Hi to all,
As some of you already know, my business involves servicing professional audio equipment including valve instrument amplifiers. Some of these amps are quite new but others are decades old.
Last week, one of the oldest examples I have ever seen arrived in the workshop carried by its proud owner. A "classic" early 1960s built Gibson GA 40 aka - the "Les Paul". It has a single chassis is fitted at the top of an open backed cabinet with a 12 inch speaker - see pic.
The amp was made in the USA for their local market and hence the 115 volt AC power system. As you can see from the schematic it has no AC supply safety earth - standard US practice at the time.
Instead, there is a "polarity switch " that connects a 0.02uF, 400volt DC capacitor to the chassis from one or other of the AC supply wires. Users are supposed to pick the one that causes the least humming noise !!
Now, this particular GA40 amp had been long ago converted to operate from a
240 volt AC supply. A 230/115 volt AUTO-TRANSFORMER ( also made in the USA ) had been installed in the bottom of the speaker cabinet. It was fitted with a 2 pin US type outlet and a short, black rubber " figure 8 " cable trailing out of a hole in cover plate. A dubious looking in-line join extended this cable via 2 metres of round black rubber cable finally ending in an Aussie 3 pin plug.Safety problems:
- The short figure 8 cable (carrying 240 volts AC ) was covered in open cracks allowing the tinned copper conductors to be seen and touched.
- The 230 /115 volt auto-transformer was wired with its common connection going to the 240 volt AC active wire.
- Because of #2, the "polarity switch" connected the amplifier's * FLOATING * chassis to either 120 volts AC or 240 volts AC via the now 45 year old, 0.02 uF, film cap.
- Such film caps are not rated to withstand 240 volts AC and commonly fail short when made to do so.
When I enquired, the owner admitted he sometimes receiving " shocks " from the amp and had no idea what the "polarity switch " was for. He also revealed that his 6 year old daughter liked to plug mic into the amp and sing through it.
Given that mics and electric guitars connect people directly to the metalwork of any amplifiers they are being used with - the whole situation was highly lethal.
Suffice to say, the old amp now has a modern 3 core lead supplying the auto-transformer, the transformer's frame is wired to the AC supply ground pin and the chassis of the amp is permanently wired to the frame of the transformer. The oversized and very dangerous 0.02 uF film cap ( referred to as a "death cap" in the USA) has simply been removed.
....... Phil