& SP61
These two are early RF pentodes of similar design but having respectively 4.0 V and 6.3 V heaters. These valves were used widely in early post-war receivers because of their availability from surplus Government stock.
Apart from low emission the major faults are mostly physical, namely intermittent metallising due to the valve becoming loose in its base, a dry joint top cap causing noisy operation and erratic synchronising, weak heater valve holder contacts due to the heavy current drawn.
The offending valve holder pins may usually be taken out and tightened without replacing the entire holder