[julie_m]

The most likely explanation is poor connections on the valve pins. Remember the heaters are in series, so the current has to flow through all of them; and if there is a break anywhere in the chain, the current will be cut off and the valves will go cold.

Both the UCH21s are involved in the radio side of things. One changes the frequency from the tuned frequency to the IF (Intermediate Frequency). From this point on, everything is working on a fixed frequency. The other uses the "H" bit as an IF amplifier and the "C" bit as an audio preamplifier. The UBL21 contains the detector diode and the audio power amplifier, and the UY21 is the power supply rectifier.

A poor connection almost anywhere will create an audible noise; a brief disconnection of the heater chain will not give the valves time to cool down, but the change in heater current may induce noise elsewhere in the circuit. If one particular valve is especially crackly, it may need its pins cleaning. Get some proper contact cleaner, and use it sparingly on each of the pins. The socket contacts are spring-loaded with a sharp edge that should scrape away any dirt and oxide as the valve is inserted and removed -- be careful doing this, as there is the equivalent of about 50kg. of air pressing on the glass on top of any forces you may exert.