In all-glass CRTs, the internal 2nd anode coating in combination with the grounded external aquadag coating form a high voltage cap to ground, about 500 pF. This filters the rectified output of the 1B3 (half-wave DC pulses at 15,734 KHz) to steady DC.

In a metal shell CRT, there is no "capacitor", so a 500 pF doorknob was added externally to perform the filtering function. Leaving it out is similar to leaving out an electrolytic in the B+ supply, although the "hum" produced will be at H frequency and will affect only the CRT.