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

Jobs to do

• Add a fuse & maybe a power switch

AWA: Fox Sound Systems, PA1003 Valve Amplifier

Branded as Fox Sound Systems, this is really an AWA PA1003.   I couldn't find any information on Fox Sound Systems, Sydney. My guess is that they were an installer who ordered unbranded amps directly from AWA for their installation work—possibly for schools or government buildings. But who knows?

I already own one of these, so when I saw this and the seller dropped the price to a reasonable amount, I snapped it up. I suppose I'll have to sell one later.

The differences between this one and my other unit are minimal. The main ones are that this amp has the optional low-impedance mic transformer on the input, and the phase inverter uses a 12AU7 instead of a 12AX7.

I recently started work on this by pulling out the microphone transformer and giving it a run with a guitar - it ripped.  

B+ ~380v

In addition to the weird impedances for 100v line systems, this output transformer can handle a regular 16ohm speaker (12 - 15 according to their published specs).

Same transformers as my other PA1003.  Which I think are Ferguson.

Model 2426 

Some nice RCA 7027A output pentodes - basically a fancy 6L6GC.  The 12X7 preamp valve and 12AU7 phase inverters are both AWA branded.  These are likely all original.

Plenty of space to work in - which is part of the reason why I like these.

It's a little grubby, and it looks like there may have been a couple of resistors replaced, but otherwise appears OK.  

7027A CLASS AB, CATHODE BIAS