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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