Smoking SEX (1.1 I think...)

[Neuronal]

A newbies first fix! - I have a circa 2009 SEX amp I built which was giving me beautiful sound for two plus years until yesterday, when I smelled smoke. I yanked the plug while both tubes were still glowing. Flipped it over, and one of the Schottky diodes had blown in half. I rechecked all the resistances (obviously I didn't check voltages), and everything checks out - is it safe just to replace the blown part? Are there any other parts I should consider replacing or pay particular attention to? I will mention that before the blown diode my power transformer gave out a subtle buzz - the vibration was enough that I had to periodically tighten down the screws on the bell cap.

Thanks in advance for your help - believe it or not, I'm excited about this - part of the reason I went down this road was that I wanted to learn how to fix my own gear, and now I get my first chance!

[Jim R.]

You should probably check the voltages and the power supply caps -- that may have been what caused the diode to go.  The buzzing noise from the transformer means it is getting overloaded somewhere, and if the rest of the circuit looks ok, that very well could have been a power supply cap failing.  Probably wouldn't hurt to replace the other diodes and look for shorts in the heater supply as well.

HTH,

Jim

[Paul Joppa]

I would replace all four of the low-voltage diodes, and carefully inspect the 10000uF capacitor for bulges - actually, they are cheap so I'd probably replace the cap as long as I'm in there, even though it's probably just fine. The resistor is tough - you can check the resistance and examine for burn marks if you like but I would not expect any problems.

Here's my theory, FWIW: because the diodes are especially difficult to solder well, one connection may have failed. The bridge will still generate enough voltage to heat the tubes, but it will conduct only half the time, carrying twice the current, and causing the remaining (working) diodes to overheat. During that interim, the heater winding is experiencing a net DC current, which pushes the power transformer into partial saturation, causing it to vibrate and buzz.

[Neuronal]

Thanks to both Jim and Paul for the advice! I will replace all the diodes, and recheck all the caps....I'll let you know how it goes! Thanks again! - Bob

[Paul Birkeland]

When you have all the old diodes and that capacitor out, pull the tubes and measure the resistance between pins 7 and 8 on one of the octal sockets to be sure there isn't a short.

To me, this sounds like a short developed (the 10,000uF cap is a possible source).

-PB