Mullard 3-3 is a quite popular 3W electron tube amplifier introduced by Mullard Ltd in 1955. A schematic and the design details of this amplifier are available in “Mullard Circuits for Audio Amplifiers” [book] and in the National Valve Museum [article]. The amplifier uses three vacuum tubes: an EF86, an EL84 and an EZ80.
It is an unusual DC-coupled stages tube amplifier
Only two stages are used and the amplifying stage EF86 is DC-coupled to the single valve output stage EL84. The control grid of EL84 is at the same potential as the anode of EF86 via R8, so the cathode potential of EL84 is about 30 volts above ground. This is a suitable value for the screen grid potential of EF86 via R9, but the most important feature of this strange and clever DC-coupling is a DC negative feedback (NFB) system from cathode of EL84 to screen grid of EF86. It works together with the AC negative feedback from the output of the audio transformer to the cathode of EF86. These improvements enhance the flatness of the frequency response and also reduce harmonic and waveform distortion, as well hum and noise.
In this project, I decided to construct a reasonable quality tube amplifier, while being as electrically similar as possible to the original Mullard 3-3 amplifier, in stereo version. I named this project 0355st, where 03 refers to the RMS power of 3 W per channel, 55 to the year 1955, and ‘st’ because it is stereophonic.
This goal has been achieved via:
- The use of quality resistors, capacitors, terminals, sockets, fuseholders, switches, connectors and transformers. I have avoided cheap counterparts.
- Drilling and milling a beautiful strong black steel chassis.
- Employing electronic parts as similar as possible to those used in the year 1955, including carbon composite resistors and a can type multi-sectioned electrolytic filter capacitor.
- Avoiding the use of semiconductors and printed circuits. The wiring is handmade.
For building this stereo version I needed to design a new power supply because the amplifier doubles the power requirements of the original. I also had to replace the EZ80 rectifier tube with an EZ81 tube.
Here you are the final schematic:
In the included material listing [parts] you have the manufacturer and part number of all the electronics components I have used. You can also see the data sheets of the electronics parts used for building this device.
I have tested it under the 230 V 50 Hz of the power grid in Spain, with a pair of Wharfedale DIAMOND 10.0 loudspeakers and it sounded like a good quality valve FM radio with very low hum. The sensitivity of the amplifier is 100 mV for 3 W rms output.
If you would like to [order] one, I can supply the drilled chassis, the whole kit, or the handmade amplifier already mounted and tested.
I am currently working on a new version of the 03’55st [rev B] which includes a line voltage selector switch for 120, 220, 230, and 240 V, and also a 6 V incandescent indicator light.