12AX7 into a pair of 6BM8's in push-pull
Outputs are 150, 300, 600 and 1200 ohms (100 volt)
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http://www.ozvalveamps.elands.com/awa.htm | Last update:
20:56 06/04/07
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Amalgamated Wireless Australia, AWA, was a major manufacturer of electronics in Australia, mainly for the broadcast radio sector, but also some more general industrial products. These included amplifiers intended for factory and school PA service. They were taken over by UK firm STC, Standard Telephones and Cables, now part of the French Alcatel group.
Contains:| (AWA) PA774 , PA826 , PA827 , PA828 , (STC) PA1001 , (AWA) PA1002BY , (AWA) PA1003Z , (STC) 6CM5 rebuild , 55W 6DQ6 , (Philips) LowLine 10 |
5 Watt
Model: PA828
Serial: 406
Date: 1960's
Source: Evan Lorden
Circuit diagramme.The amplifier itself is very conventional; 6AV6 mic pre, grampohone pickup input with level and top cut, and two 12AX7 stages into a single 6V6.
The output is arranged to drive 1, 2, 3, or 4 by 600-ohm loads.
The give-away is the power supply, 240VAC mains and 12 volt DC (from a car battery). The tranny has both a 240 volts and 12 volt input winding. The 12 volt DC is chopped by a mechanical vibrator.
So we have a small amplifier that will run from a car battery and drive a few 600-ohm speakers, so it's intended for portable use, and to drive long speaker lines. Obvious uses would be small surf life-saving carnivals, school sports days and the like, driving a few metal horns on poles.
The vibrator itself is a heavily-weighted reed arranged much like a buzzer or bell, with coil and an interrupter contact to make it oscillate.
This carries another pair of contacts that allow the 12 volts to be alternately switched to either end of the 12 volt winding. This is called an asynch vibrator, and has to be followed by some sort of rectifier, in this case the 6X4.
There is another form called the synch or self rectifying vibrator. This switches the high voltage side with yet another set of contacts, making a mechanical rectifier.
There was once a lot of gear around that used vibrators for the HT, including many early valve car radios, grand sets for off-grid farmhouse use (which may be for 24 or 32 volts DC), taxi two-way radios, etc.
Vibrators are prone to contact pitting, but with a little care can generally be wangled open and repaired. Contact timing and adjustment is critical and some care should be taken after a repair that the switching is clean and symmetrical.
Source: The Oscilloscope Book, E.N.Bradley, Norman Price 1951.At one time there were solid-state replacements available, and in this case it wouldn't be too hard to figure out a suitable s.s. power chopper.
The “standby/operate” switch, S1, turns off the vibrator and HT while leaving the heaters running, as a power economy measure.
Model: PA1001
Serial: AB 4031
Date: 1960's
From Craig Dean.
I have just acquired/rescued (via e-bay, $4.99) a 1960's school/auditorium 100w STC valve amp model PA1001. It's a rebadged AWA, the original AWA logo is still visible under the solid black line on the front panel.
Photos are of its initial condition prior to replacement of most of the caps, suspect resistors and damaged wiring.
The corrosion on the power and output transformer turned out to be only relatively benign surface corrosion, so the $5 bet paid off well.
It now works with some minor tweaking and fault isolation still required, (a minor HT insulation breakdown of one of the connectors or wires, residual AC ripple, and fitment of suitably matched EL34's). Alternatively it can be fitted with 6L6/KT66's.
I plan to use it as a guitar amp for the time being while I decide how best to restore it.
The control line-up is very typical of PA's for factory and office service. In this case it has two microphone inputs with volume controls.
Next is a volume control for the Phono-Radio input. This reflects the common practice of feeding a tuner or record turntable (phono) into the PA for background music, “BGM”, between announcements.
The Radio input connects to a socket under the chassis and Phono to a socket on the same side as the mic inputs.
Generally these amps were installed somewhere in the office, frequently at the front reception desk, and it was not unknown before the days of Muzac™ for the receptionist to double as the factory DJ, changing records.
The dimples are plugs in holes. This indicates that the basic chassis metalwork was also used for other models with different facilities, perhaps one with a built-in radio tuner.
The switch selects either the Phono (turntable) or Radio input.
The tone control would apply to the BGM but not the microphone inputs, and would normally be a simple top-cut type control.
The terminal strips are to select the required output impedance and to connect the speaker circuit(s). The required strappings are shown in the table. In a large installation this may be 100-Volt Line. Note the terminals, upper right, marked “A” and “E”. These also suggest a model including an AM tuner, being the Antenna and Earth terminals respectively.
The Dymo™ tape marking gives away the four cathode fuses as a retro-fit. Cathode fusing is not a good idea.
The Output Limiter pre-set may be related to ALC/Ducking.
The next two adjustments are for the bias adjustment to allow idle current setting for each pair of output valves.
The “AC Bal” will be a heater circuit balance or “hum-dinger”.
Finally a switched bass cut control for situations where the speakers are minimally baffled and excessive bass drive would result on cone flapping on breath-pops, or to cut turntable rumble.
Things start in the lower-left. The screw locking rings for the two microphone connectors can just be seen in the left side.
The first stage is built on a sub-chassis often both mechanically and electricially isolated from the main chassis against physical microphonics and hum-loops, generally using rubber grommets as mountings.
The two capsules next to the preamp stage are mu-metal screened isolation transformers for each microphone, typically something like 600 ohms to 10K ohms, and may even provide balanced inputs. Sometimes the input circuits also bring in a keying line as a contact from a remote mic to allow remote PTT.
The following preamp and phase-inverter stage are above, and in the centre is the output transformer connected to the rear output selectors. Note the “A” terminal isn't connected.
Below-centre are a number of small holes. These may be for mounting of coils when a tuner is included. Note also the underchassis vent slots around the output quad sockets. Passive cooling is still good design and the idea is to use the heat from the output valves to draw air from below chassis.
The holes in the front covered with fine metalmesh are actually intended for a small loudspeaker for local monitoring.
The socket far right below the mains transformer is a connection point for such a speaker when fitted.
The switch labeled “Monitor/On” on the far right [front panel] selects the monitor speaker which is connected via the socket on the chassis directly behind this switch. This socket is connected via the switch through a 22ohm resistor to terminal 1 and 12 of the output transformer.
Click for full size 150kb jpg.The deposited dirt maps the convestion air flows, and indicates this amp spent a long time operating in one place. Most of the components would now be suspect, particularly the few waxed paper capacitors scattered about. Note the burnt screen resistor on the second output socket from the top, common with a 6CA7/EL34 screen grid failure.
The two larger caps on the right are a voltage doubler network, the power transformer is not centre tapped. The Radio input connects to a socket under the chasis and Phono to a socket on the same side as the mic input.
Similar to Conqueror
50 Watt PA. 2x 6CA7/EL34's(?)
Source: Steve Kain 08/02/06
Clean underchassis, but some of the components may be past it.
Note the burned screen resistors (again) just left of o/p valve sockets; two 47 ohm heater balance resistors on the upper output socket.
One insulated electro, one normal, in series, and two diodes in series below - a doubler HT supply on the right.
Your guess is as good as mine as to what the 24 volt AC outlet was intended for.
Controls close-up 16kb jpg
Model: PA1003Z
Serial: DL 7738 - 770
Date: 1960's
Source: Murray and JodiAnother very conventional amp like its kin, but this particular one effectively has the heater for the preamp as the output pair cathode resistor! If this is someone at AWA being very cunning, I'm afraid it totally evades me.
I'd love to know if they are all like this, or only this unit has been modded. m&j have kindly provided a circuit trace, but for once I'm not putting it up until another one turns up like this, 'cause I suspect this one may only be somebody's bodgy fiddling.
This amp sounds incredibly nice. It's been fully rebuilt and reconditioned by an experienced tech, and is ready to give years of service to the tone-conscious guitarist. Truly a match for any 'boutique' amp, but a genuine hard-wired article from the '60s, not a modern clone.
The tech who rebuilt this amp says;
The STC amp has been modified and has received outstanding appraisal from one of Adelaide's old guitarists. He said that this amp performed much better than his American amp which used 6L6's in push pull ! I was truly appreciative of his comments.
This one is an almost identical circuit to the Playmaster. There are two 6CM5 high mu pentodes in the output, an EF86 preamp and a 12AT7 phase splitter. All standard guitar and speaker jacks. Controls are on/off switch, pre-amp gain, main gain, and tone control. There are two spare NOS 6CM5's also plugged into the chassis, and they are included. The output specs quickly, 50Hz 25W and 1kHz 38W
Source: Neil Rote, Grouse Guitars
Source: DunxyAnother small factory PA using 6GW8's (centre) for about 10 watts out.
The thing that looks like an upside-down film can with an octal base, left rear, will be the microphone input coupling transformer.
Note the blackening on the rectifier glass envelope, far right. This is caused by a long period of electron bombardment and is the sign of a valve that has been in service for a long time. In extreme cases this darkening will become silver, a bit like the getter.
It means the valve is old but I've seen equipment with dozens of 12AX7's like this still working fine.
Source: Dunxy
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