AX40
and AX100
Introduction
The mid seventies brought some updates to the WEM ER and PA heads, with public address duties going to the AX and guitar to the GX series. They were soon overtaken by changes in the music scene and the 40watt versions don't seem to have lasted long. The AX was eventually joined/replaced by more versatile, multichannel powered mixers. Still useful for small bands and solo performers though. Apparently voiced for PA work, they don't really suit guitar reproduction quite as well as the old PA/ER (personal opinion). But then why should they?
The 1975/6 catalogue extract below is courtesy of the WEM Owners Club at http://wem-owners.com/ . Lots of really interesting Watkins and WEM reference material.
Catalogue Specifications
Input Sensitivity:
Catalogue just says "high impedance" with no levels specified.
Bench test with volume at max and tone controls at 12o'clock:
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Input 1 - 6mV
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input 2 - 12mV
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Input 3 - 6mV
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Input 4 - 12mV
If both inputs of either pair (1+2 or 3+4) are used, both inputs switch to 6mV.
Slave Output:
640mV @ 600 ohms
Power Output:
40watts into 8ohms (AX40)
100watts into 8ohms (AX100
Note - there is no mention of how the power is measured.
What you get at clipping with a 1kHz sine wave into a dummy load is:
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33watts rms into 6ohms
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28.5watts rms into 8ohms
Dimensions:
Both versions - 185/8"w x 71/8"h x 11"d
Weight:
AX40 - 18Ib approx
AX100 - ??
Price in 1975:
AX40 - £84.00 (about £625 in 2024)
AX100 - £110 (about £818 in 2024)
For a discussion about the Bulgin power connectors, have a look at the ER40 page HERE.
PRINTING
Wix only provides webp images at the moment. To make a hard copy of a drawing :
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Left click when the magnifying glass icon appears and wait for the picture to clear.
(On a touch screen, tap on the image) -
Right click and 'save image as' to somewhere as a webp file.
(On a touch screen, touch and hold down to view the menu) -
Open the saved file with 'Photos' (Windows 10) or whatever app works for you.
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Use the print menu to print to A4 plain paper.
HUM and NOISE
In all WEM heads in this configuration (ERs, PAs, AXs), coupling between the mains transformer area and the front preamp board tends to cause hum even with the volumes right down and some units were fitted with a metal screen between the two sections. If the front panel is unscrewed and laid flat, the hum generally quietens down (just to prove the point). It really isn't worth spending hours on the test bench trying to get rid of it.
The other characteristic they all have in common is noisy volume pots at switch on. While all of the caps are busy charging up there can be two to three volts DC across the pots. It normally cleans up after about thirty seconds and isn't a problem. If the volume pots continue to crackle and Deoxit doesn't work, it's worth trying to rebuild them. They're almost impossible to find and good quality when they were new. Occasionally, fitting new low leakage 6u8s can help slightly.
Preamp
As in the ER and PA series, the 40watt and 100watt versions share a common preamp, the one difference being the value of the series resistor feeding the zener voltage regulator. The new circuit gets an isolating capacitor on the slave output - no more DC. The tone circuits have changed to a Baxandall arrangement in the feedback loop of a one transistor gain stage.
WEM AX40 and AX100 Preamp Schematic
WEM AX40 and AX100 Preamp Board Layout
AX40 Power Amplifier
Electrically, the AX40 power amp section is the same as the ER/PA40s with a few updates to component types. Mechanically it's much simpler. Main features are:
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No mains voltage selector
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A new circuit board with the output device power resistors on-board instead of on a tag strip
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A grounding link to the speaker socket / mains input mounting plate
WEM AX40 Power Amplifier Schematic
WEM AX40 Power Section Layout
WEM AX40 Power Amplifier PCB Layout
AX100 Preamp
See AX40 Preamp section above.
AX100 Power Amplifier
Sorry, haven't got one to look at at the moment.