mikey383
Well-known member
Opened up the Dual yesterday to do a bias mod....while I was in there, I figured I would do the serial loop mod as well. Started with the loop first, then did the bias mod.
While I'm trying to set the bias using the output transformer shunt method, I find that I can't get any combination of tubes to match from side to side. One side consistently gets ~90 mA, and the other side gets ~45mA. So I pulled the 2 outer tubes and checked again. Both sides get around 45mA. Swap to the other set of tubes. Same thing.
Ah ha. Now I'm getting somewhere. A bad tube has been ruled out, and I'm ready to start looking at bad solder joints. I start looking at the tube socket on the side that reads low, and this is what I find.
Sorry for the crappy cell phone pic. In case you can't tell, that's a blown screen resistor on the far left (from back of amp) socket. I checked it with my DMM, and sure enough, it's dead.
I don't know how long it's been running like that, but I only used the 2 inner tubes since I bought the amp. I went back to 4 tubes for the last 3 shows I played, but it just didn't sound right with 4 tubes. It sounded ok, but not good. Now I know why.
Now I just have to find a resistor to replace it.
While I'm trying to set the bias using the output transformer shunt method, I find that I can't get any combination of tubes to match from side to side. One side consistently gets ~90 mA, and the other side gets ~45mA. So I pulled the 2 outer tubes and checked again. Both sides get around 45mA. Swap to the other set of tubes. Same thing.
Ah ha. Now I'm getting somewhere. A bad tube has been ruled out, and I'm ready to start looking at bad solder joints. I start looking at the tube socket on the side that reads low, and this is what I find.
Sorry for the crappy cell phone pic. In case you can't tell, that's a blown screen resistor on the far left (from back of amp) socket. I checked it with my DMM, and sure enough, it's dead.
I don't know how long it's been running like that, but I only used the 2 inner tubes since I bought the amp. I went back to 4 tubes for the last 3 shows I played, but it just didn't sound right with 4 tubes. It sounded ok, but not good. Now I know why.
Now I just have to find a resistor to replace it.
