AWA: Model PA1003 25w PA Valve Amplifier

Yes, I bought another vintage PA head & yes, this is another non-pedal related post.  

This one is larger than the previous one at 25 watts, although I'll see where this lands once I eventually convert it.  I may shave a bit of voltage off so the output doesn't have to work as hard, as I'd like to keep the RCA valves running as long as possible. 

The PA1003 is part of a series, with its siblings being the 100 watt PA1001 and the 50 watt PA1002 with a few variations of each model available.   They all look similar on the outside, but I'm not sure how similar they all are under the hood.  The larger models look more complicated and use different valve sets by the look of it.

When I get around to it, I'll link a record of the modding process somewhere.   Long story short, it now has Fender style preamp, TMB from a Bassman and the power amp stage is unchanged.  Negative feedback has been removed.   It sounds great, and has a tonne of output.

AWA MODEL PA1003 25 WATT VALVE PA AMPLIFIER     

Similar to the AWA PA774, it has one high-impedance microphone input, one P.U. channel (for old school record players) with a tone control for the P.U. channel.  There's a bass cut on the rear, intended for when the amp was used with horn speakers that can't handle low frequencies.

25 watts, push-pull arrangement using RCA 7027A valves - these are kind of a higher spec 6L6GC.

There were multiple optional extras available for this, which apparently included the power on/off switch.  Fortunately there's at least space for one on the front that's already labeled, with a pre-drilled hole that has a cover on it that can easily be removed.

I'm a little disappointed that this one didn't have the 6" x 4" monitor speaker installed, cause that would have been a nice addition.  Oddly, despite having no power switch, the switch for the monitor speaker is installed.  That being said, I'm glad that the optional radio tuner was not installed, cause I would have ripped that out.

It has a number of options available for speakers - a few high impedance 100v lines and the option for a single 12 - 16 ohm speaker.

Side-mounted inputs for a high-impedance microphone and phono. 

The power transformer is most likely a Ferguson based on looks - they were commonly used in Australian amplifiers.   I can't find it anywhere in their catalog, so it may have been a special order by AWA, or maybe it's not Ferguson?

I’m sure the output transformer would have been from the same supplier - as above, I can’t find it in any catalogs. 

The strange looking cardboard tube capacitor towards the rear did not measure well, and I was sure that it would need to be replaced - after brining it up slowly on the variac and giving some time to reform, it's fine now.  The Ducon in front measure right on spec.  

The additional holes in the chassis are for mounting optional extras, in this case, the holes are for a radio receiver.  They may come in handy later, if I ever need to mount anything else, such as a choke or maybe some filter caps.  

RCA 7027A output pentodes and a Brimar 12AX7.  There's also another pre-amp valve, an ECC83 that the manufacturers name has worn off (it's in a slip in metal shield).

Plenty of room inside, and encouraging to see it full of mustard caps, which are working fine, no leakage, so they can stay.

 A lot of components are on tag strips, and well spaced out which is very handy.   The resistors are mostly carbon film and look to be in good shape - can’t see any signs of heat stress.  

The only part that I can see is broken is the switch for the bass cut, and given that it’s in the negative feedback loop, which I have removed anyway.

SCHEMATIC   

The schematic below is very similar to the PA1003 that I have, but not identical.  I actually spent a good deal of time tracing it before I found the schematic below…    anyways, tracing is always a good learning experience.

Mine doesn't have the mic transformer and there are a few minor differences in values around the mic gain stage.

There's a wealth of information available here, care of Tim Robbins - I believe the schematic above is also by Tim, but I'm not certain of this.   The schematics included are of the PA1003 with his modifications.  Side note: He also has a guide to modding PA heads, Australian ones in particular; the theory probably applies to any weird old vintage PA amp.

This amp does have one unusual thing going on - the heaters for the 2 x 12AX7 are used as cathode bias for the output pentodes.  There's a Silvertone that does a similar thing, and some Fisher hi-fi amps.  This provides DC to the pre-amp heaters instead of the usual AC voltage, which in theory should make for quieter operation.  There are some downsides, including a slow warm-up and limits on what output valves can be used.

AWA PA1003 ADVERTISEMENT

RCA 7027A DATA