FOX: Valve PA head, Serial 360

Here are a few photos and specs of a recent acquisition.  A small Australian made PA head, branded as Fox, Sydney.  It required minor repairs; I bought it as not working.  It turned out some components were not connected, so an easy fix.  

It's a very cool little amp - and despite its size, it's pretty loud, and it sounds great.  Really nice clean and edge of break-up tones.

FOX PA AMPLIFIER.  SERIAL NUMBER 360

Controls:  Gram, Tone, Mic (all 1meg pots)

Gram (turntable) screw-on inputs on the front

Mic input missing

Multi-pin speaker output on the side (McMurdo 5pin)

No power on/off switch or fuse.

My favourite feature - the pilot light on a little lampost.  

Preamp:  6AU6 pentode (7pin)

Phase invert and power:  2 x 6GW8 / ECL86 

OUTPUT TRANSFORMER

A&R marked 2676

Push-pull with a centre-tapped primary. Primary DC resistance measured 176 Ω total, with 76 Ω and 100 Ω from the red centre tap to each end

1–2 with 8 Ω → 8.30 kΩ

2–3 with 8 Ω → 8.85 kΩ

1–3 with 32 Ω → 8.57 kΩ

Pins 4 (black) and 5 (brown) are used for the negative-feedback connection; pin 4 is grounded and pin 5 connects through the 47 kΩ feedback resistor.

Secondary: black, purple, blue, brown, white 

POWER

Power transformer: A&R Type 1885-80 MA.  

285.0.285

220ohm 5w resistors in series with AC supply to rectifier anodes

6V4  EZ80 rectifier - I think.  It may be the higher rated EZ81

B+ 290v

SCHEMATIC

I traced the amp, but did not completely finish measuring / jotting down notes on the transformers - job for another day.

note:  pots reversed compared to amp.  i.e.  pins 1, 2 & 3 the other way around

REPAIRS / MODS

Minor repairs made

• Reconnected 220ohm resistors to PT secondaries
• Reconnected a cap and resistor that had ends lifted from the board  
• Removed random heater wiring that was not connected to anything 
• Installed Mic input jack
• Cleaned pots - so dirty that they did nto work in some positions 

Mods

• Added 33k grid stopper on the preamp.  Dropped 4m7 grid leak down to 1meg 
• Reduced 820k grid leaks on 6GW8 down to 270k
• Reduced 100n cap on phase inverter down to 12n
• Replaced bypass caps
• Changed tone control to Fender Princeton / Deluxe one knob
• Added a power switch and a fuse