A Marshall JCM900 Breaking Valves was the cause of this amp’s failure. It travels around a lot and it has the original 1990’s valve retainers on the output valves.
The original ones fitted to this 4102 model are shown below. These are fitted over the 5881’s to hold them in place. The issue is that an amp is well gigged and in and out of vans suffers a lot of vibration. These holders rub on the 5881’s and crack them.
The solution is to replace them with the claw type. This stops the wear on the glass.
One valve became cracked and broke, it shorted out and blew the fuses plus a screen resistor. The amp is dead ! A relatively simple fix. The retainers were duly replaced and the bias set up. While the amp was out of the case the very noisy master volume pots were replaced. The result was a happy amp. Now we should have no more 5881’s shattering on the road.
Marshall JCM900’s use 5881 output valves. Tung Sol 5881’s sound superb in these amps Check them out!