"While the industry was doing its best to move the format forward or to convince the amp-buying public that that was what it was doing, at least one great artist after another was discovering the unmitigated glory of a cranked 5F6-A Bassman, and the way that one of these rough-edged old bass amps from the late 50s could sing like sweet hellfire when you injected a good six-string guitar and found its sweet spot.1 ... Teaming the pre-CBS Fender Stratocaster and the late 1950s tweed Fender Bassman amplifier has been a dream for most every guitarist at least once in their playing career, and more stars have used this combination of gear - or something extremely similar - to make memorable music than perhaps any other coupling of guitar and amp."2 -Dave Hunter

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

Pointing downward, from left to right, are a GZ34 full-wave rectifier, two 5881/6L6 beam power tetrodes, two 12AX7 dual triodes, and a 12AY7 dual triode. From right to left, they implement a first-stage preamp, a second-stage voltage amplifier and DC-coupled cathode follower, a long-tailed-pair phase inverter, a fixed-bias push-pull power amp, and a high-voltage DC power supply.

First-Stage 12AY7 Preamp

The Bassman 5F6-A preamp contains two voltage amplifiers, one for the bright inputs and one for the normal inputs. Each uses a 12AY7 triode with a shared 820Ω cathode resistor and 250μ bypass capacitor. Only the bright channel is shown here.

Fender Bassman 5F6-A schematic of first two stages

The normal channel lacks the 100pF bright bypass capacitor.

The #1 input jack connects to a classic gamma network comprised of a 1MΩ grid leak resistor and a 34kΩ effective grid-stopper resistance comprised of two 68kΩ resistors in parallel. Across the network there is unity (0dB) midrange gain and very little treble attenuation due to Miller capacitance. The purpose of the 34kΩ grid-stopper resistance is to attenuate only radio frequencies.

A 820Ω cathode resistor in the first stage is shared by two triodes, which doubles the current through it. According to Ohm's Law, the resistance needed to create the same cathode voltage doubles when only one tube is used, so the equivalent resistance for one triode is 1.64Ω. Only half the capacitor value is needed for the same bypass performance: 125μF.

According to the 12AY7 calculator, for a plate supply voltage of 325V, a 100kΩ plate load resistor, and a 1.64kΩ cathode resistor, the DC grid bias is -2.7V. Fender's measured value is close: -2.5V.

Fender Bassman 5F6-A first stage DC operating point

The calculator shows a DC plate-to-cathode voltage of 157V. Fender measures 148V. The DC current load on the power supply is 1.65mA per 12AY7 triode.

The green AC load line is based on the 1MΩ AC load established by the volume control. It indicates that the grid voltage can swing from 0V to -8V, so the bias is warm and input headroom is 2.7V peak (+5.6dBV), well beyond the output of a typical pickup.

The Cathode Bypass Capacitor calculator shows that the capacitor fully bypasses the cathode resistor - there is less than 0.01dB bass attenuation compared to midrange.

Fender Bassman 5F6-A preamp first stage cathode bypass capacitor response

By today's standards 250μF is extreme overkill. Because it bypasses only 2.5V, however, the voltage rating is low, so a large capacitance value does not represent a huge extravagance.

The Preamp Gain and Output Impedance calculator shows that the unloaded gain and output impedance are 30.9dB and 20kΩ, respectively.

Fender Bassman 5F6-A preamp first stage unloaded gain and output impedance

The output impedance and 1MΩ volume control form a voltage divider with a voltage "gain" of 1MΩ/(1MΩ + 20kΩ) = 0.98 (-0.2dB). The loaded gain, measured from the grid of the first triode to the top of the volume control, is therefore 30.7dB.

The Coupling Capacitor calculator shows that 0.02μF provides an adequate bass response.

Fender Bassman 5F6-A first stage coupling capacitor bass response

Gain at 82Hz is down by less than a tenth of a dB. Gain at 10Hz, well below audio, is down by 2dB, which contributes to inter-stage plate supply decoupling.

Second Stage 12AX7 Preamp

The 820Ω cathode resistor for the second stage is not bypassed by a capacitor, creating negative feedback from cathode degeneration. This reduces gain and increases input headroom.

According to the 12AX7 calculator, for a plate supply voltage of 325V, a 100kΩ plate load resistor, and an 820Ω cathode resistor, the DC grid bias is -1.2V, which matches Fender's measured value.

Fender Bassman 5F6-A second stage DC operating point

The bias is warm, reducing the DC plate voltage to only 181V, which facilitates a reduction in grid-to-ground voltage for the DC-coupled cathode follower driven by this stage. DC current load on the power supply is about 1.4mA.

The DC-coupled cathode follower represents a very light load. This means the AC load line nearly coincides with the DC load line, which can make it difficult to identify the DC operating point. Under these circumstances, it is a good idea to enter an arbitrarily low resistance for the AC load. 123kΩ is used here.

According to the red DC/AC load line, for the grid-to-cathode voltage to reach 0V, the grid-to-ground voltage needs to swing positive by 1.2V plus an additional (2.22mA-1.19mA)(820Ω) = 0.8V, i.e. 2V peak (+3dBV), which represents input headroom.

The Preamp Gain and Output Impedance calculator shows that the unloaded gain and output impedance are 32.2dB and 59kΩ, respectively.

Fender Bassman 5F6-A preamp second stage unloaded gain and output impedance

Because of the light load, the loaded gain is the same: 32.2dB.

The greatest treble attenuation due to Miller capacitance occurs with the volume control set to maximum. Ignoring the relatively large 1MΩ resistance of the potentiometer, the series resistance that forms a low-pass filter with the Miller capacitance is 20kΩ+270kΩ = 290kΩ

According to the Grid Stopper Resistor calculator, there is 4.4dB attenuation at the extreme upper limit of treble. Overall the frequency response is flat, particularly for bass because of DC coupling and an unbypassed cathode resistor.

Fender Bassman 5F6-A second stage grid stopper treble attenuation

Cathode Follower

The cathode follower is DC coupled to the driving stage.

Fender Bassman 5F6-A cathode follower schematic

According to the Cathode Follower calculator, this stage has an output impedance of only 615Ω, making it perfect for driving a frequency-dependent load that demands a lot of current.

Fender Bassman 5F6-A cathode follower gain and output impedance

Attenuation is only 0.14dB. For typical guitar amplifier applications, unity gain and a zero-ohm output impedance can generally be assumed without a significant loss of accuracy.

Tone Stack

Fender's 3-knob tone stack architecture is perhaps the most copied electronic circuit in music electronics.

Fender Bassman 5F6-A tone stack schematic

Parts values vary. Marshall's JMP50 Model 1987 "plexi," for example, increases the treble bypass value to 500pF and reduces the 56kΩ series resistor to 33kΩ. These modifications reduce midrange scoop for less insertion loss.

Here is the tone stack response (dB gain versus frequency in hertz) with the middle control at 50-percent rotation.

Fender Bassman 5F6-A tone stack response with controls at 50-percent rotation

There is about 12dB insertion loss at midrange and about a 12dB range for the bass and treble controls.

Long-Tailed-Pair Phase Inverter

The phase inverter is a classic long tailed pair.

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

To compensate for imbalance between the two phases, the plate load resistor value for inverted phase is reduced to 82kΩ. The 10kΩ + 5kΩ = 15kΩ tail carries the current of two triodes, so the equivalent for one triode is 30kΩ. Adding this to the average 91kΩ plate load, the total resistance in series with the tube is 121kΩ plus the effective cathode resistor value. Since it also carries the current of two tubes, the latter's effective value is double the resistor's 470Ω value: 940Ω. The 12AX7 calculator indicates that the DC grid bias is -1.4V. Fender measures -1.5V.

Fender Bassman 5F6-A LTP DC operating point

DC current load on the power supply is 1.47mA per triode, for a total of about 3mA.

The Long Tailed Pair calculator indicates that the stage has a gain of about 28dB with a slight imbalance between phases that contributes to 2nd harmonic distortion.

Fender Bassman 5F6-A LTP balance

The Phase Inverter Bass Response calculator shows that the hefty 0.1μF coupling capacitors provide a flat response.

Fender Bassman 5F6-A LTP bass response

Bass response is down by only a fraction of a dB compared to midrange.

Push-Pull Power Amp

The power amp has two 5881 beam power tetrodes in Class AB push-pull. Modern implementations often use the 6L6GC.

Fender Bassman 5F6-A power amp schematic

The primary impedance is 4kΩ center tapped. For Class B, this value is divided by 4 to get an AC load line impedance of 1kΩ for one tube. (Class A uses a factor of 2.) For Class AB designs, the transformer operates like Class B as the tube approaches the knee. The factor of 2 or 4 accounts for how the windings of the opposite phase affect the impedance across the entire primary, plate-to-plate. For Class B, the opposite tube is in cutoff, so no current flows in its winding and the power amp is effectively using only half the primary at a time.

Here is a 1kΩ load line (red) for a 432V plate supply superimposed on the published plate characteristics for a 6L6GC.

Fender Bassman 5F6-A power amp load line

It crosses the knee of an imagined curve (green) for a 430V screen and a 0V grid.

A swing from 87V, 345mA to 432V, 0mA creates an RMS output power of 0.5(432V-87V)(345mA-0mA) = 60W. Actual output power is somewhere in the vicinity of 45W because of screen voltage sag.

References

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

2Dave Hunter, Guitar Rigs: Classic Guitar & Amp Combinations, (San Francisco: Backbeat Books, 2005), p. 58.

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