Sunday, January 10, 2016

Mullard 3-3 Amplifier Project (Part 1)

Mullard 3-3 is quiet 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 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 6J8 pentode. Both of these valves can directly use with this circuit and those values are available for lesser price than EL84, EF86 tubes.

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

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

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

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

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