Another addition to my AWA valve PA family. I hesitated on this one at first—mainly because the power stage is a bit of a beast, and I’ve already got two from the same line. But the price was right, and apparently, I have no resistance when it comes to valve PA heads.
This one’s a 50-watter from the mid-sixties, with a particularly complex output stage—rumoured to resemble a McIntosh hi-fi design. I’ve no idea how it actually works yet, so there’s some learning ahead… or I might just leave it alone and enjoy the mystery.
- 2 x mic inputs, 1 x phono / radio input with tone control
- 12AX7 preamps, EF86 mixer, 12AX7 phase inverter, with a 12AU7 driving the 7027A in the power stage
- Solid state rectifier / voltage doubler ~500V B+
- Fixed bias, with rear panel access to bias controls
- Optical compressor / output limiter driven from an OT wind, applied to the EF86 mixer across all input channels
- Bass cut switch
- Power transformer marked 52481A
- Output transformer: unknown.
AWA MODEL PA1002B, 50W VALVE AMPLIFIER
These photos are from the eBay listing
The output transformer in this amp is a bit different from the usual guitar amp arrangement. Rather than having simple 4, 8 and 16 ohm speaker taps, it uses a sectional secondary made up of multiple windings and taps that can be linked in different ways.
With 10.37VAC applied to the primary, the measured secondary voltages were 1-3 = 0.945VAC, 4-6 = 0.946VAC, 7-9 = 0.943VAC, and 10-12 = 0.942VAC. That tells us each full secondary section is essentially identical, with a turns ratio of about 10.37 / 0.945 = 10.97:1. Because transformer voltage ratio follows turns ratio, and impedance reflects by the square of that ratio, each section reflects its load back to the primary by roughly 121:1
That matters because a push-pull pair of 7027A valves usually wants to see a plate-to-plate load in roughly the 6k to 6.6k region under common Class AB conditions. RCA’s data shows typical plate-to-plate loads of 6600 ohms, 6000 ohms and 6500 ohms for several fixed-bias examples, with cathode-biased examples also shown at 6600 ohms.
So from the valve side of things, the transformer makes good sense. The unusual part is the secondary. This was not designed as a normal guitar amp speaker secondary, but as a higher-impedance PA/distribution type output with multiple linking options. In practical terms, to get anything even vaguely close to a regular speaker impedance, all of the secondary windings need to be paralleled.
I should have looked more carefully at the photos. That's red plate on one 7027 and the other doesn't have much getter left.
AWA PA1002 SCHEMATIC
ACCESSORIES & ADVERTISING
As these were primarily intended for commercial use, a number of configurations are accessories were available - including preamps removed, so the amplifier could be used in a large distributed system.
FURTHER READING
Manual and installation instructions
Aussie Guitar Gear Heads. (does not like Safari security settings by the way, Chrome works)
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