AWA: Model PA872, 20w Valve PA Amplifier

AWA Model PA872, PA head from the early to mid-sixties.  I have its little brother, the PA774, so had to grab this.  I think they look pretty cool - I love the giant knobs and of course the schematic on the back of the amp is a bonus.   

• Inputs / Controls:    Mic level, phono level and tone.  Bass cut switch on the back.  Phono is the old school high output type, so it skips a gain stage.   

• Valves:  5V4G rectifier, 2 x 12AX7 for preamp and phase inverter, 2 x KT66 for the power amp. 

• Output transformer:  5k P-P.  Various outputs for 300, 150, 75 and 40 ohms.  A single 12 - 15 ohm speaker can be placed across the 150 & 75 ohm winds.  NFB wind is 50 ohms.

• Power transformer:  Marked 50483

• Voltage:  B+ is about 320v.

• Power amp: Push-pull, cathode biased power amp stage.   20 watts

• Phase inverter:  Floating paraphase inverter

• NFB:  Negative feedback from the 50 ohm transformer winding.  Bass cut incorporated into the network.

• Preamp heater voltage:  The preamp valve’s heaters are in parallel with the cathode resistor in the power amp stage - meaning they’re running on a DC supply to try and keep hum from down.

Further and significantly more detailed information on the PA872 by Tim Robbins can be found here

AWA PA872, 20W VALVE AMP

Schematic screen printed on the back, just like its little brother.  Terminals from the various secondary winds on the output transformer.   

I think they're A&R transformers, which seem to be pretty typical for AWA.  

The large filter caps don’t belong.  The 100uf is about double what the rectifier can handle.   It’s a bit cramped and messy in there, so I’m going to have to really think about how to approach this.  

The 1A fuse had been replaced with a 50A automotive fuse.  That’s a good way to smoke an amp.   The mains electrical cable also has to go too - the insulation feels stiff and perhaps a bit fragile.  Of course it needs an on/off power switch too.

AWA PA872 ADVERTISING & SPECS

Advertising above care of Kevin Chant

image care of the Internet Archive

TRANSFORMER: A&R 2504 output transformer

Rewired the one of the secondaries on this A&R 2504 output transformer from this Steanes Sound System S311 PA head for an 8 ohm speaker.  

This is very much a note to self for future reference:  approximately zero other people will be interested in this. 

Two bifilar winds in parallel were used for negative feedback (marked FB).   Wiring them in series increased the impedance to ~8 ohms.   Linking the Not my genius idea unfortunately - full credit to Tim Robbins and the work he has published on converting Australia valve PA amps.     

Normally the secondaries are as follows

1    FB

2    FB (just under 2ohms)

3    0 Common

4    125

5    250

6    500

6K P-P, 25w

Paired with EL34 

I’ll tidy up the wires later.   Had some other stuff to do - cyclone on the way….   

ETONE: Gold 12" Guitar Speaker

Etone speakers are a bit of a hidden gem in the guitar world, with their roots firmly planted in Australia. Manufactured in Sydney, these speakers were often considered the local (and cheaper) alternative to Celestion.

Unfortunately there isn’t much detailed information available on the specs, but it's widely believed that they have a ceramic magnet and handle around 40 watts of power. 

This particular speaker dates from 1975, as suggested by its code (thanks to the comment from Peter F). While the previous owner mistakenly marked the model with "975," that’s actually the date code, not the model number.

One of the most interesting things about the Etone gold speakers is their unique construction. Unlike most speakers, the frame and basket are one solid piece, which means the magnet is mounted from the front directly into the bell.

I have two Vox AC15 amps, the newer AC15C1X with a celestion blue and an older AC15CC1 with a wharfdale GSH12-30 speaker.  Naturally the wharfdale was the one that I swapped out - I prefer the etone over the wharfdale.  The wharfdale wasn't terrible, it just wasn't a lively sounding speaker by comparison (if that makes any sense to you).  

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.

I recently started work on this by pulling out the microphone transformer and giving it a run with a guitar - it ripped.  I'm now wondering how much I want to mod it for guitar...  I might try and keep change as minimal as possible.

B+ 400v

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.

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.  

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.