[Amps] SB-220 flash over in power supply - Help and results

[Dave G. ve9cb@rac.ca]

 Thank you to Victor 4X6GP, Jim K8OZ, Bill VE3NH, Tom W2CO and Dave NR1DX for their responses here on the reflector.  Thanks also to mark K1RX for his on-air observations.
Many of you suggested that I should increase the bypass capacitor at the HV end of the plate choke.  I have done so, and I must confess that is one step I omitted to follow in AG6K's Jan 1989 article on adding 160m to the SB-220.  I added the capacitor Rick suggested in his article, .0047uF.  Mine is rated for 8kV, much higher that Rick recommended.
I also checked over the balancing resistors on the caps of the B+ supply, and confirmed that they all still have their nominal values of 30kOhms each.  .  
The arcing in the power supply has stopped, and I continue to produce about 600w out with about 80w of drive.  The tubesa re of unknown age 
However, I appear to have a new problem: significant hum on my signal.  This suggests to me that if there weren't problems in the B+ supply before, there are now.  The HV meter on the amp shows nominal key-up values of 2kV (CW/Tune position) and 2.5kV (SSB position).  
An obvious solution may be to buy one of the two kits from W8CQ's Harbach Electronics to replace the current ancient filter caps.
Do any of you have other suggestions?
73,
Dave VE9CB-------- 
    On Saturday, December 14, 2019, 12:12:31 a.m. AST, Dave G. ve9cb at rac.ca <ve9cb at rac.ca> wrote:  
 
  Hello all:

I am looking for some advice on some arcing I am seeing in the B+ supply of my Heathkit SB-220.
I never used the amplifier before I bought it at a flea market in Spring 2019.
I have modified the amp to add 160m, following Rick Measures' (AG6K) excellent article in January 1989 QST.  Among the many things I did ti the amp, I replaced the stock Heathkit plate choke with the plate choke used in Ameriton amplifiers (part number 10-15197.  I thought was a better way than to winding my own choke.   
I now get 600w out of the amp on 160m with about 80w of drive on the Low B+ position (2kV).  I run the amp from a 110vac, rather than 220vac.  It will ultimately be used in a remote location where only 110vac is available.    
While keying in CW. I see sparks and flash-overs about 25% of the time. These flash-overs are between the leads of the filter caps in the B+ supply and the chassis wall of the amplifier.   
I suspect stray RF is the culprit.  I believe the Ameritron plate choke is sufficient, because it is used in all Ameritron amps that include 160m.  
So, what should I do to eliminate this arcing?  Would adding a layer of insulation on the interior wall of the chassis opposite the filter caps be enough?  Is this just applying a band-aid to cover up a more serious problem?
I have not tested the amp on other bands to see if it happens there, too.  The primary use of this amp will be for CQ 160m CW contest operations from Labrador (VO2).  
73,
Dave VE9CB