He is right about the VHF parasiics.
A new capacitor will not stop it from arcing.

Look here for answers and parts to fix it.

http://www.somis.org/

I have installed his Low VHF-Q parasitic
suppressor retrofit-kit in my own SB220
and it has never arced.

Chris wrote:
Hi Larry
It doesn't mean that the cap is bad because it arcs, Try installing a
better parasitic suppressor on the tubes first.

I'm looking for the TUNE variable capacitor for a Heathkit
SB-220/SB-221 Linear Amplifier. The one that's in the one I just
picked up has been arc'ed so badly that even after carefully filing
away the burnt/melted parts, it still arcs occasionally. The cap is a
250 pf. variable cap, and the Heathkit part number was 26-131. I
haven't looked at a Heath SB-200/SB-201 manual to see if those amps
also use the same Tuning cap.

If anyone has a junked/parts unit that has this variable cap
available, please let me know.

Larry