Voltage issue with High Current C4S

[Toobuzz]

OA = 145V
OB = 91V

 

[Paul Birkeland]

OK, likely the only issue with your amp is that high OA voltage on the center PC board.  That can happen from any of the following and this is the order I would use to debug them:

1.  The solder joint at pins 4/5 on the 9 pin socket isn't solid and one half of the tube isn't heating up.  This will also manifest as only half of the tube glowing when you look at it. 
2.  The solder joint on the center leg of the MJE350 feeding OA isn't 100% soldered and working properly.  Just reheat it until you see the solder wick into the board to be sure.
3.  Possibly an LED isn't working properly, but they don't break on their own.  A voltage of 145V on OA and a DC voltage across an unlit LED of about 12V DC would indicate a failed LED.  It's really important that when you see an LED not illuminating that you don't fixate on it and resolder it to death.
4.  Some other wire is broken or loose. 

 

[Toobuzz]

I found the issue and I’ll spare you a pic.  12au7 rolling made the pins move and the lead attached to the center pin of the right channel snapped.  I’ll put a little slack in my replacement.

 

[Paul Birkeland]

You can also install a wire between pins 3 and 8 for the time being to get the amp back up and running. 

 

[Toobuzz]

Thanks for the tip!  Do you think it would be wise to order some new resistors and a little more wire (in addition to the LED) and clean up that 12au7 socket. It’s definitely the sloppiest part of what I’ve done here. I was thinking the heater wire is fine but maybe redo everything else attaching to the 12au7 lugs?

 

[Paul Birkeland]

I would just replace the LED and enjoy the amp.