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WEM logo.jpg

AX40

WEM AX100 Front View.jpg

and AX100

WEM AX40 Front View.jpg

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.

75-76 Bell Extract.jpg

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:

  • Input 1 - 6mV

  • input 2 - 12mV

  • Input 3 - 6mV

  • 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:

  • 33watts rms into 6ohms

  • 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)

WEM AX40 Side View.jpg
WEM AX40 Rear View.jpg

For a discussion about the Bulgin power connectors, have a look at the ER40 page HERE.

PRINTING

Magnifying glass.jpg

Wix only provides webp images at the moment. To make a hard copy of a drawing :

  • 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.

  • 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 Preamp Schematic.jpg

WEM AX40 and AX100 Preamp Schematic

WEM AX40 and AX100 Preamp Board Layout

WEM AX40 Preamp Board Layout.jpg

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:

  • No mains voltage selector

  • A new circuit board with the output device power resistors on-board instead of on a tag strip

  • A grounding link to the speaker socket / mains input mounting plate

WEM AX40 Power Amplifier Schematic

WEM AX40  Power Amp Schematic.jpg

WEM AX40 Power Section Layout

WEM AX40 Power Section Layout.jpg

WEM AX40 Power Amplifier PCB Layout

WEM AXA40 Power Amp Board.jpg

AX100 Preamp

See AX40 Preamp section above.

AX100 Power Amplifier

Sorry, haven't got one to look at at the moment.

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