Tweet
This amp is suffering some ghostlike symptoms. Only when the amp is warmed up does it have a fizzy noise when ANYTHING is tapped including chassis. I checked the bias just to check and one side was 45ma and one was 21ma. In the past I'd figure one coupling cap or grid resistor from PI to be bad. I cannot tell as if I measure grid voltage it is the same for both output tube grids. HOWEVER, the voltage is fluctuating on both grids. It looks like a charging and discharging cap somewhere as my meter on DC moves up and down from about .1 volts to 1 volt. I then decided to check the cathodes and they are fluctuating too. They are moving up and down between 9.6v and 10.6v. It takes about 2 or 3 seconds between min and max voltages. My wild guess is a bad bypass cap but this is a pcb and I'd need to take out board to get at it. The cathode resistor seems fine. Does this sound right to anyone?
-
-
I would closely inspect for a cold solder joint and maybe reflow some or all solder joints in each section (pre amp, power supply, PI ...). You may have already done this, but I would replace each tube one at a time. Some amp companies run EL84's so hot that they soon fall apart inside causing all kinds of noise. -
Ok thanks I'll check it out.Comment
-
Ok I resoldered EVERYTHING on the board.. lotta joints there. The bias is now at 40ma when it should be no more the 25ma. There is still 10v across the cathode resistor... I'm suspecting the bypass cap and am gonna take it out to see if bias goes back down.Comment
-
I pulled the bypass cap and no change. The tubes are both running at 40ma... The plate voltage is 338v, screen voltages are at 330v and grids at 0v. I cannot figure out why these tubes are running so hot... I'm thinking of increasing that cathode resistor but it doesn't make sense that the factory set the tubes running at this current. anyone have any ideas?Comment
Comment