AWA: Model PA872, 20w Valve PA Amplifier (Serial Number 94)

I posted another AWA PA872 a while back, but realised I never actually shared this one.  It had been sitting on the shelf for months - one of those projects that was nearly there, but never quite finished.  I finally got around to wrapping it up this weekend.

This looks to be an earlier unit, although schematically the same. The serial number is much lower (#94), and the general aesthetic appears earlier.

Someone had already had a go at converting it—let’s just say what I found inside is a good reminder to be cautious with amps of unknown history.  I never even turned it on - deathtrap.

This one came to me as a friendly trade. The previous owner hadn’t worked on it themselves, and was unaware of the condition—they were just clearing out a few projects they knew they were never going to get around to.

With no real baseline to work from, I stripped it right back to the sockets and started again.  It’s now rebuilt as a 5E3-style Deluxe, with a few tweaks to taste.

The stock 5E3 uses 100n coupling caps on the preamp.  I dropped the bright channel right down to 22n, and trimmed the normal channel slightly to 82n.  For the tone control, I’ve gone with stock 5n on the bass side and increased treble from 500pf to 1n, for a slightly different response.  The results are good - two distinctly different, but useful channels.

I chased a pretty stubborn hum for a while - in the end, it came down to grounding.  Moving the power transformer ground to the same point as the filter caps fixed it immediately.  We're only talking a distance of about 80mm.

There was a previous attempt to “fix” the hum, which involved adding 2 x 220uf filter caps to the power supply—so it’s no surprise the valve rectifier was dead on arrival.  Yes - 220uf, not 22...  

AWA MODEL PA 872 - 20 WATT PA AMPLIFIER

I really like the blue hammered finish, and someone in the design team really nailed the look.  

Schematic screened onto the back of the amp - a nice touch for serviceability.  Serial number 94.

The amp can run a 15 ohm speaker without any dodgy rewiring of the output transformer, so that's always handy.

I patched over some holes for a couple of can capacitors, as they were dead, and not conveniently placed for what I wanted to achieve.

Not my most amazing work to date, but it is solid, and it sounds great.

FURTHER READING

Tim Robbins PA872 conversion 

My previous post for the 872, which has full tech details