Blown valves in valve amplifiers

A blown valve is by far the most usual cause of valve amplifier problems. Valves can blow at any time and can last years or months. If you have problems with your valve amplifier always check the valves first.

If the problem is with a particular channel (ie left or right speakers) you can easily swap the valves from left to right as the left side valves control the left channel and so on. If the problem then moves to the other speaker then you have found the fault.

In general the smaller pre-amp valves tend to last longer and when they go they start to make banging or popping noises. Since the larger power amp valves are using more current, they tend to be more spectacular when they go and can often blow a fuse at the same time.
On the SE88i if popping or banging can be heard through both channels it is likely to be the centre 12AX7 valve.  

Please make sure the amplifier is NOT connected to the electricity supply before removing or installing the vacuum tubes.

If you are removing valves, wait until they have cooled down before attempting to remove them.

What do you see if a valve blows?

1.Valves can be quite dramatic if they blow with loud crackling noises and a very bright glow from the valve as the valve consumes itself.
2.A valve can also blow with little or no visible effect.
3.Sometimes, if the vacuum in the valve is lost through tiny hairline cracks, you will see that the silver coating on the inside top of the valve will disappear or go clear.

example of a blown KT88

Smaller  pre-amplifier valves

The smaller pre amp valves tend to start making strange noises when they are on their way out. The can be loud popping or banging noises. On the SE88i, if the centre small 12AX7 valve is about to go you will hear loud popping and banks through both speakers.
You can check the other two pre-amp valves by swapping them over to see if odd sounds or problems swap from left to right channel.
When one of the pre-amplifier valves blows it is likely the amplifier will continue to operate, however, you will only hear sound from one speaker.

Installing and removing  valves

To remove a valve, use a dry cloth to grip the old valve by the base, pull the valve vertically upwards. The valve is simply pressed into the valve holder so there should be minimal resistance.

It is common for a blow valve to also blow the internal fuse, when this happens the amplifier will not turn on and there will be no blue light on the front panel the internal fuse is located in the power socket on the rear of the amplifier.