CIRCUIT DESIGN OF THE FENDER BASSMAN 5F6-A

The Preamp

The Fender Bassman 5F6-A feeds both the bright and the normal channels through the gauntlet of two amplification stages, a cathode follower, and a tone stack on their way to the phase inverter. This contrasts with the Vox Top Boost AC30 which reserves all the extra treatment for just the bright channel. The low-gain input jacks for both channels are connected to an inverted-L network that acts as a voltage divider to attenuate the input signal by half, or 3dB.

Fender Bassman 5F6-A low gain input jack

The high-gain input jacks connect the guitar circuit to a gamma network formed by the grid resistor RG and the grid-stopper resistor RGS.

Fender Bassman 5F6-A high gain input circuit

The resistance Ro is the output impedance of the guitar and represents the ability of the guitar circuit to drive the amplifier input. An output impedance of zero ohms signifies that the guitar is a pure voltage source that can supply any amount of current to the next stage. A non-zero output impedance represents the decrease in voltage magnitude that occurs depending on how much current is required. The other parts values are

RGS = 34k
RG = 1M

(RGS represents the effective value of two 68k resistors in parallel.) The gamma network exploits the capacitance between the tube's electrodes to attenuate radio frequencies, as demonstrated by our Grid Stopper Resistor Calculator. When we use the 12AY7 voltage gain of 35 (that we will determine shortly) the calculator shows that the -3dB attenuation frequency is above 100kHz and the gain is down by only a tiny fraction at 32kHz, so this is excellent treble response even by high-fidelity standards.

First Stage DC Operating Point

Here is the first preamp circuit for the normal channel:

Fender Bassman 5F6-A normal channel preamp circuit

The parts values are

RL = 100k
RV = 1M
CG = 0.02uF
RK = 820
CK = 250uF

The bright channel adds a bypass capacitor to the volume control, which we will discuss in a moment. At DC all the capacitors are open circuits, so the bright and normal channels have identical DC operating points. Under DC conditions the cathode resistor RK carries the current of both 12AY7 triodes, so its effective resistance per triode is 1.64k, double its nominal value of 820 ohms. For a plate supply voltage of VPP = 325 and a plate load resistor of RL = 100k the load line (red) and grid lines (blue) for RK = 1.64k are plotted here:

Fender Bassman 5F6-A preamp load line and DC operating point

The lines intersect at a DC grid bias voltage of minus 2.8 volts. The DC operating point for the bright and normal triodes is thus defined by a quiescent grid voltage, plate voltage, and plate current of

VGQ = -2.8 volts
VPQ = 157 volts
IPQ = 1.65 milliamps

Voltage Gain and Output Impedance

When two triode amplifiers share the same cathode resistor we want to ensure that RK is fully bypassed by CK. Otherwise the two triodes become a common-cathode differential amplifier instead of independent amplifiers. The 250uF capacitor effectively shorts all audio frequencies to ground, thus completely bypassing the resistor and providing maximum gain. This is demonstrated by our Cathode Bypass Capacitor Calculator. The calculator value RG refers to the grid resistor of the following stage, because it represents the load that the preamp is required to drive. In the case of the Bassman preamp this is a 1M volume control. The calculator shows that the voltage gain is 34.5 for for all audio frequencies.

The output impedance of the first stage, which represents its ability to drive a load, depends on the plate load resistor value RL and whether the cathode resistor RK is fully bypassed. Our Preamp Output Impedance Calculator shows that the first stage has an output impedance of 20k. This compares to 38k for a 12AX7, which has a higher amplification factor for more gain. The 12AX7 also has a higher plate resistance, however, which makes the gain more susceptible to being dragged down by the load of the next stage.

The calculator shows that the unloaded gain for the Bassman's 12AY7 preamp is 35, representing the voltage gain that is achieved if the preamp is disconnected from the volume control. The preamp's 20k output impedance causes the voltage gain to sag to 34.5 when connected to the next stage. This is not much attenuation, because the 1M volume control doesn't demand much current. If the load were an electron-hungry tonestack then the situation would be quite different. Nevertheless, for big loads the Bassman's 12AY7 fairs a bit better than a 12AX7 and substantially better than the Vox AC30's 12AX7 which uses 220k for the plate load resistor RL.

Coupling and Bright Boost Capacitors

The coupling capacitor CG blocks the high DC voltage at the plate but is designed to be large enough to pass all audio frequencies on to the next stage. If the value is too small then the stage suffers bass attenuation. If it is too large then the amp could succumb to unwanted effects like blocking and motorboating. Our Coupling Capacitor Calculator shows only 2dB attenuation at 10 Hertz, well below the lowest note on a guitar with standard tuning, which is 82 Hertz. The capacitor value is substantially more than what is necessary for good bass response for this stage. Designers need to keep in mind, however, that bass and treble attenuation are cumulative over multiple stages. This is one of the reasons why tone stacks generally include bass and treble boost.

When the volume control is at maximum the 100pF bypass capacitor CBP is shorted, so the bright channel has the same response as the normal channel.

Fender Bassman 5F6-A bright boost capacitor circuit

At lower volume settings the control becomes a voltage divider that attenuates its input. The bypass capacitor shorts the top of the control at high frequencies and thus provides treble boost (or perhaps more accurately stated, less attenuation for treble frequencies). When the volume control is set to 50 percent our Bright Boost Capacitor Calculator shows that at low frequencies the signal is attenuated by 6.2dB. (The extra 0.2dB attenuation is caused by the output impedance of the driving stage.) Treble boost begins to get significant above 1kHz. Many amplifiers use 220pF or 500pF in this position, which allows more high frequencies to bleed through. The Marshall Model 1987 uses 0.005uF.

References

1Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Ed., (Seattle: Pentode Press, 2009).

2Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).


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