Mosfet cathode follower question

[tubenit]

I've used a mosfet cathode follower in a handful of different amps over the last maybe 7-8 yrs and it's always worked just fine.  I can't tell a difference between the mosfet or a 12A_7 triode in that position.

Having said that, I now have had one "fail" in the sense of the "cathode-like" side of it went bad.  I was using a 12AY7 triode before the mosfet CF and after. 

I realize that the mosfets have a part that can be heat sinked to a chassis, but I've never done that before and up until now still had no issues.

Is it possible to still heat sink one with it being mounted on a turret board?   

I am wondering if the 12AY7 prior to the mosfet played a role in this possibly drawing more current then something like a 12AT7?  Was it an error of choosing the wrong tube,  not heat sinking it or just a part that went bad?

My current solution is to have V1a be gain stage 1 .............. then V1b & V2a come next for the 2nd gain stage.  I've done this before also and not had any issues with it  (vs using a dual triode in the 2nd gain stage with the CF).


Thoughts?   With respect, Tubenit

[tubeswell]

The IRF820 in the TO220 package has a 1W power rating (as far as I can remember), and if you’re using a 100k+ source resistor, you shouldn’t exceed this power rating with standard tube amp voltages. You can get single TO220 heat sinks for these with various ratings (but unless you use insulation, the heat sink with be at the Drain voltage). More common problem with these is excessive Gate-Source voltage leading to failure (which is why the 5V zener is important)

[pdf64]

The lead length between the gate stopper body and the gate should be as short as is feasible. I think it may be better for the gate protection zener to be at the input side of the gate stopper, rather than the gate side.

[PRR]

Where did you get that MOSFET?

That "burn" is IMHO more likely a factory defect than in-use damage. There are lots of junk semiconductors on the market. Did you get "a good deal" on it? Or is it from a reputable supplier?

[sluckey]

I think your MOSFET just failed on it's own. I'd just replace it. I don't think heat was an issue in your circuit but just in case...

Search Amazon for "TO-220 heatsink". I like this one for board mounting because it's small and also includes all the necessary hardware to properly mount your MOSFET keeping the heatsink electrically insulated. I don't think you even need heat sink grease for this type insulator (not sure though).

     https://www.amazon.com/Makeronics-Insulator-rubberized-Transistor-International/dp/B083Q9KCH1/

[tubenit]

Gentleman, I appreciate all the input.  Thank you!   Will look into that heat sink approach.


I want to double check and make sure my thinking is headed the right direction.   Regarding how one would wire up a mosfet into a circuit similar to a triode, I've always thought of :


source = cathode


drain= plate


gate = grid 


Am I thinking about this correctly?      With respect, Tubenit

[shooter]

as long as you're not being tested, your thinking/understanding is fine.

[sluckey]

Am I thinking about this correctly?

Yes, even if you are being tested.   

[tubenit]

Thanks!  I am grateful for the help. 


With respect, Tubenit




UPDATE:   The mosfet failed because I had it wired up incorrectly.  I would not hesitate to use one again as a cathode follower

[glass54]

Hi TubeNit
I too have had dodgy Semis. I only buy from reputable sources (in Aussie and all has been good in the last decade or so).
You could try an IRF840, may give you slightly better "free air" heat performance but probably not worth the effort.
I have used both 820 and 840 successfully. A compact TO220 heatsink may be of use as well.
+1 with PDF, get that Zener close. And as others have stated, you may have just got a poorly handled or compromised device.
One thing for sure, it's not your workman, beautiful soldering 
I'd only worry if it happened again 
Kind regards
Mirek