[Amps] SB-220 parasitic suppressors

[Phil Clements]

: [Amps] SB-220 parasitic suppressors


> One reason that an SB-220 can develop VHF parasitics is the 
> following:  the original suppressors use 2-watt carbon composition 
> resistors.  These resistors tend to increase in value when they age, 
> and especially when they are heated. If an SB-220 has been used 
> heavily, especially on 10 meters or -- worst case -- 
> on 10 (or 11!) meter AM, the resistors will have gotten quite hot 
> and may have changed significantly in value, or even opened up.
>
> If the resistors are discolored, they should definitely be replaced.

Sage advice, Vic....
My standard procedure here is to check the value of the suppressor 
resistor on a time schedule, along with cleaning the components and 
chassis, and a thorough visual inspection throughout the amp. As Rich 
and several others have pointed out, the Rsupp can change value 
without any outward signs whatsoever. It can be the result of age, 
over-heating, glitches, or _____.( fill in the blank) It is a chore 
because one end of the resistor must be lifted to measure it. I have 
had several Rsupp's over the years that appeared pristine, but broke 
when gently unsoldered. Those of us that use Globar resistors need to 
make sure the clips are clean and good contact is made between the 
clip and resistor body. With the price and availability of tubes 
nowadays, a maintenance schedule makes sense to me, and is cheap 
insurance. While you are at it, don't forget to clean the fan/blower 
blades and oil the motor bearings.


(((73)))
Phil, K5PC