Fullscreen HTML5Med Res (??MB)HTML4

By Brandon Speedie Fender “Tweed” Bassman 5F6-A guitar amplifier from 1958 The Bassman is one of the most legendary guitar amplifiers in history, despite originally being designed for bassists. This model from the late 50’s is notable as the circuit was directly copied by Jim Marshall in the JTM45, the first ever Marshall amplifier. T he early 1950s was a revolutionary time in the live music industry. The preceding 20 years had seen a slow shift away from big bands to smaller groups, enabled by electric amplification. The instrument of choice was the hollow-body electric guitar, which could replace an entire orchestral section. Hollow-body guitars weren’t without their problems. With performances getting louder, guitarists were increasingly having tone and feedback difficulties. Leo Fender read the situation beautifully, and in 1950, introduced the Fender Telecaster (originally named the Fender Broadcaster). Its solid body solved the problems hollow body players were facing, and its small size and light weight made it an instant hit with guitarists of the era. It was the first commercially successful solid-body guitar and is still one of the most popular to this day. The Telecaster kickstarted Fender’s 90 Silicon Chip luthier (stringed instrument manufacturing) business. The following year, he introduced the Precision Bass as a replacement for the upright double bass. Again, it was hugely popular. Its much smaller size, electric amplification and fretted fingerboard were groundbreaking. It is another Fender design that has stood the test of time. The next year, Fender introduced the Bassman, a 15-inch (~380mm) speaker cabinet with a built-in amplifier. Originally targeting Precision Bass players, the Bassman soon found favour with other instruments, including guitarists. The Bassman received various upgrades over the following years, including switching from a single 15-inch driver to four 10-inch (~250mm) drivers in 1954. In 1957, they added a middle EQ control to the famous Fender “tone stack”. The models with 10-inch speakers are considered by many to be the best guitar amplifiers ever made. The 5F6-A is the last update Fender made to the 1950s Tweed Bassman. Circuit analysis Valve amplifier chassis can retain very high voltages even when unplugged. Care should be taken when working on these devices. The Bassman circuit is shown in Fig.1. The input circuitry is centred around a 12AY7 dual triode configured as two independent common-cathode voltage amplifiers. The instrument is connected via TRS plugs to one of four inputs, split across two channels (“normal” and “bright”). The #1 inputs apply the input signal to the grid of the respective triode through a 68kW grid stopper resistor, with a 1MW grid leak resistor. The #2 inputs are for higher input signals and thus apply a pad through the action of the 68kW voltage dividers. Both channels share an 820W cathode Australia's electronics magazine siliconchip.com.au degeneration resistor, bypassed by a 250μF capacitor for increased AC gain. With plate loads of 100kW, this first stage provides a voltage gain of 32.2 times for the #1 inputs and 16.1 times for the #2 inputs. The outputs of these two amplifiers are AC-coupled by 20nF capacitors into variable voltage dividers formed by the 1MW potentiometers. The output signals from the two pot wipers are mixed before being fed to the following stage. The bright channel includes a 100pF treble bleed resistor in parallel with the volume control. This has no effect at full volume, but as the volume is reduced, there is more treble bypassing the volume pot, making the audio ‘brighter’. This capacitor is the only difference between the normal and bright channels. The second stage uses another dual triode, the venerable 12AX7. The first half is a voltage amplifier very similar to the input stage, except using 270kW grid stopper resistors (which also perform the channel mixing) and no capacitor bypassing the 820W cathode degeneration resistor. This stage’s voltage gain is 20.7. Its output is fed into the second half of the 12AX7, this time configured as a cathode follower with a 100kW resistor from the cathode to ground. The Fender Tone Stack The output of the cathode follower feeds perhaps the most copied circuit in audio electronics, the BMT (bass, mid, treble) Fender Tone Stack. The treble control is, somewhat unusually, a high pass filter made from the RC combination of the 250pF treble bleed resistor and the 250kW pot, giving a cutoff frequency of around 2.5kHz. The pot effectively works as a blend control; it has treble frequencies at the top of its range and bass/mid frequencies at the bottom. This blending also introduces some distortion, as the treble frequencies are not phase-aligned with the bass/mid. The resulting harmonics and intermodulation are colloquially called “Fender shimmer”, a desirable effect. The bass control is made from the 1MW audio (logarithmic) taper pot connected as a rheostat, in combination with a 20nF capacitor. This forms a high-pass filter with a variable cutoff frequency between 8Hz and 318Hz. siliconchip.com.au Fig.1: the Bassman circuit is fairly elegant, using just three twin triodes, two power pentodes and one rectifier valve. The first two twin triodes provide preamplification and are followed by a passive tone control network. The signal from that network is applied to the grids of the third dual triode, which provides more gain and acts as a phase splitter, driving the pentodes in a pushpull configuration. The four speaker drivers are wired in parallel. “PRES” stands for presence, a Fender specialty that boosts upper-mid and treble. Australia's electronics magazine April 2024  91 which shunts high frequencies to ground via the 100nF capacitor. The feedback loop therefore has a variable frequency response, which provides a treble boost when the control is up. Output stage Photo 1: the rear of the cabinet, showing the open-backed design and four 10inch (~250mm) Jensen speakers. Remarkably, this example still has the genuine tweed. In comparison, earlier models of the Bassman amplifier used just one 15-inch speaker and were rated at 26W. This image and the lead photos are reproduced with permission from truevintageguitar.com The mid control is effectively a swept band-stop filter, a low-pass and a high-pass filter connected in series. The low-pass filter has a cutoff frequency of 142Hz, formed by the 56kW tone slope resistor and the 20nF mid capacitor. The high-pass filter is thus formed by the mid capacitor and the 25kW potentiometer. With the mid control fully up, the cutoff frequency is 318Hz. With the control fully down, all mid frequencies are blocked. All three controls strongly interact with each other, as shown in Fig.2. Note the notch in the mid frequencies with all controls at the middle of their range, as shown by the red curve. This is to compensate for the response of electromagnetic pickups, which typically over-emphasise mid frequencies. Phase splitter The output of the tone stack feeds into another 12AX7 common-cathode amplifier, configured as a long-tailed pair. This stage provides voltage gain to recover signal attenuated through the tone stack and also produces phase-inverted signals for driving the push-pull output stage. The signal is AC coupled through a 20nF capacitor and fed to the grid of 92 Silicon Chip the inverting amplifier. With an 82kW plate load and shared 15kW tail, the voltage gain is around -21.9 for the inverted output. The gain is slightly higher for the in-phase output at 22.6 times, given the 100kW plate load. The other side of the long-tailed pair receives negative feedback from the secondary side of the output transformer. This signal is fed to the grid via a 27kW feedback resistor and 100nF AC-coupling capacitor. What is that 5kW pot doing? That is the so-called “presence” control, The two outputs from the phase splitter are derived from each side of the long tail pair and thus are roughly phase-inverted replicas of each other. These signals are AC-coupled through 100nF capacitors to the grids of the output stage tubes, with 220kW bias resistors to -48V DC. The screens are pulled up through 470W, an increase on the 100W used on earlier versions. Original versions of the 5F6-A used 5881 output valves, but most examples these days will have the more common 6L6 beam tetrodes. These tubes are arranged with grounded cathodes and plates directly connected to the output transformer, which drives its four paralleled speakers at 2W. The maximum output power is around 45W RMS. Power supply Power is derived from the 8087 mains transformer, which includes a centre-tapped 325-0-325V secondary, plus separate 5V and 6.3V filament heater supplies. The 325V secondary is full-wave rectified by a GZ34 and locally filtered by some bulk capacitance and a choke. This produces the nominal 430V HT supply, as well as the lower 385V and 325V supplies for the preamplification stages via series dropper resistors. Distributed capacitance at each preamp stage provides further filtering. The -48V DC supply for biasing the output stage comes from a selenium Fig.2: the Tone Stack frequency response with all controls individually swept. The red curve is with all knobs at 12 o’clock. There is no impedance buffering, resulting in strong interaction between the controls. rectifier operating on a separate transformer tap. This part runs hot and is a common source of failure. Typically, it will be replaced by a modern silicon rectifier diode such as the 1N4007 (1000V 1A). Perhaps the most interesting part of the power supply is its poor transient response and relatively high output impedance. When driven hard, the HT supply will sag by as much as 60V. This strongly interacts with the output stage, increasing distortion. While many circuit designers might consider this unacceptably poor performance, musicians love it! It is for this reason that valve (vacuum tube) amps have a reputation for sounding good when turned up loud. The valve amplifier sound doesn’t just come from this poor regulation, though. The soft overloading properties of the output valves naturally play a part; they don’t just go hard into clipping at higher volume levels but tend to compress the sound first. The speaker transformer is also an important part of the valve sound as it can introduce a lot of (desirable) distortion as the core starts to saturate. Another contributor to the ‘valve sound’ is the non-linearity of the preamplification and tone control stages, as there is no feedback around any of the triodes. So their non-linear transfer function and inherent quirks will ‘colour’ the sound. Another trick guitarists often use is to play their guitar Silicon Chip kcaBBack Issues $10.00 + post January 1995 to October 2021 $11.50 + post November 2021 to September 2023 $12.50 + post October 2023 onwards All back issues after February 2015 are in stock, while most from January 1995 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com. au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 We also sell photocopies of individual articles for those who don’t have a computer near the speakers, in the magnetic field, which induces feedback into the pickup. An interesting feature of the power supply is the “ground switch” that connects one of the incoming AC lines to Earth via a 50nF capacitor. It is set for minimum noise. One reason for this is that Active (Live) and Neutral can sometimes be swapped, so this switch can allow you to ‘find’ the Neutral and locally Earth it for higher AC frequencies. There’s also a standby switch that disconnects the HT but leaves the valve heaters powered. This way, the amp can warm up without producing any sound and is ready at a moment’s notice. Chassis layout The chassis layout is quite neat, as shown in Fig.3 and the photos. While not indicated on the circuit or chassis layout diagrams, you can see from the photo that the 12AY7 and 12AX7s in the input stages are fitted with shield cans to reduce hum and buzz pickup. Most of the resistors and capacitors are mounted on one long dual tag strip and wired to the valves, pots etc using point-to-point wiring – see Photo 3. There are a few resistors and capacitors soldered directly to valve socket, pot or switch tags. 1959 Bassman reissue The original Bassman has proven so collectable that in 1990 Fender began reselling the 5F6-A as a 1959 reissue. The circuit is largely original, except some changes to use less expensive or more readily available parts. The changes are: • The GZ34 rectifier valve was replaced with a plug-in solid-state dual common cathode rectifier. As a result, the HT rail voltages increased from 432/430/385/325V to 491/490/477/383V. This would have Fig.3: the chassis layout for the 5F6-A guitar amplifier. You can find a more legible version of this diagram from https:// robrobinette.com/5F6A_Modifications.htm siliconchip.com.au Australia's electronics magazine April 2024  93 Photo 2: the Bassman chassis. This amplifier is entirely original except for NOS replacement valves and a new power lead. Reproduced with permission from truevintageguitar.com increased the maximum output power by a few watts. • The 20nF capacitors were changed to 22nF and 250μF to 220μF. • The two 1MW volume control pots at the input were changed from logarithmic to linear types • The 56kW resistor in the tone control network changed to 100kW and the upper 20nF capacitor was increased to 100nF. • The 10kW biasing resistor for the final 12AX7 stage changed to 6.8kW. The 5kW bias adjustment potentiometer with the 100nF capacitor from its wiper to Earth was replaced with a 25kW potentiometer in series with the 100nF capacitor, both shunted by a 4.7kW resistor. • In the power supply, the 8μF capacitor filtering the +325V HT rail was changed to two 22μF capacitors in parallel, while the filter capacitor for the +385V rail was changed from 20μF to 22μF. • The 20μF filter capacitor for the +430V rail changed to two 47μF 350V capacitors in series, with 220kW resistors across each. The two 20μF filter capacitors for the +432V rail became two 100μF 350V capacitors in series, also with 220kW resistors across each. The JTM45 Across the Atlantic, Jim Marshall was selling the 5F6-A in his small music shop. In the early 1960s, the store was frequented by Pete Townshend of The Who. Pete bemoaned the expense of Fender’s equipment and encouraged Jim to make amplifiers locally in preference to the imported American product. The result was the Marshall JTM45. Electrically, it was almost identical to Fender’s 5F6-A Bassman. The only notable changes were the use of a 12AX7 in the first stage (rather than Fender’s 12AY7) and some minor tweaks to component values. The overall effect was an amplifier with similar performance to the Bassman but with higher gain and brighter voicing. This amplifier therefore tended to go into overdrive sooner, a characteristic Townsend had requested. Marshall amplifiers have since become known for this high gain, high distortion “British crunch”. Legacy Photo 3: the featured amplifier still has the original Astron filter capacitors and was manufactured using point-to-point wiring. 94 Silicon Chip Australia's electronics magazine Fender and Marshall are giants of the industry. A large proportion of contemporary music has been performed using equipment from these manufacturers. Even today, any live performance with guitars will likely feature Fender and Marshall gear. Remarkably, their genesis is in these three inventions by Leo Fender in the early 1950s: the Telecaster, Precision Bass, and Bassman amplifier. SC siliconchip.com.au