[Amps] A Meeting Ground

[R.Measures]

On Nov 4, 2004, at 9:25 AM, StephenTetorka at cs.com wrote:

> Hi All:
>
> This radiant discussion on parasitics invites me to ask a question: 
> "where does all the 'mish-mash' go to when a chassis ground is 
> employed?"

Steve --  In most g-g amplifiers, the grid-I meter shunt R is the only 
DC current path from the –HV to chassis gnd.  Thus, when a +HV 
flashover to gnd occurs, the potential across the grid-I meter shunt R 
spikes to several kV negative unless there are glitch diodes across it. 
   In a SB-220, R3 (0.82-ohm, 2w) is the grid-I meter shunt R.  When a
SB-220 has a parasitic oscillation there are two currents that can 
damage R3:  1.  since there is an unloaded condition during a VHF 
oscillation, there is a  maximal-burst of grid current.   2.  the 
parasite-related arc that makes the big-bang is from the +HV to gnd, so 
the 25uF HV filter C discharges c. 100J (a.k.a. 100 watt-seconds) 
through R3 - which typically blows it apart.
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org