[Amps] SB-220 parasitic suppressors

[R.Measures]

On Nov 3, 2004, at 2:04 PM, Vic Rosenthal wrote:

> 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.

Vic -- The suppressor resistors in my SB-220 toasted during operation 
on 7MHz.  I replaced them with new resistors and they got toasted too.  
I did the math and there was no way that 7MHz energy should have been 
able to do that with a 60 or so nH L across each R.  I observed that my 
SB-220 had a slight second-dip in plate-anode current as I tuned 
through resonance on 40m.   These things started me thinking.  Several 
years later I concluded that Murphy's theorem that things are not as 
simple as they first look is usually true.
> ...
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org