Dilshan R Jayakody’s Web Log

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.

Few months back I got an old Heathkit IO-12 Oscilloscope from my friend and at the time of delivery this unit is not under the working conditions. Heathkit IO-12 is Value based laboratory oscilloscope with following technical features, Frequency Response : 3 Hz to 4 MHz ≅ 5 MHz Sensitivity : 0.025V/inch Rise time: 8µS Sweep Range : 10 Hz to 500 kHz When I remove the casing of the oscilloscope I found several damaged components (specially capacitors) and wires. After making the careful observation and testings I decided to rewire the whole oscilloscope with new components except the following, 12BH7 Dual Triode 12AU7 Dual Triode 6AN8 Triode Pentode 6AB4 Triode 1V2 ½ Wave Rectifier 6X4 Full Wave Rectifier 6J6 Dual Triode Main Transformer Front end controls (including switches, potentiometers, connectors and indicator) Most of the inductors Cathode Ray Tube, etc As mentioned in the above list all the Values, transformers and some of the critical components are in good condition. Only t...