Monday, February 22, 2016

Mullard 3-3 Amplifier Project (Part 2)

This is a second article related to Mullard 3-3 Amplifier Project and in this article we introduce HT transformer and HT power supply related to this amplifier. As mentioned in previous post, power supply unit of this tube amplifier is constructed using 400V 5A bridge rectifier, 220µF (400V) and 82µF (400V) electrolytic capacitors.

Homebrew HT transformer and PSU

The most vital component of this power supply is HT transformer and due to limited availability we construct this transformer by ourselves. We construct this transformer to obtain following voltages and primary winding is design for 230V AC mains lines.

Winding Voltage Current
HT Winding 210V 100mA
Filament Winding 6.3V 1.5A

Winding parameters for this transformer is listed in below tables:

Core Size 1 inch2
Winding Turns Wire
Primary Winding 1725 32 S.W.G
Secondary Winding (210V) 1650 34 S.W.G
Secondary Winding (6.3V) 44.5 21 S.W.G

To reduce noises we build shielding layer in between primary and secondary winding with 32 S.W.G enameled copper wire and connected it to transformer laminations/ground.

To get stability and safety we recommended to apply isolation tape layers in between each winding of this transformer.

Winding details of this transformer is calculated using Python script available in here.

During our tests, this transformer and PSU performs extremely well with our Mullard 3-3 amplifier unit and we really impressed with its higher audio quality.

Please note that electrolytic filter capacitors in this PSU hold lethal charges. Be sure to discharge them before working on PSU or amplifier unit. To discharge, connect 10K - 100K resistor to output terminal and leave it for few minutes.

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