STUDENT PROJECT: Class AB Push-Pull Vacuum Tube Guitar Amplifier Project – Dec 23, 2007 1 Abstract—Analysis of a vintage Class AB Push-Pull audio
amplifier is presented. Armed with the understanding gained
from this analysis, techniques used by engineers of the past, and modern circuit analysis tools a redesign and
improvement to this revered amplifier is produced. Using
primitive electronic components such as vacuum tubes,
magnetic transformers, and passive components, this
improved design is realized and constructed.
Index Terms—Class-AB, Audio, Electron Tube
ANY MUSICIANS AGREE that the class AB vacuum
tube electric guitar amplifier was perfected in the
1950’s with the design of the Fender Bassman 5F6-A.
Over the years since its inception, many manufacturers
have attempted to improve this circuit, yet the basic layout has been largely unchanged. The purpose of this project is
to understand the design of this classic amplifier, assess
what differences I would like in a guitar amplifier, and
using Spice simulation, attempt to adjust the circuit so as
to achieve the desired sonic results, thereby developing a
Fig 1. High level block diagram of the 5F6-A
Please Refer to Fig A1 (pg XX) for detailed schematic
of the Fender Bassman 5F6-A.
A. Ear Analysis
Bud Purvine, a Local magnetics engineer and
owner of the Onetics Transformer Company, was kind
enough to allow me to come listen to his original
Bassman. This amp immediately has a very familiar
sound, and it is understandable why this is considered a
favorite by many guitarists. It has a very clear and bright
sound that ranges from clean to mildly distorted when
turned up and played hard. It is labeled to put out 50
watts, has a very sensitive EQ control, and uses four 10”
speakers in a closed wooden enclosure.
Based on my impression, I chose the following
criteria in modifying the Bassman:
• More volume
• A stronger low end response
• More capability for distortion
• Less noise/ac hum
• More simplistic input network
In designing a modified amplifier, each stage of the circuit would need to be visited.
B. 12AY7 PreAmp
Class AB Push-Pull Vacuum Tube Guitar
Amplifier Analysis, Design, and Construction
STUDENT PROJECT: Class AB Push-Pull Vacuum Tube Guitar Amplifier Project – Dec 23, 2007 2 Fig 2. Preamp Circuitry
The Bassman preamp employs the use of two distinct
preamp channels. Both inputs see a passive gamma
network before interfacing a common cathode voltage
amplifying stage using a 12AY7 medium μ triode. Each
of these stages is loaded by a passive network which
includes a potentiometer tapping a portion of the output
voltage signal to ground.
Load line analysis of the plate current characteristic
curves of the 12AY7 reveals the operating point and small
signal characteristics of each stage. It’s worth noting that since bias current from both parallel stages is shared in the cathode resistor(RK), the effective cathode
resistance(RK,eff) of each individual stage is seen as twice the value of RK.
Fig 3. Load line associated with 12AY7 preamp stage
Fig 3 above displays the manufacturer’s published
typical anode characteristics of the 12AY7. The DC load
line in red is composed of the resistive relationship
between the HT supply of 325V and the load resistor
value of 100kΩ. The blue grid line is composed of the
relationship described in (1).
The choice of the intersection of these lines as a bias
point yields small signal parameters as shown in (2) –
μ = (4)
p μ = gm× r (4.1)
Mid-band gain(G) is calculated from this circuit using
(5) by noting that the triode behaves like a voltage
controlled voltage source loaded by a voltage divider
between the load resistor and plate resistor.
= μ (5)
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