Circuit Analysis of the Fender Bassman 5F6-A
Fender designs the Bassman in 1952 to be an amplifier for its new Precision Bass. The amp has a 15-inch speaker with a closed, ported cabinet and uses a pair of beam power tetrodes for lots of volume. Throughout the decade it is modified and improved, culminating in the model 5F6-A.
Ironically it doesn't quite make it as a bass amp, but the musical tones it creates from a guitar make history. The Bassman stack and long-tailed-pair phase inverter with negative feedback become the foundation for many great rock and blues sounds.
"From Fran Beecher with Bill Haley, and Bo Diddley with his two extra speakers (pointing backwards!), to Bruce Springsteen and Jimmie Vaughan, from recording studios that keep one as their "house" amp to the weekend warrior who escapes the Monday-Friday grind answering to no one but his guitar and amp, the 4x10 Bassman is to this day a sanctuary of tone for many. Those that desire one give up whatever it takes to obtain their holy grail." -John Teagle1
Many other amplifier manufacturers create new Bassman-derived designs that themselves become the legendary instruments of a rock generation.2
"Marshall copied Fender's basic circuit - virtually no difference at all. They even lined up the jacks the same way, and the on/off switch, the standby switch. I think the whole motivation for those early Marshall amps was to copy the more expensive Fenders so blokes on a pension in England could afford to buy one for their kids." -Aspen Pittman
"Fender was first, and the original Marshall was basically a dupe of a Fender Bassman. You can't reinvent the wheel." -Zakk Wilde
"Obviously we looked at the Fender amps, because they were my favorite amplifier, and the Bassman seemed to be nearer the sound that people were talking about ... so we were influenced by it, but after all, there is nothing new in valve technology; it's all been done before." -Jim Marshall
Following its debut, the 5F6-A circuit is tweaked, enhanced, and re-introduced under many different logos and the late-1950's original becomes one of the most collected guitar amplifiers of all time.
Evolutionary Design
New product development is a process that is accelerated by a legacy of successful products. Here is a modern example.3
"Representing the fourth collaboration between Dr. Z and Brad Paisley, the DB4 is the outcome of the country star's request for a rich and warm sounding British-voiced amp. Dr. Z founder Mike Zaite says he started from scratch with this design, deciding early on a U.S.-made N.O.S. 5879 pentode tube for the preamp."
The subtext here is that most professional designs are not started "from scratch." Instead, they evolve from previous designs. Likewise, the 5F6-A represents evolutionary development based on a history of modifications to a basic architecture. The early narrow-panel Bassman takes the basic 5B6 architecture, adds fixed bias, increases feedback, and adds a second tone control. The 5F Series adds a 3-knob tone stack and modifies the feedback circuit.
Each of these system modifications is made in the context of similar changes to the rest of Fender's product line. From the 5B6 to the 5F6, for example, the Bassman transitions its phase inverter from paraphase, to split-load, to long-tailed pair. The Twin 5C8, 5E8, and 5F8 take the same design path.
The Sum of its Parts
There is no single component that defines the performance of the 5F6-A. Moreover, there are so many elements that contribute to its performance:
- a "Bassman" tone stack driven by a DC-coupled cathode follower with a very hot bias,
- a long-tailed-pair phase inverter with an overcompensated plate load resistor for the inverted phase,
- just the right amount of feedback,
- relatively high signal levels compared to headroom upstream of the power amp,
- a tube-rectified power supply with just the right amount of squish,
- a beefy output transformer, and
- the right cabinet and speakers.
These elements and more come together to create an amp that sits in the sweet spot of a palette of tones that range from warm jazz to gritty blues to classic rock to alternative country.
Legacy
The tweed Bassman does not have as many clones or kits as the venerable 5E3 Deluxe, but when you count the tweaks, mods, and adaptations of the original design then the numbers become staggering. Jim Marshall and Ken Bran famously copy the Bassman in its entirety to create the JTM45. From there Marshall modifies the circuit to accommodate European parts, then further tweaks it to create the legendary Plexi. The Bassman's long-tailed pair lives on in the Mesa/Boogie Mark I, Egnator Tweaker-40, and the Peavey EVH 120. Its DC-coupled cathode follower and tone stack sit prominently in the signal path of the Soldano SLO, Traynor YBA-1, and Vox AC30 (without a middle control). The Bassman's gamma-network front end is the hands-down favorite of builders to this day.
"The Bassman has made its presence felt, in design terms, in more tube amps than any other amplifier." -Dave Hunter4
Here is a photo of the inside of an original 5F6-A chassis, courtesy of our friends at Technical University Berlin.
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 250μF bypass capacitor. Only the bright channel is shown here.
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.
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.
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 31dB and 20kΩ, respectively.
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 30.8dB.
The Coupling Capacitor calculator shows that 0.02μF provides an adequate 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.5
Second Stage 12AX7 Preamp
The 820Ω cathode resistor for the second stage is not bypassed by a capacitor, creating negative feedback from cathode degeneration.6 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.
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 32dB and 59kΩ, respectively.
Because of the light load, the loaded gain is the same: 32dB.
The greatest treble attenuation due to Miller capacitance7 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 the output impedance of the driving stage plus the grid-stopper resistance:
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.
Cathode Follower
The cathode follower is DC coupled to the driving stage.
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.
Attenuation is only 0.14dB. For typical guitar amplifier applications, unity gain and a zero-ohm output impedance can generally be assumed for a cathode follower.
Tone Stack
Fender's 3-knob tone stack is perhaps the most copied electronic circuit in music electronics.
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.
Based on SPICE simulation,8 here is the tone stack response (dB gain versus frequency in hertz) with the middle control 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.9
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.
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.
The Phase Inverter Bass Response calculator shows that the hefty 0.1μF coupling capacitors provide a flat response.
Bass response is down by only a fraction of a dB compared to midrange.
Push-Pull Power Amp
The 5F6 has 100Ω screen resistors. The 5F6-A increases their values to 470Ω to provide better protection against radiofrequency oscillation.
The change also affects power amp performance at full power. The screen voltage is 430V at idle. Let's use two points of reference for full power:
- If the voltage drop across the screen resistor reaches 30V, creating a 400V screen, the plate swings to a minimum value of 102V (blue dot on the red 1kΩ load line shown below). The screen current swings to 43mA (blue dot on screen characteristics shown next).
- If there is no voltage drop across the screen resistor, the plate swings to 90V (green dot) and the screen current swings to about 55mA or 60mA (beyond the top of the screen characteristics).
According to Ohm's Law, 43mA creates only a 20V drop across a 470Ω screen resistor to create a 410V screen. On the other hand, 55mA or 60mA creates a 26V to 28V drop for a 402V to 404V screen, so the actual screen voltage is somewhere between these limits. Based on these reference points, we can estimate that the screen swings down to about 405V at full power, which gives us the red dot on the plate characteristics shown here.
The dot marks a 100V plate and a plate current of 330mA. Output power is about
This estimate assumes that DC power supply voltages do not sag because of higher average plate current. At full sag output power is significantly less.
References
1John Teagle and John Sprung, Fender Amps the First Fifty Years, (Milwaukee: Hal Leonard, 1995), pp. 71-72.
2Tom Wheeler, The Soul of Tone, (Milwaukee: Hal Leonard, 2007), p. 171.
3Art Thompson, "Dr. Z DB4," Guitar Player, August 2016, p. 74.
4Dave Hunter, Guitar Rigs, (San Francisco: Backbeat Books, 2005), p. 69.
5Dave Hunter, Guitar Rigs: Classic Guitar & Amp Combinations, (San Francisco: Backbeat Books, 2005), p. 58.
6Richard Kuehnel, Fundamentals of Guitar Amplifier System Design, (Seattle: Amp Books, 2019), pp. 100-113.
7Richard Kuehnel, Guitar Amplifier Electronics: Basic Theory, (Seattle: Amp Books, 2018), pp. 61-65.
8Richard Kuehnel, Guitar Amplifier Electronics: Basic Theory, (Seattle: Amp Books, 2018), pp. 71-72.
9Richard Kuehnel, Guitar Amplifier Electronics: Circuit Simulation, (Seattle: Amp Books, 2019).
