Skip to main content

Mullard 3-3 Amplifier Project (Part 1)

Mullard 3-3 is quite popular 3W tube amplifier introduced by Mullard Ltd in 1956. A schematic and design detail of this amplifier is available in "Mullard Circuits for Audio Amplifiers" book and in National Valve Museum article. This amplifier is based on EF86, EL84 vacuum tubes, and EZ80 full wave rectifier tube. In this project, we decided to construct this original Mullard 3-3 Amplifier with some slight changes and commonly available electronic components.

Top view of Mullard 3-3 amplifier with 6P14P and 6J8 tubes.

In our prototype, we replace the EZ80 tube with 400V 5A bridge rectifier which is commonly available in many electronic spare parts shops. Also, we replace EL84 with 6P14P pentode and EF86 with a 6J8 pentode. Both of these valves can directly use with this circuit and those values are available for a lesser price than EL84, EF86 tubes.

Except for the above changes, we slightly change some values of resistors and capacitors due to their unavailability in the local market. Those modified values listed in the below table:

Component Value
R4 100Ω (1W)
R13 470Ω (5W) + 100Ω (5W) resistors in series
R15 470Ω
RV1 1MΩ
C2, C5 330pF / 2kV
C6, C9 47µF / 400V
C8 22µF / 100V
C10 10µF / 400V

The next most important component is the Audio output transformer. Due to limited availability, Audio output transformer is now expensive to purchase and costs more than Rs. 3000 (22USD at the time of writing). Because of this reason, we decided to construct the output transformer by ourselves and its input parameters and winding details are listed below:

Parameter Value
Supply voltage 300V
Output power 3W
Primary impedance 5000Ω
Output impedance 8Ω
Core size 0.3 inch2
Primary winding 6435 turns using 36 S.W.G enameled copper wire
Secondary winding 257 turns using 27 S.W.G enameled copper wire

For the above calculations, we use audio output transformer winding calculator script, which is available in here.

We assemble this amplifier on home-made aluminum chassis and tag-board is constructed on a PCB. As shown in photographs for some components we use point-to-point wiring.

Bottom view of the prototype amplifier.

In our test arrangement, we power this amplifier with our lab PSU and the results were astounding. For this test setup, we use the 8-inch 8R full range speaker and PC sound card as an audio input source.

In the next stages, we plan to construct mains transformer, PSU and cabinet for this amplifier.

Comments

Popular posts from this blog

Enable WebRTC on QtWebEngine for Raspberry Pi 3

WebRTC is a web technology to enable peer to peer communication in real-time. It mainly uses to create video conferencing and chat applications using web browsers. In this post, we describe how to enable this technology in QtWebEngine on Raspberry Pi 3 platform.

QtWebEngine is an embedded browser component which comes with the Qt framework. This component is based on Google Chromium browser and it supports most of the Chromium features including WebRTC. In PC, WebRTC applications run smoothly on QtWebEngine component. But in Raspberry Pi platform situation is different and none of the WebRTC application is work with the QtWebEngine. The only thing which we can see is a black box in an HTML5 video tag area. At the time of writing this problem exists in Qt version 5.6, 5.7 and 5.8.

BMP180 based USB atmospheric pressure monitor

We initially developed this USB atmospheric pressure monitor to study some operating characteristics of Bosch BMP180 sensor. BMP180 is a low-cost sensor for measuring barometric pressure and temperature. According to the datasheet this sensor can use to measure pressure ranging between 300hPa to 1100hPa. This sensor is introduced a couple of years back but still, it is popular due to lower cost and simplicity of its interface.


We did this unit to test the BMP180 sensor more accurately and to study its behaviors. This unit is based on PIC18F2550 microcontroller and the main reason to select this MCU is because of its built-in USB 2.0 interface.


To display sensor calibration data and it’s readings we did small windows application. This application display and plot temperature and pressure readings captured from the BMP180 sensor.

This unit is programmed to work as a USB HID device and no special device driver is required to use this device. We test this unit in Windows 10 environment.

Programmable light controller

The main objective of this project is to design a maintenance free and low-cost light which automatically turns on and off at the predetermined time of the day.

To meet the above requirement I designed this controller using ATmega8 MCU and DS1307 RTC. The driver stage of this light controller is intended to work with commonly available 7W LED modules.


The core component of this programmable light is ATmega8 low power CMOS microcontroller. The main reason to select this microcontroller is it’s lower cost and higher availability. Except for the above two reasons this microcontroller also bundled with a rich set of peripherals which including 23 GPIOs, 3 independent timers, Two-wire serial interface, EEPROM, etc.

Apart from ATmega8 microcontroller, this system uses DS1307 real time clock to maintain system time. Like ATmega8, DS1307 is also a very popular RTC in the market.

This controller is designed to work with a 24V DC power supply. The main reason to select 24V is that most of the…