[Amps] SB-220 PROBLEM

[Gudguyham at aol.com]

Just when you thought you have seen just about every problem with a  
particular amp, you see a new one that baffles.  In this case the problem  is this.  
Amp blows the .82 grid shunt resistor.  Replace resistor  with new one.  As a 
precaution after the repair attempt to bring amp up on  a variac.  As soon as 
the input voltage is raised on the variac, the plate  meter indicates a reading 
and at the same time, the grid current goes negative  when the multimeter is 
in grid position.
 
Troubleshooting notes:
1. Removed tubes.....same thing
2. Isolated power supply from all metering circuitry, DC voltage OK
3. Checked 3600 ohm series plate current meter resistor....OK
4.  Checked 1 ohm plate meter shunt resistor....OK
5.  Replaced .82 grid  shunt resistor....Ok
6. Meters themselves test OK
7. Removed 120 vdc bias supply voltage.....same thing
8. Shut down primary voltage to filament transformer....same thing
9.  Isolated power transformer checked secondary AC...OK
10. Removed rectifier board checked for solder bridges etc...All OK
 
SO I have checked just about damn thing and still same problem.   The power 
supply is operating fine as I did isolate it from the metering  circuitry and 
it is working OK as soon as I connect the metering circuitry the  plate meter 
indicates a reading with only a few primary volts coming from  variac.  Don't 
know where to look next as I checked everything  already.  What have I missed?  
Suggestions?   Lou



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