[Amps] SB-220 flash over in power supply

Thomas Walsh w2co at comcast.net
Sat Dec 14 09:20:55 EST 2019


Use a grid dip meter and make sure the plate choke is not
resonant in circuit with tubes in place on the freq. you want
to operate on. Actually whenever you change the plate choke
this should be done because it is unknown resonances.

The other thing to check is the filter capacitor equalizing
resistors to make sure none are open. That would cause
much higher voltages on those caps..
de W2CO


TW @
A.R.S. W2CO


On Dec 13, 2019, at 21:12, 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
   
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