The Fender Bassman 5F6-A Circuit

Here is a photo of the inside of an original 5F6-A chassis, courtesy of our friends at Technical University Berlin.

Fender Bassman 5F6-A, chassis photo

The 1st-Stage 12AY7 Preamp

The Bassman 5F6-A preamp contains two voltage amplifiers, one for the bright inputs and one for the normal inputs. The preamp is designed to boost the relatively weak signals from the guitar pickup and to suppress radio frequency interference. The boost comes from a medium-mu 12AY7 triode with a fully bypassed cathode. (In designs based on the 5F6-A this tube is often replaced by a high-mu 12AX7 triode.) The RF suppression results from 68k grid stoppers in combination with the Miller Capacitance of the tube. There are four inputs: fully amplified normal and bright channels (the #1 inputs) and attenuated normal and bright inputs (the #2 inputs).

Except for the highest audio frequencies, where Miller Capacitance becomes a factor, the input impedance is 1 megohm at the #1 inputs and 136k at the #2 inputs. Based on a graphical analysis of the tube's AC characteristics at the DC operating point, the 12AY7 amplification factor is estimated to be 49.1 and the plate resistance is equal to 29.9k. The voltage gains are thus -32.2 and -16.1, respectively, depending on which input is used. These are not the gains that are achieved when connected to the next stage, however, because the output impedance is significant: 23k.

Fender Bassman 5F6-A preamp circuit

The Marshall JTM45 copies this circuit in its entirety but substitutes a high-mu 12AX7 tube for greater gain. The Marshall Bluesbreaker and other early models also use the Bassman preamp. The Marshall JMP50 Model 1987 splits the normal and bright channels into separate circuits. Interestingly, however, Marshall intentionally keeps the normal channel cathode resistor at 820 ohms. It would need to double to 1.64k to provide the same DC operating point as the Bassman, because the 5F6-A resistor carries the DC current of two triodes.

The Marshall JMP50 Model 1987 Preamp

The 2nd-Stage 12AX7 Voltage Amp

The next stage in the Bassman 5F6-A centers around a 12AX7 tube containing two triodes, the first of which is another voltage amplifier. Unlike the previous preamp its cathode resistor is not bypassed by a large capacitor.

Fender Bassman 5F6-A voltage amp circuit

The input impedance at maximum volume is 351k. From a graphical analysis of the DC operating point the triode amplification factor is estimated to be 100 and the plate resistance is 59k. This puts the voltage gain at -20.7 when the active channel's volume control is set to maximum and the inactive volume control is set to minimum. The output impedance is significant at 59k, but the input impedance of the next stage is almost infinite so the voltage gain is not significantly reduced by the connected load. The following graph (from reference 1) shows the frequency response for the bright channel input (solid) and the normal channel input (dotted) for volume settings of 100%, 50%, 25%, 12.5%, and 6.25%. This response is increased by 23 dB when we add the voltage gain of the triode.

Fender Bassman 5F6-A bright channel response

Note that at maximum volume there is no difference between the 5F6-A bright and normal channels because the volume control shorts out the 100pF bypass capacitor. The treble boost increases substantially, however, at low volume control settings.

The first Marshall JTM45 copies the Fender Bassman 5F6-A voltage amp in its entirety. Later versions increase the grid stopper resistors to 470k and provide more bypass capacitance in the bright channel input. The changes increase the difference between the two channels: the bright channel is brighter and the normal channel has more treble attenuation.

The Marshall JMP50 Model 1987 Voltage Amp

The 3rd-Stage 12AX7 Cathode Follower

The second 12AX7 triode is wired as a cathode follower, which has no voltage amplification but provides a low-impedance source for the tone stack that follows it. It thus acts as a buffer to isolate the voltage amplification stages from the tone stack.

Fender Bassman 5F6-A cathode follower circuit

Since almost no current flows through the grid we can consider the input impedance to be infinite. Based on the DC operating point the triode amplification factor is estimated to be 93 and the plate resistance is 50k. This puts the voltage gain at +0.984, representing a slight loss. The output impedance, however, is a very low 531 ohms.

The 12AX7 cathode follower circuits in the Marshall JTM45, Bluesbreaker, and JMP50 Model 1987 are identical to the Bassman 5F6-A.

The Tone Stack

The Fender Bassman 5F6-A offers the guitar play a wide range of tone settings from three separate controls: bass, midrange, and treble. The tone stack is driven by a 12AX7 cathode follower circuit with a very low output impedance capable of providing a source voltage that isn't easily dragged down by a varying load. It drives a long-tailed-pair phase inverter with a very high input impedance. These two factors, a low impedance source and a high-impedance load, make the tone stack's frequency response relatively independent of the preceding and following stages.

The passive tone stack manipulates response through frequency-selective attenuation. In the extreme with all of the tone controls set to zero, for example, it induces a whopping 15dB loss at midrange and even more at bass and treble frequencies. The attenuation is mitigated by the combined gain of the Bassman's voltage preamps and twin-triode phase inverter. With what amounts to an additional voltage amplification stage and a zero-gain cathode follower circuit, the 5F6-A tone stack requires the equivalent of a complete 12AX7 dual triode for adequate drive voltage. Such is the technological price of placing an incredibly rich tonal palette in the hands of the guitar player.

Fender Bassman 5F6-A tone stack circuit

From top to bottom the potentiometers control treble, bass, and midrange. The input impedance varies depending on the tone control settings and has a worst case lower limit of 46k. There is no voltage gain in the tone stack and depending on the control settings the losses are substantial. The worst-case (highest) output impedance is 1.275 megohms. This occurs at low audio frequencies when all the tone controls are at maximum. For higher frequencies and different tone settings the output impedance is well below a megohm.

Graphs of Fender Bassman 5F6-A Tone Stack Frequency Response

The design of the tone stack in most Marshall amplifiers is the same as in the 5F6-A, but the component values often differ. JTM45 stacks use both 270pF and 220pF capacitors instead of the Bassman's 250pF treble bypass. They also use 0.022uF instead of 0.02uF for the other bypass capacitors. Given part tolerances and the closeness of the values, however, these modifications are probably the result of component availability rather than an effort to fine tune the frequency response. The Marshall Bluesbreaker is similarly configured. The JMP50 Model 1987 substitutes 33k, 500pF, and 0.022uF instead of 56k, 250pF, and 0.02uF, which gives less midrange signal attenuation and narrows the band of frequencies defined to be midrange.

5F6-A versus JMP50 Tone Stacks

The Long-Tailed-Pair Phase Inverter

The Bassman 5F6-A phase splitter provides further voltage amplification and creates two outputs of opposite phase to drive the push-pull power amplifier.

Fender Bassman 5F6-A long tailed pair phase inverter circuit

At high frequencies with the presence control at maximum the input impedance gets as low as 1.9 megohms under open-loop conditions, which is still much higher than the output impedance of the tone stack. When the presence control is at minimum the input impedance increases to more than 2.7 megohms. Negative feedback increases these values even more. Based on the DC operating point we estimate the triode amplification factors to be 101 and the plate resistances to be 57.7k. The voltage gain for the inverted output (connected to the 82k plate resistor) is then -21.9, taking into account the additional load of the push-pull power amplifier input. For the in-phase output the gain is +22.6. Feedback from the output transformer is connected to the phase splitter via a 27k resistor. The input impedance presented to the feedback signal is 31k at minimum presence and 27k at maximum presence for high frequencies. The voltage gains for the feedback input are +3.4 and -3.5 to the inverted and in-phase outputs.

The original Marshall JTM45 phase splitter is an exact copy of the Bassman 5F6-A long tailed pair. Some versions substitute 0.022uF for the 0.02uf input coupling capacitor and 4.7k instead of 5k for the presence control without significant changes in performance. The Marshall Bluesbreaker also uses this design. The JMP50 Model 1987 decreases the value of the coupling capacitors to the push-pull power amp from 0.1uF to 0.022uF.

The Push-Pull Power Amp

The Bassman 5F6-A power amplifier is a key factor in creating the amp's unique sound because its nonlinear response creates tones not found in the input signal. The 5F6-A originally shared the power load between two 5881 tubes, but new-production versions of the 6L6, which have nearly identical characteristic curves, are commonly used instead. In our analysis we use the widely available 6L6GC. The 5F6-A output transformer is model number 45249, with a 4k primary and a 2-ohm secondary.

Fender Bassman 5F6-A push-pull power amp circuit

The following graph (from reference 1) shows, for open-loop conditions, the voltage at the 5F6-A output transformer's 2-ohm secondary versus the AC input voltage at the upper tube grid. Also shown is a straight-line approximation and a third-order approximation for the curve. By open loop we mean that the 27k resistor at the base of the phase splitter has been disconnected from the 2-ohm transformer output and grounded.

Fender Bassman 5F6-A power amp voltage gain

The Bassman's nonlinear behavior can clearly be seen in the output as deviation from a straight line. The third-order approximation, however, is almost indistinguishable, which indicates that most of the nonlinearity can be described by the product of a constant and the cube of the AC input voltage. The output voltage as a function of the input voltage v is thus closely approximated by Av3+Cv, where A and C are constants. We used linear regression to estimate the values of the constants at two operating extremes: no power supply voltage sag and maximum power supply voltage sag. The first condition occurs when the amplifier has been operating at very low power levels and then experiences a sudden increase to maximum power. The second condition represents the steady-state condition where the amplifier has been operating at maximum power long enough for the power supply voltage to sag to its minimum level. At maximum sag we conclude that A = -4.2x10-5 and C = -0.17. When there is no sag A = -2.9x10-5 and C = -0.25. Voltage sag thus increases the magnitude of the nonlinear coefficient A and decreases the magnitude of the linear coefficient C. Sag therefore increases nonlinear distortion.

Using a third-order approximation, the 5F6-A's open-loop, third-harmonic distortion is 9 percent (maximum supply voltage sag) and 5 percent (no sag). Under closed-loop conditions with the presence control at minimum the Bassman's harmonic distortion is significantly less due to negative feedback.

The Marshall JTM45 power amp is a direct copy of the Bassman 5F6-A circuit except for one very important change: the negative feedback signal is taken from the 16-ohm transformer output instead of the 2-ohm output. This almost triples the feedback voltage, which reduces nonlinear distortion and flattens the amplifier's frequency response at the cost of less overall gain. Later Marshalls, including the JMP50 Model 1987, use EL34 pentodes, so although the push-pull, class AB circuit design is the same (except for component values and supply voltages), the tube change dramatically alters its characteristics.

The Power Supply

The Fender Bassman 5F6-A power supply consists of a GZ34 full-wave rectifier tube and a series of low-pass filters that supply DC power to each of the amplifier stages. The two output plates are by far the biggest current consumers. Because the pentodes are operating in push-pull they are relatively immune from power supply ripple and are placed the furthest upstream in the filter chain where the most ripple exists. The preamps, with their low-level audio signals, are the most susceptible to hum and thus receive the most filtering.

Fender Bassman 5F6-A power supply circuit

Because the 5F6-A power supply uses a full-wave rectifier, the fundamental frequency of the AC hum is 120 Hz. At that frequency there is 42dB of hum attenuation at the pentode plate supplies, 83dB at the screen supplies, 120dB at the phase splitter plate supplies, and 156dB at the preamp plate supplies. There is less attenuation if the amplifier is powered by 50Hz, as would be the case in Europe.

A sudden increase in audio power level, from zero signal to maximum power, causes the 5F6-A class AB power amp to draw much more current. Because of resistance in the Bassman's power supply transformer windings and in the GZ34 rectifier, the increased current load causes the DC output voltage to drop. The power supply's large filter capacitors and choke are able to temporarily supply current to the screens and plates, which causes the voltage to gradually sag over time. Their reaction time in response to a change in load is an important characteristic of the amplifier. The following graph (from reference 1) shows how the DC screen supply voltage sags during the first half second in response to an instantaneous increase in signal amplitude from zero signal to maximum power.

Fender Bassman 5F6-A power supply voltage sag

The Bassman's response consists of three components added together. The first component represents the steady-state screen supply voltage sag at maximum power. The second is a transient component that decreases exponentially with time. It has a time constant of 0.022, so after 22 milliseconds it is reduced to 37 percent of its start value. The third component, representing an underdamped condition, decreases exponentially at a slower rate, 0.72, and has a damped frequency of oscillation of 13 Hz. The oscillation is caused by the interaction of the choke with the filter capacitors. The three separate components are broken out in the next graph (also from reference 1).

vacuum tube power supply voltage sag

The graph shows the steady-state plus first transient, steady-state plus second transient, and total response over the first 200 milliseconds.

The Bassman 5F6-A power transformer is model number 8087. It has a 325-0-325 secondary, which means that it has two opposite phases that are 325 volts RMS (460 volts peak) and a center tap that is connected to ground. The transformer has separate secondary windings that supply 1.9A at 5 volts to the GZ34 filament and 2.7A at 6.3 volts to the triodes and pentodes. The 5F6-A choke is model number 14684.

The Marshall JTM45 power supply is identical to the 5F6-A except for component values. The grid bias supply in particular is fixed and taken from the same transformer tap as the Bassman. The JMP50 Model 1987, however, shifts the grid supply to one phase of the high-voltage secondary, eliminating the need for a lower-voltage tap, and adds a variable resistor to enable the bias voltage to be adjusted.

The Marshall JMP50 Model 1987 Grid Bias Supply

References

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

2Michael Doyle, The History of Marshall, (Milwaukee: Hal Leonard Corp., 1993).


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