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 the previous post, the 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 the HT transformer and due to limited availability, we construct this transformer by ourselves. We construct this transformer to obtain the 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	1inch2

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 applying isolation tape layers in between each winding of this transformer.

Winding details of this transformer are 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.