Phoenix Amplification Phoenix Amplification by British Music Strings Limited.

British Music Strings (a London string maker) was founded in 1918. a London string maker [Source: Vintage Drum Guide]

I came across this 1977 Phoenix Valve Pre Guitar Amplifier on eBay:  Perspex front.  These were endorsed by Ritchie Blackmore and were made in the United Kingdom.

It has a Valve Pre stage matched with a solid state amplifier, there is an AUX control which either hints at it having an aux circuit for adding effects.