changing the plate load resistor on the inverting part of the phase inverter (V4B) to 180k should balance the feed to the output tubes better, a la the "classic" AA864 & AA165 Bassman and AB763 ***Reverb amps. [Source]

    Distortion Channel(top)

    Apparently, Fender tried to fix the original crappy channel with another crappy one.
    On the cathode of V3, after the 4.7K cathode resistor, they changed the circuit from just a 3.3K resistor to a 2.2K R paralleled by a diode into a coil (marked 01194 4 on the scheme). I changed the 2.2K resistor to a 1 meg ohm. It changed the whole channel for the better. It is not a heavy metal distortion, kind of a soft distortion. [Source]

    Controlling amount of boost by a potentiometer(top)

    One end of a 39 Ohm resistor is soldered to one end of the 100 Ohm resistor in the feedback loop (located in the upper right corner of the black component board). The other end of the 39 Ohm resistor goes to the right switch terminal on the old distortion potentiometer. Then one blue wire goes from the left switch terminal of the potentiometer to the rightmost terminal on top of the potentiometer. Another blue wire goes from the middle terminal on top of the potentiometer to the grounded terminal on the master volume potentiometer. That's about it!

    You can now continuously adjust the amount of negative feedback by turning the distortion knob. You can also completely turn off the boost function by turning the distortion knob all the way counterclockwise (thereby shutting off the embedded switch).

    Of course, you will have to disconnect the components associated with the not-so-popular distortion circuit before you can use the distortion potentiometer for its new purpose. But that's a different story [Source]

    [​IMG]

    reverb upgrade kit (top)

    2 to 3 spring upgrade [Source]
    Reverb upgrade kit

    Moving the input signal for the reverb driver (top)

    As others have pointed out with the Super Twin Reverb, the reverb itself does not sound very good for many settings on the five-band equalizer. The reason for this is that the input signal to the reverb circuit is taken directly after the tone stack preamp, but before the input stage of the equalizer. The reverb return signal is then mixed with the eq:ed Dry signal which gives rise to some nasty frequency clashes in many cases. To remedy this problem, I took the reverb input signal from the equalizer output (the rail from the tube anode that connects all equalizer potentiometers). And suddenly I had something that sounded very much like a nice blackface type of reverb ... [Source]

    Moving the connector jacks for the reverb tank to get rid of noise and oscillation problems (top)

    The reverb was still not as clear and Bright sounding as a real blackface/silverface one, which turned out to be due to a 0.022uF shunting Capacitor on the return jack from the reverb tank. Once I removed that Capacitor, the brilliance was back. And so was an unwanted oscillation that manifested itself when the reverb control was turned upwards 4-5. After some debugging, I realized that the oscillation was caused by too long wires from the reverb circuit tubes and the connector jacks for the reverb tank. So, my remedy was to drill new holes for these jacks as close to the tubes as possible. Now, the reverb sounded as great as I wanted it to! [Source]

    Changing 110/120 or 220/240 volts(top)

    for the benefit of those across the pond (or those who are lucky enough to cross it regularly), these come with a dual primary in the PT, so they'll work 110/120 or 220/240 depending on how you hook them up; and if you want to get real slick, wire the PT primaries to a heavy-duty 4PDT switch (and in a perfect world the ground switch and the convenience outlet to a heavy-duty 3PST to take them completely out of the question when operating 220) and you're world-Wide with a flick of a switch (well-2). See the right side of the schematic. [Source]

    Adjusting guitar amp eq bands to bass bands(top)

    Not worth it! The eq points are so similar that it doesn't really matter which one you have. [Source]

    Replacing output transformer for 4 and 8 ohm(top)

    Mercury Magnetics Studio Bass replacement (FSTB-OM)

    Wireing current output transformer for both 4 and 8 ohm(top)

    At one point Fender started using the same transformer in both the Studio Bass and Super Twin and only had a speaker output jack connected to one of the taps depending on which model it got installed into, so you could modify the amp to have both taps [Source]

    Rewire output tubes to triode mode of operation(top)

    Rewire 4 out of the 6 output valve sockets to reconfigure 2 pairs of 6L6GC to triode mode of operation. The remaining pair stay wired as tetrodes. At 90 watts this amp will still be very loud, but useable with a warmer, more musical sound. [Source]

    FAQ(top)

    What are the differences between these amps?(top)

    Studio Bass
    • 8 ohm output transformer
    • No distortion control
    • No reverb control
    • Active eq bands 1850, 1000, 390, 190, 80
    • came in a 115 combo
    • labeled as 200W
    • Tube V2: 12AU7A, Tube V3 7025

    Super Twin
    • 4 ohm output transformer
    • distortion control
    • No reverb control
    • Active eq bands 2300, 1250, 485, 235, 100
    • came in a 212 combo
    • labeled as 180W
    • Tube V2: 12AU7A, Tube V3 7025

    Super Twin Reverb
    • 4 ohm output transformer
    • distortion control
    • reverb control
    • Active eq bands 2300, 1250, 485, 235, 100
    • came in a 212 combo
    • labeled as 180W
    • Tube V2: 6CX8, Tube V3 6C10

    What accounts for the 20W difference in output?(top)

    The output watts may be a label only difference. It's been suggested the power rating difference between 180w STR and the 200w SB are just the result of Fender using a different (higher) distortion (THD%) rating for the SB. [Source]

    Is there a difference between the two input jacks?(top)

    Two inputs jacks but only one real channel. You can't jumper the two like you can with 2 channel amps like the older Bassmans or ampegs. The amp can be used like a 2 channel amp with the foot switchable active eq. [Source]

    How can the date of manufacture be determined?(top)

    http://www.superiormusic.com/page195.htm

    Studio Bass Amp was made from 1977 - 1980. [Source]
    A7+5-digits - 1977
    A8+5-digits - 1978
    A9+5-digits - 1979
    F0+5--digits - 1980
    F1+5--digits - 1981

    Potentiometer and capacitor date codes can be used. how?

    Can an 8 ohm speaker load be used with the Super Twin or Super Twin Reverb?(top)

    Yes, 8 ohms will be fine. On a fender one can safely go 100% in either direction. For the best tone and most efficient power output, 4 ohms is recommended. Matching ohm load is more important with tube amps than Solid state amps. [Source]

    Why does the distortion sound so bad?(top)

    Try it loud. REALLY loud. You might like it better. It's a very dated distortion sound, and great for the kinds of music by the people who played the amps. Ted Nugent is an example artist. It's definitely good at ONE thing, and only that one thing. But yes, the general consensus is that if it sounds like Butt-Ass-Crap, the distortion circuit is working just fine. [Source]

    Where can I get parts?(top)

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