AWA: Fox Sound Systems, 25w PA 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.

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 A&R transformers as my other PA1003.

Model 2426 - which is not listed in any A&R catalogs that I can find.  Probably a special order, similar to an existing A&R product.

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.

Given the values provided in the specs, I really doubt that this is a 25w amp.  It's more likely to be at least 30, and maybe heading towards 50w.

Plenty of space to work in - 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.  

UTILITY: Speaker Cab Impedance / Speaker Switch

When I set out to build a proper test cabinet for amp building, I came across a great switch wiring setup that perfectly suited my needs. With the parts I had on hand, I was able to put together a versatile cab without risking damage to my favourite speaker cabs.  There are of course commercial products that achieve the same thing, and sometimes more.  But this is a DIY blog right.  

Why a test cab?  I already own several cabinets that I really like, however, I was a bit concerned about accidentally destroying a vintage speaker.  A dedicated test cab allows me to safely experiment with different speaker loads without worrying.

What You’ll Need

• A twin-speaker cabinet with 8-ohm speakers
• A 3-pole, 4-position rotary switch (Make sure it's a sturdy one, ideally rated at a couple of amps)

There are four switch positions, covering the most common loads:

• The first two switch positions isolate the individual speakers, providing an 8 ohm load.  Handy if you have two different speakers in a cab that you want to isolate.
• Position three is both speakers in series, providing a 4 ohm load
• Position four is both speakers in parallel, proving a 16 ohm load

I kind of rushed into throwing this together, and now I want this to rework this to put it in sequential order...  4, 8, 8, 16.    

A Word of Caution

WARNING: If you wire this incorrectly, you could fry your amp’s output transformer - a costly mistake.  Always triple-check your wiring with a multimeter before plugging in your amp.  If you're unsure or don’t have the tools to verify your work, do not attempt this.

4-WAY SPEAKER / IMPEDANCE SELECTOR

The wiring diagram I used was posted by YellowBoots on the Tele forum

TEST CAB

The cabinet I’m using is a late '70s Peavey Deuce combo, minus the amp.  It houses two 12-inch CTS speakers, which I think are rated at 50W to 75W each.

It's quite a well built twin 12 combo cab, proper ply etc instead of the awful stuff they use these days in most amps.  I have it sitting on wheels, so it's easy to drag around the workshop.

Yes, I will eventually label it properly, but for now, this will do.  

I gave the new test cab a run with this little AWA PA 774 amplifier.  I modded it to have a Fender style preamp with bass and treble controls, cause I only had three knobs to work with.  It's a cool little amp and sounded great through the cab.  As I'm not a fan of the look of the old peavy cabs, I pulled the aluminium strip and logo off the front. 

VOX: Tone Bender

Vox tone bender - two knobs and three silicon transistors.  It's kind of a silicon MKII with clipping diodes on the output.  It’s loaded with a Jen FZ III circuit,  which I wasn’t aware that it was ever used in a Vox.   

Photos from someone that grabbed a fuzz from me recently - Initially I had no idea what the circuit was, so I thought I'd post it here.

Philips: 956C Valve PA Amplifier, 30 watt

This is an unbranded Philips or Steanes model 956C valve PA amplifier - it's very similar to the Philips 956B that I have, which is why I bought it.

The amplifier was unbranded and came from a batch of various types labelled as "Fox Sound Systems" in Sydney, Australia. I suspect that Fox was an installer rather than a manufacturer, as I haven't found any evidence of them producing equipment—they don’t seem to exist as a brand. It’s possible that this was part of a government contract, as these amplifiers were commonly used in the public school system.

PHILIPS / STEANS SOUND SYSTEM 956C VALVE PA HEAD

VALVE LINE-UP 

1 x EF86

1 x 6N8

1 x 12AX7

2 x EL34

GZ32 Rectifier

INPUTS / OUTPUTS / CONTROLS

Inputs:          Pickup & Microphone 

Controls:      PU Volume, Mic Volume, Bass & Treble 

Output:        Speaker binding posts / banana plug

TRANSFORMERS

Power transformer:   A&R, Type 1687B 

B+   I'm guessing it's about 420 or so volts.  

Output transformer:  A&R, Type 2503

Output impedance:  300, 150, 100, 75, 15

Looks pretty clean inside - likely has not been worked on.  Apart from the odd wax capacitor, I can't see any glaring issues.  

Part of the reason that I like these, is that there's a decent amount of room to work with inside the chassis and it has a few options on the back for things like adding a power switch and speaker jacks.

Not sure what's going on with the impedance selector and the speaker output posts.   It's not a standard set-up, I need to trace the circuit.

Valves are mainly Miniwatt, made in Great Britain, Holland and Australia.  A couple of Japanese EL34s - which I was initially a little disappointed to see, until I read that they're Matsushita valves, made on Mullard machinery and sound the same as the European Mullards.

GUITAR AMPLIFIER CONVERSION

I rebuilt it to resemble the EF86 channel of the Matchless Phoenix—similar to a DC30 but with EL34s. Why? The valve lineup was already similar, the number of knobs matched, and I liked the look of the circuit.  It turned out to be a great decision because I’m really happy with the result.

The signal path is as follows:  Vox EF86 preamp > rotary switch with a variety of capacitor values > volume pot > 12AX7 long tail phase inverter > master volume / treble cut > power valves 

COLORSOUND: Sakata Hybrid One Knob Fuzz Box

Macari's often release pedals that are minor variations on the same circuit - in this case the "Sakata" Colorsound hybrid one knob fuzz has quite a few value changes compared to the previous hybrid they released, which was closer to a stock fuzz box.   Macari's describe the Sakata as being tuned closer to a MK1.5 

I was going to add this to my previous post about the Colorsound hybrid fuzz box, but I thought there are enough differences to deserve it's own post.  For one, this is a positive ground effect, with the previous is a standard negative ground set-up.

Personally, I would not pick up the soldering iron until I'd tested this on a breadboard.  Being a small circuit it will be quick and easy to do, and you can test a few values / transistors to see what you think.

COLORSOUND "SAKATA" ONE KNOB FUZZ 

Transistors: BC179C for Q1 and an STC TK22C for Q2.  Both PNP transistors.  Like any fuzz face style circuit, transistor choices in this circuit will matter, as will the bias resistors.

Q1:  The BC179C lists hfe in the 380-800 range - it's the PNP compliment to the BC109C if this helps you think of alternatives or as to how it might behave in the circuit.

Q2:  If you can't access a TK22C, try an OC71.  Without knowing the leakage and hfe range of the TK22C they selected, there will be some guess work here, even if you have some on hand (note my point above re breadboarding).

The bias resistors on Q2 look like 520 ohms and 5k6.   The feedback resistor is 100k.  These values are important.  

The input cap is an electrolytic - I can't read the value, which isn't all that important, as once the input cap is over 1uf differences become minor (stock value on the fuzz box is 100n).   A MK1.5 is 4u7 so I'd run with that.  If you don't have a 4u7, 2u2 will be close enough.

Same with the bypass cap that's paired with a 1k resistor.  I can't see the value, it's most likely 22u, which is what appears in a MK1.5, Fuzz Face etc.   

Output cap is 100n 250v.   Stock value is 220n.