Mofeta FLAMING HOT TRANSISTOR 😬

[andyjperez]

So I just finished putting together my MoFeTa today plugged it into my amp for a minute before I left for work and it sounded pretty good. Came home and plugged it into my modeler so I could listen to it with headphones, I was hearing a very faint chirp, so I popped it open and figured I’d fiddle with the bias pots and see if that would help the offending transistor. Went to adjust the Bass trimpot and GODDAMN!!! One of the 2N5457s in the bass sector was METHOD ACTING the part of a tube!! Hot as hell!! So I immediately unplugged it and brought it over to my bench, replaced the offender and checked all nearby solder joints, gave it another spin. Still hotter than a meth pipe on payday!! So I unplugged again and turned the trimpot all the way down, let it cool, then plugged it back in to see if that’d have an effect. Still Blazing like a high school stoner in a Honda civic. Wondering if anyone has run into a similar problem? Just looked at the schematic and it looks like it’s Q4, not much going on there, looks 18v in from the charge pump and the trimmer, and one resistor to ground, so I’m not sure what I should be checking. None of the other components are getting warm, just ol’ 2N6L6 over here! Any help would be much appreciated, in the meantime I’m going to go look for some tiny eggs to fry.

 

[andyjperez]

I've checked my layout ten times before postign it here..
Do you have a breadboard?

Same circuit, different layout.

I DO have a few breadboards, so I’ll get on this now. Sincerely, thanks so much for your help. I really really appreciate you taking the time.

 

[temol]

Here's simplest breadboard setup for Vp measurement. You can add a switch to disconnect the battery.
Mind the pinout!

 

[andyjperez]

So it’s all but confirmed that my problem is these shitty, most likely bootleg, Amazon transistors. I whipped up a tester on some vero according to your second layout and tested them, ran the numbers against that runoffgroove.com page you sent me, and the values were so off from the ranges presented that they were essentially nonsense. Also did the direct test with the 9v on breadboard. And got .469v (nice).
To further my hypothesis I picked up one of those $15 component testers, while sketchy in and of themselves, it seems to read every resistor and cap accurately to my DMM’s results, it also had no problem reading various PNP and NPN transistors, and a 2n7000 mosfet (see pic) but when confronted with either the 2n5458 or the 2n5457s from the package I was using it reads them as resistors. (Or if flipped around, inductors).
I ordered some new ones from Smallbear that purport to be either deadstock Motorola and/or Fairchild, so hopefully those work out and I can get on with my life! I’ll keep ya posted. In the meantime, avoid the “Bridgold” Amazon transistors, as they seem to be shit on three sticks.