This week we look at a recent repair for a regular customer, known in these pages as Jimmy James. Jim is an amp collector and his specialty is low-wattage valve (tube) amps, both vintage and contemporary, typically up to about 10 watts in audio output. Jim supplied the Matamp C7 which we discussed in our blog back in April 2012. That particular blog has been read many hundreds of times world wide, so we know there is a strong interest in such amps.
Jim has acquired a pair of WEM “Westminster 10″ 1×12 combo amplifiers, presumably manufactured in the UK during the late 60’s or early 70’s. We have chosen to look at amp serial # CW46047, as this example is in much better condition cosmetically, and also is loaded with a very good sounding vintage alnico magnet speaker, which appears to be original. In fact, apart from the valves, the whole amp appears to be completely original. There are no labels anywhere on this amp to identify the model name or number, the only way we found out this is the Westminster is by comparing the schematics available on the ‘net to the actual amp. The original Westminster models from the early 60’s used a valve (tube) rectifier, whereas this amp has a solid-state full-wave rectifier.
Jim’s amp was actually working, but needed microphonic valves replaced and attention to any electrical safety details, which included a new 240V 3-pin plug. The pots and jacks were due for cleaning, and the amp was unstable at higher settings of the ‘treble’ pot. It turns out the bodies of the three pots were no longer earthed (grounded) – earthing the pots back to the board cured the instability problem. The cathode bias bypass capacitor for the output valves was replaced, having been affected by the heat from the cathode resistor.
The whole amp is built on a steel plate behind the front control panel, which includes power and output transformers and a printed circuit board (p.c.b.) which supports all components excepting the two input jacks. The controls are volume, bass and treble – with the on/off switch part of the ‘treble’ pot. This assembly is fastened to a number of captive bolts, and the whole thing lifts out fairly easily for servicing, although on an amp of this age brittle wiring sometimes has a tendency to break off.
The amp design is based on a pair of 6BM8 (ECL82 in Europe & UK) valves (tubes), which comprise a triode and a power pentode in the same envelope. The performance of the triode is similar to that of a single triode within a 12AX7, and the power pentode has a maximum anode dissipation of 7 watts, with a maximum anode voltage of 300V for either section. So, the 6BM8 is similar to the 6GW8 discussed in previous blogs (beware: with different pin connections), but with a slightly lower rating for the pentode section, allowing for a maximum audio output power of around 10 watts in Class A/B push-pull (cathode bias), as compared to around 14 watts for the 6GW8. Untold thousands of 6BM8 valves were manufactured back in the day for consumer electronics applications, and unlike the 6GW8 they are still in production. The triode sections are typically used for phase-inverter applications, etc.
The preamp utilises a single 12AX7, is voiced somewhat like an early 60’s Fender amp, and could be described as low gain. Jim’s pair of Westminster 10’s are the very first WEM amps we’ve seen after decades of amp work, so obviously they were never imported into Australia in commercial quantities. The WEM amps have their own unique cosmetic appeal, including a very distinctive woven speaker grille cloth.
WEM are probably best known globally for their Copicat tape echo units, and David Gilmour famously used WEM 4×12 cabs with his Hiwatt amps in Pink Floyd.
Although WEM produced a range of UK built all-valve guitar amplifiers for many years, it would appear they achieved their greatest successes with sound systems for large concerts, virtually inventing British concert PA systems. The story of WEM and British concert PA has been related in a number of books and publications. IR.