Mofeta FLAMING HOT TRANSISTOR šŸ˜¬

andyjperez

New member
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 Iā€™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 thatā€™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 itā€™s Q4, not much going on there, looks 18v in from the charge pump and the trimmer, and one resistor to ground, so Iā€™m not sure what I should be checking. None of the other components are getting warm, just olā€™ 2N6L6 over here! Any help would be much appreciated, in the meantime Iā€™m going to go look for some tiny eggs to fry. 0593EFB7-B27D-4F5E-9AE2-9042EDD973B3.jpeg 1D5B515A-183B-4BAA-81B2-04A3B56CA5A0.jpeg 7001B763-A0A8-4C27-86BB-66743C1768C1.jpeg
 

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Solution
Here's simplest breadboard setup for Vp measurement. You can add a switch to disconnect the battery.
Mind the pinout!
1608110341732-png.8452
Holy shit good eye!! I was just measuring the bias at the pads and I couldnā€™t get anything below 10v and that particular transistor was just chilling at 16v even turned all the way down. Which is weird because I couldnā€™t find 20k trimpots so I used 25k wonder if thereā€™s other 3k3s I fudged, probably if there are multiples because I pre-pull all my components, test them, then tape them down to the parts sheet. Guess Iā€™ll check now after I replace that one, thanks for putting me on the right track!!
 
So, fixing that one resistor kept the offending transistor from heating up, theyā€™re all still biased hot though, 11-ish volts with the trimmers rolled all the way back. Also the oscillation is still there and when ā€œtwiddling the trimmersā€ the bass transistor is definitely the culprit. Sounds like Clara Rockmore is getting down in that little fucker. Only other thing I can think of is to swap the trim pot. ..maybe all of them?
 
Check and make sure the trimpots are the right value (it should be printed on the top edge of them), they aren't likely all defective.

Where did you get your transistors?
 
Not that I am a pedal troubleshooter, but I am a service tech electrician who has to troubleshoot awful combinations of software and hardware problems for building control systems.
So I have seen and fixed enough issues to know that it is not usually the item showing symptoms that is the problem. Rarely do transistors or ICs get hot because of an internal issue. Temol pointed out the first thing I thought and looks like he was right. That resistor being 100 times smaller let a lot of current flow.
So after you replaced the R10, did you slap in yet another fresh transistor at Q4?
And by design, looks like ditchin the trim pot would let a lot of current flow through Q3 as well?
Granted, Robert and Temol know better than I on these matters.
 
So, fixing that one resistor kept the offending transistor from heating up, theyā€™re all still biased hot though, 11-ish volts with the trimmers rolled all the way back. Also the oscillation is still there and when ā€œtwiddling the trimmersā€ the bass transistor is definitely the culprit. Sounds like Clara Rockmore is getting down in that little fucker. Only other thing I can think of is to swap the trim pot. ..maybe all of them?
As Robert listed, Check that the Trimmer has 203 stamped on the side = 20K
 
Three possible reasons come to mind why you can't set the drain voltage lower.
1. Trimmer value different than it should be (for example 10k instead of 20k).
2. Incorrect values of source resistors (higher than on the schematic).
3. Transistors (parameters other than recommended, e.g. Vp too low, fakes)
 
Hey All, thanks for the input! As I think I mentioned the trim post are 25k not 20 (because I couldnā€™t find any) but I doubt that would be the problem as itā€™s going in the OTHER direction, as far as the transistors and their quality goes, Iā€™ve used these same transistors in other builds (a couple of univox Square Waves) and they were fine. However I DO have a multimeter capable of testing transistors. I just have no idea the values I should be looking for. D9B1C720-AF6D-403E-A50B-A0063F35D35E.jpeg
 
Awesome Iā€™ll check that now, thank you!
Hey so I built myself a little tester according to that schematic, best I could anyway. But the readings Iā€™m getting are way different and again the JFET is heating up like a southern Baptist minister in a highschool parking lot. Mind taking a peek and checking my work for errors. You guys have been really helpful, super appreciated! D8374724-C293-4733-B837-89A0E21F0D26.png 8931F544-E432-4954-AC18-4340C63762D0.jpeg
 
It looks like wiring errors and a transistor not plugged in properly.

Here's the pinout for 2N5457.

View attachment 44013

And here's correct wiring.

View attachment 44014
Excellent, thank you!

Also, in one of the threads you recommended me on biasing issues I ran into a comment left by someone else having the identical (remaining) problem Iā€™m having with regards to the transistors not biasing below ~11v on his mofeta build, Iā€™m going to see if I can search his post history and see if he ever resolved his problem.
 
It looks like wiring errors and a transistor not plugged in properly.

Here's the pinout for 2N5457.

View attachment 44013

And here's correct wiring.

View attachment 44014
Well, itā€™s becoming clear to me that these ā€œBridgoldā€ transistors are the problem, remade the tester to your spec, turned it on and the transistor just popped and fried. Going to look for some higher-quality non-Amazon JFETs
 
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