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21KV sounds low. Do you have a service manual with the correct spec? If it is low try to adjust the HV reg give the manual specified HV....If it won't go that high see if it goes higher with the HV regulator top connection removed...If it exceeds posted specs with the HV reg removed then the next step is to start servicing the regulator circuit to get it into a state where it can hold the HV on spec.
Low HV due to reg issues strains the fly in shunt reg sets like most RCAs, and accelerates cathode wear out in the CRTs of all sets...It also has no merrits to balance it's risks.
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