Some forum members had a shorted tube – here is a picture of how it looks like (credit to AudiogoN member ‘amfibius’) when the event happens:
1. The short (lighting show you have seen) on 845 tubes is caused by residual air in the vacuum tubes act with active materials on the cathode. When active materials (electron emission coating on the cathode) become detached from the cathode under extreme heat (1000V), it will act with the residual air in the tube causing a short (lightening arc). The residual air may not be present at factory final testing when the tube leaves factory, but during shipping/handling/or usage with time, very tiny amount of air could leak into the tube, especially these large sized vacuum tubes.
2. Very few tube factory still make 845 tubes nowadays – as far as I know, only Shuguang in China and KR Audio in Europe. KR 845 tubes have quality problems too such as red plate caused by over current, so Shuguang 845 tubes have been used on most modern 845 tube amps. Shuguang made a few different design 845 tubes in the past – including a metal plate 845C. The metal plate 845C tubes were considered best sounding among all past models but the metal plate soldering cannot handle extreme heat well, so historically there have been very high percentage failure rate (nearly 50%) so the metal plate 845C tube has disappeared from the market now.
3. When a 845 tube has an internal short, it will produce crackling noise to the speakers, but such a noise (if the amp volume was not extremely high) usually is not of extreme frequency and will not damage the speakers, but it could cause panic to the users when seeing the lightening and sound at tube failure. In an event of tube short, the amp’s main fuse will be blown to protect the main transformers in less than a second, so usually the amplifier will continue operate fine and no other parts will be burnt. The tube short happens internally in the glass enclosure – the internal pressure will never be more than air pressure in the environment because tubes are vacuumed (less air pressure inside than outside even a leakage), there will never be a glass explosion happening at the event. When too much air has leaked into the tube, the silver coating inside the tube will turn grey or white indicating the tube is no longer vacuum sealed, ie not usable anymore.
4. This tube failure event is unpredictable – but it’s not to be scared. Other type of power output tubes such KT88 and 300B do not operate at as high voltage as 845 (450V vs. 900-1000V), so consumers rarely see tube failure as often with those tubes as with 845. Once a 845 has a short (light show and sound), do not use it again in other amps – such a tube should be disposed.
5. NOS RCA 845 tubes are considered the best quality on the market, but the technology then has been somehow lost with time. Shuguang is continuing research and improving on their 845 tubes, but that will take sometime to become reality.
I hope this helps 845 tube users who have been ‘shocked’ by the lighting show Not to be scared when the lighting happens – you have just witnessed nature at work from a close distance inside a glass tube.
Rachel @ Grant Fidelity