Mullard 5-20 Amplifier
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.
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.