AX40
and AX100
IMPORTANT: These drawings and notes are provided for interest only and are not guaranteed to be accurate. Use at your own risk and discretion. The equipment shown contains lethal voltages and should only be worked on by people with suitable training and experience. Schematics and layouts are typical but will vary.
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 was from the WEM Owners Club's very interesting collection of Watkins and WEM reference material. The site's no longer operating unfortunately. Hopefully it might make a comeback.
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 (bless 'em 😖) have changed their image format - again - to avif just to make them awkward to print. Unless you want to buy a converter, the easiest way to make a hard copy is:
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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). This will display a higher res file. -
Right click and 'save image as' to somewhere as an avif file.
(On a touch screen, touch and hold down to view the menu) -
Upload the saved file to your Google Drive.
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Open the file and use the print menu to make a hard copy or save as a pdf.
HUM and NOISE
It wouldn't normally be a problem on stage but in a very quiet environment there's an excessive hum coupling between the mains transformer area and the preamp board. If the front panel is unscrewed and moved out, the hum generally quietens down (just to prove the point).
The culprit is the Echo Send/Return jack socket. The contacts and shorting link are a perfect loop aerial for 50Hz pick up (it's not the twisted pair wiring). It might seem like overkill but, if you want to cure it, it's very easy to fit a local screen secured under the socket. The one in the photograph is made of very thin steel sheet and reduced the output hum and noise from 22mV to 8mV rms. What was left was mostly normal background hiss.
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 were 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
The AX40 and AX100 preamp sections are identical apart from the zener diode series resistor. See above.
AX100 Power Amplifier
Sorry, haven't got one to look at at the moment.