Bill Farmer

Random thoughts on random subjects

Mullard 5-20 Amplifier

by Bill Farmer. Categories: Electronics .

Many years ago one of my brothers’ friends brought me a faulty military surplus valve amplifier which used 807 valves for the output. It appeared to have suffered a flashover which had destroyed much of the wiring and components inside the chassis, but the tagboards appeared to be reusable.

I had a copy of a Mullard amplifier book, Mullard Circuits for Audio Amplifiers at the time, which I subsequently lent so someone who never returned it. Just recently I found a copy online.

So I stripped the amplifier and decided to rebuild it using the Mullard 5-20 design. There were the two 807 tetrode output valves in push-pull, an EF37A pentode with a top cap for the grid, an ECC34 double triode, and a double diode rectifier valve. Apart from the 807s this lined up with the Mullard design with older octal valves instead of the newer B9A ones.

Mullard 5-20

The mains transformer and output transformer had no markings, but I was able to identify the connections by injecting from a 6.3V filament transformer and connect them correctly.