Valve amplifier kit

Flawed valve amplifier exploded!

While the boss is away the muppet does play.

When I first set out repairing electronics items I would occasionally find a TV with a valve or two in them, but most of the time there were plenty of transistors in them. At night school the city and guilds course largely ignored the vacuum valve technology and concentrated on bipolar transistor circuits.

Once I started working I partnered up with Ken Barton a engineer with many years experience. We made a good team as I was confident on the newer technology such as addressable fire alarms, while Ken knew plenty about the valve based public address systems and flag and relay fire alarm systems.

I had never built anything using valves before, so I bought a little amplifier board kit which in truth is only a pre amplifier stage.

This little 2 valve kit uses relatively low voltages and with the blue light emitting diodes under the tubes looks cute.

After building the amplifier I hooked it up and left it running on test. An hour later there was a loud bang. Something hit the ceiling and there was a magic smoke smell all over the room.

After unplugging the amplifier I found a capacitor had ruptured in a very explosive way.

This kit had come with a schematic so I decided to try and trace the problem. I thought maybe I wired something in the wrong way around!

As it turned out I had all the parts in the correct way a round and in the correct locations. The fault happened as the supplied capacitors had very little head room for over voltage. A small increase in the mains voltage was likely enough to explode the capacitor.

I replaced the capacitors with higher voltage rating ones and the amplifier was placed in a clear acrylic case. I have no real use for it as a amplifier but maybe I will incorporate it in another project some day.