WEM ER40 PA40 ER100 and PA100 | backfromthesixties

ER40, PA40, ER100 & PA100

(Plus SL40 & SL100)

Two pairs of identical twins from the Sixties and Seventies. The ER and PA versions of each type seem to have been identical (including the price) apart from the model number on the front panel. Bit of mystery there.

Both types are great guitar amps but, at first sight, one volume control per channel and hi/lo inputs would be disappointing if you wanted more than two mics . If inputs 1 and 2 are used at the same time, though, their sensitivities become identical, so four mics are a possibility. Main vocalist in one channel and backup vocals in the other seems workable. If there are more than three people in the band who can sing you're very lucky.

The 100 watt versions seem to have got the slave link system first, leaving the 40s to catch up a bit later. Early 40s won't have the extra jack socket on the front panel - not that they ever got round to labelling it anyway. One quirky feature to be aware of is the almost 5v dc on the slave output tip.

The SL40 and SL100 are just the power output stages and power supplies from the ER and PA models and were designed to be daisy chained to drive extra sets of speakers. Either of the SLs can be used with any PA/ER amp (40 or 100). They don't have to match. They can also be driven by any equipment with a line level output.

RIGHT: Just make sure that every amp in the chain has the power output in the same phase. Things can get a bit odd otherwise.

This PA100 came with the speaker wires reversed.

Not a problem until you link it to another amp or attach a scope ground to it.

Just as a side note, the fuse holders in series with the speakers on 40s are an endless source of amusement when they corrode and cause intermittent output.

Mechanically, SLs are connected in series, as described in the catalogues. Electronically, though, the inputs are all driven in parallel from the main amp. The IN and OUT sockets are just joined together internally, so each unit puts an equal load on the output impedance of whatever's driving the stack. Just like the speaker outputs, it doesn't matter which front panel socket you use.

All models are silicon solid state and they're all designed to run into a six ohm load for full output.

Now everybody knows that real men don't need the instructions (and you're not likely to find any anyway) but, just in case, this is what the options were:

In real life, all three outputs are wired in parallel, so any speaker can go into any socket. The amps are quite happy into an open circuit and you can use whatever speaker system you like as long as the total impedance is more than 6ohms. The power output obviously drops off as the impedance goes up.

Twelve and six ohm cabinets are getting hard to come by these days, but two sixteens or an eight will do the job. Don't try it with two eights though.

On the subject of power output, don't expect to get a continuous 40 or 100watts rms with a sine wave. 30watts and 75watts would be doing well at clipping.

Power Connectors

Power Connections

If the chassis still has the original mains power inlet, it takes a Bulgin S.A. 2111 (90 deg) or S.A. 1862 (straight) cable mounted, mating socket. And before somebody starts arguing that it's a plug, this is the Bulgin catalogue definition:

Just out of interest, chassis mounted Bulgin plugs from this era usually had split pins that mated with solid, tubular sockets on the cable end. Watkins didn't seem too bothered and you can find split or solid pins at random.

If the amp needs a PAT certificate, it needs to have this system removed and an IEC connector fitted. A Bulgin PX0579 can be used (after the panel is opened out slightly) if it's fitted sideways. They're available from Farnell, RS, Mouser and Digi-Key among others. The original tabs were generally 6.3mm (1/4").

Some older amps are also going to need a short wire link made up to ground the panel metalwork.

PX0579/28 2.8mm solder tabs

PX0579/48 4.8mm solder tabs

PX0579/63 6.3mm solder tabs

PX0579/TERM Screw terminals

RIGHT: A classic, dodgy Bulgin cable. Earth socket completely missing and the whiskers waiting to trip out the venue mains panel.

LEFT: Open the panel slightly at the arrows to fit a PX0579.

The PA40

Above: Early versions won't have the slave link jack socket to the left of the neon lamp.

Catalogue Entry: PA40

"Exactly as PA.100 in appearance and facilities" (See below) "but with less power output."

Input Sensitivity: 4 & 8mV high impedance (This is probably a mistake & should be 7 and 14mV)

Slave Output: 640mV @ 600 ohms

Power Output: 40 watts R.M.S. into 6 ohms"

Dimensions: 171/2" w x 7" h x 101/2" d

Weight: 18Ib

Price in 1970: £58.00

The ER40

Catalogue Entry: ER40

"A solid state, general purpose amplifier developing 40 watts R.M.S. for use with any instrument - Guitar, Bass, Keyboards etc. This is a thoroughly reliable and trouble free unit capable of extremely high, clean levels of volume.

Two channels with one high and one low sensitivity input on each channel. Treble boost and separate Volume, Bass and Treble controls for each channel. Output for connecting Slave amplifier.

Input Sensitivity: High 7.5mV @ 1KHz, Low 15mV @ 1KHz

Input Impedance: High

Slave Output: 640mV @ 1KHz (under full power)

Output Impedance: 6 - 8 ohms

Output Power: 40 watts R.M.S. into 6 ohms

Mains: 110 - 220 - 240 volts A.C."

Dimensions: 171/2" w x 7" h x 101/2" d (44.4 x 17.8 x 26.7cm)

Weight: 18Ib (8.16Kg)

Price in 1970: £58.00

The SL40

Catalogue Entry: SL40

"Exactly as SL.100 in appearance and operation but with 40 watts of output power.

Input Sensitivity: 640mV high impedance

Power output: 40 watts rms @ 6 ohms

Power amp response: Flat 30Hz - 16KHz

Mains: 110 - 220 - 240 volts A.C."

Dimensions: 171/2" w x 7" h x 101/2" d (44.4 x 17.8 x 26.7cm)

Weight: 17Ib

Price in 1970: £51.00

The PA100

Catalogue Entry: PA100

"A reliable and consistant unit which may be expected to give trouble free performance even when driven for long periods at full power. Two channels with two inputs ( one high and one low sensitivity to each channel ). Presence booster operated by slide switch. Separate Volume, Bass and Treble controls for each channel. Output for slave connection.

(As with all WEM units, the speaker leads are supplied with the amplifier and not the speaker enclosures.)

Input Sensitivity: 7 and 14mV high impedance

Slave Output: 640mV @ 600 ohms

Power output: 100 watts rms @ 6 ohms

Power amp response: Flat 30Hz - 16KHz

Power consumption: 230 watts

Mains: 110 - 220 - 240 volts A.C."

Dimensions: 171/2" w x 7" h x 101/2" d (44.4 x 17.8 x 26.7cm)

Weight: 23Ib

Price in 1970: £98.50

The ER100

Catalogue Entry: ER100

"Exactly as ER40 in appearance and facilities" (See above) "but with power output of 100 watts."

Dimensions: 171/2" w x 7" h x 101/2" d

Weight: 23Ib (10.5Kg)

Price in 1970: £98.50

The SL100

Catalogue Entry: SL100

"A Slave power amplifier. Connected in series with the PA.100 this unit will feed 100 watts into it's own pair of columns. Further slaves may be connected in series with each other to reach extremely high powers. The interconnecting is done quite simply with a short lead connected to standard jack plugs.

Input Sensitivity: 640mV high impedance

Power output: 100 watts rms @ 6 ohms

Power amp response: Flat 30Hz - 16KHz

Mains: 110 - 220 - 240 volts A.C."

Dimensions: 171/2" w x 7" h x 101/2" d (44.4 x 17.8 x 26.7cm)

Weight: 22Ib

Price in 1970: £86.50

Chassis and Cabinet Variations

RIGHT: The back of an early 40 showing an old chassis with captive cables, a steel bracket to wrap them around and clips to store the cabinet attachment bolts.

LEFT: The same chassis in a new style case with an old label. The speaker / mains panel is at the side, to the left in this illustration.

RIGHT: A later 40 chassis / case / label combination

The early label shows the old UK mains lead wiring colours of red, black and green. New regs in mid 1969 saw flexible appliance leads change to Brown, Blue and Green/Yellow as shown on the later back panel.

Examples with a hole from top to bottom at each end of the case would have been supplied as part of a head / cab combination with two long bolts to hold them together.

SCHEMATICS & LAYOUTS

These are just snapshots from a long production span.

Layouts, wire colours and component values are going to vary.

PRINTING

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.

Front panels and power sections are interchangeable between ERs and PAs of the same power rating, but NOT between 40s and 100s without changing the resistor on the preamp board that feeds the zener diode voltage regulator (Edit. The resistor value also varies with the type of zener fitted). Component ID numbers shown below follow the factory where available, but the power sections are mostly specific to these drawings. Preamp numbers start at 1 and power components start at 101. Con1 and Con2 on the preamps mate with Con101 and Con102 on the power amps.

HUM and NOISE

In all WEM heads in this configuration, coupling between the mains transformer area and the front preamp board tends to cause hum even with the volumes right down and a few units were produced with a metal screen fitted between the two sections. If the front panel is unscrewed and laid flat, the hum generally quietens down (yes I know you can't use it like that). Since it's only a (small) problem in a studio setting and nobody's going to notice on stage, it's not 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.

Preamp Schematic

WEM ER40, PA40, ER100 & PA100

Later Version From PA40 Ser No. 15063