[Heathkit] SB-220 Resistor # R3 Blown

[kc7hkp at comcast.net]

HI Guys
I am bring a SB-220 Amp back to life, I put power to it with the tubes out and meter showed 2700 volts.
So i put the 2 Tubes  in  and resister R-3,  2 Watts at .82R had blown, according Rich Measures page its the resistor grid current meter shunt resistor and is the only path for the High Voltage to go to chassis from the filter bank.
Does any one know why it may have blown, ?
It  has to do with the interlock also.
easy to replace but why it broke is got me worried, i am not a Tech man so i need some ones help here.
Thanks
George

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KC7HKP 
George Yazzolino 
14801 NE 20th Circle 
Vancouver, Wa. USA ,98684 
Grid CN-85 
kc7hkp at arrl.net