Left Channel Died

[Paul Birkeland]

Does the high voltage at OA now follow a tube? 

[Kevin]

No, it's still the left channel (B side) reading 300v. Lead 16.

[Paul Birkeland]

Alright, so you have 1.25V at pin 9, this is drawn across a 1.27K resistor, which means that 1mA of current is flowing where there should be 2mA.

1.2V of bias on the tube and 300V at the plate would indicate 40mA of current drawn based on the curves for the 6DJ7.

Either you have shorted transistors in the C4S, or you have lost the ground reference for the grid on that 6FJ7.  The grid for section 1 is pin 10 on the 12 pin socket, in is connected to pin 8 by a 220 ohm resistor, then is wired to the volume pot.  If any of those connections are loose or poorly soldered, you will lose your ground reference and your voltages will go whacky.  This can include all of the connections at the pot itself, so a broken ground wire up by the pot could cause this issue.

If you have shorted transistors on the C4S, this could be confirmed by setting your meter to measure continuity (it will beep when you touch the probes together), then check each pair of legs on each transistor on the C4S board to look for shorts.  If you find shorted transistors, then the immediate concern would be why they shorted, which would go back to a poor solder joint. 

[Kevin]

The ground reference connections are good. I resoldered them anyway just to be sure. I think you're onto something with the transistors. None of the PN2907A seem to be shorted. However, the MJE5731A on the right (the affected channel) might have a problem. When set on continuity, when I test the middle and right lead, it doesn't straight beep like a real short, but i hear rapid clicking from the meter. This does not happen with the MJE5731A on the left. Could it be a bad transistor?

[Paul Birkeland]

Transistors rarely go bad on their own, so it could be a blown transistor that was cooked from a different issue.

[Kevin]

Would faulty speakers cause a transistor to blow? I noticed some weird things going on with my Emotiva amp going into fault mode when hooked up to these speakers. The amp doesn't go into fault mode when I unplug the speakers.

[Paul Birkeland]

You could wire the binding posts together and play the amp at maximum level for a year and the amp wouldn't really care.  One of the big differences between our amps and just about all others is that they are ridiculously stable. 

When I have had this problem in my own gear, it will be either because something is loose or because a lead that I trimmed off a resistor or some other part has not fallen all the way out of the amplifier and has shorted something out.  There are also some unfortunate builders (especially lately) who have used plumbers' or jewelers' flux on their kits, and that will definitely take its toll on the solid state parts.

[Kevin]

This amp was built before I even played around with silver solder. I used the regular 40/60 solder for this amp.

Should I try to swap out the MJE5731A? I'm looking on Mouser and Digikey, and both sites are saying that the part is obsolete. There's the MJE5731AG. Would one of these work? Or should I swap out the whole C4s board?

[Paul Birkeland]

The AG will work.

[Kevin]

I forgot to post an update, but changing the transistor worked great. Speaker and headphone output work fine.

Some additional updates/information regarding my setup... I was using the SEX to power my desktop speakers, the Klipsch Promedia 2.1 system. I attached pictures below to show they're meant to be set up and how I wired them to make them work with a speaker amp.

When I hooked my Emotiva A-100 amp to the speakers, and they worked for a little while, but the fault eventually triggered. Eventually, the Emotiva amp would just go straight into fault mode after being turned on. I reached out to Emotiva, and they sent me a new amp and the SAME THING happened. The amp would work for a bit, then go into fault mode. Eventually, the amp would go straight to fault mode after being turned on.

I ended up buying a new pair of DIY speakers from parts express and have been using the SEX amp with it since and haven't had any issues. Was there something inherently wrong with the way I hooked up the speakers that could have broken 3 speaker amps? Or might there be something wrong with the speakers?

[Paul Birkeland]

You can hook pretty much anything you want up to the binding posts of the SEX and not run into problems.  The only things I wouldn't recommend hooking up would be something like a car battery, a power cord from the wall, etc.