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-   -   Mosfet breakdown (http://www.diyaudio.com/forums/tubelab/231106-mosfet-breakdown.html)

RuiR 1st March 2013 08:53 AM

Mosfet breakdown
 
Hi

I'm having a problem with my SSE, the mosfets start to degrade and lose their function with time and i don,te konw what causes it.

I already searched but i couldn't find any related problem :apathic:

Last time i instaled new ones i had at R14/R24 around 300v and 200v at R13 but 225v at r23... it slowly degraded and after a couple of hours i had 300v at R23 and after the other one degraded to 300v at R13...

What can cause this? I don't know what i'm doing wrong :confused: I can only think in poor soldering technic, my iron is a 15W only and i might take too long soldering :


Thank you for your imput

JonSnell Electronic 1st March 2013 09:05 AM

Do they recover when cold?

RuiR 1st March 2013 10:16 AM

Quote:

Originally Posted by Harleyjon (Post 3390774)
Do they recover when cold?

I made the mistake of removing them and ask later, but no, i remember reading the voltage i think in cold and it was the same at R23 and R13, 300v

However it was very noticeable a sound degradation after a few minutes after runing it from cold :bulb:


Thank you :up:

JonSnell Electronic 1st March 2013 10:23 AM

The reason I ask is because MosFets act like a valve. When the MosFets warm up their gain goes down lower and lower to act like a self preservation system. The idea being as they heat up, the gain reduces therefore the current flow reduces and the dissipation reduces lowering the generated heat. Same as a valve, if you overdrive a valve the anode will glow and the electron flow reduces allowing the valve to cool down.
That is why valve amplifiers have that unique sound.

JMFahey 1st March 2013 10:28 AM

Quote:

I don't know what i'm doing wrong
Well, for starters you are posting no schematic ;)
WHAT are you talking about?

RuiR 1st March 2013 10:32 AM

Quote:

Originally Posted by Harleyjon (Post 3390846)
...
That is why valve amplifiers have that unique sound.

It plays beautifully while the mosfets work.

I will order another 5AR4 valve too. I don't think this one is faulty but i will play safe.

P.S I used once thermal paste between the mosfet and the heatsink, later i didn't with the same final result... do you advise using?

RuiR 1st March 2013 11:57 AM

Quote:

Originally Posted by JMFahey (Post 3390852)
Well, for starters you are posting no schematic ;)
WHAT are you talking about?

Posted in tubelab forum and thought everyone was aware of this design, my fault. You can check it here: Simple SE schematic

It must have to be related with something i'm doing wrong with the mosfets :s I will take a photo of them in the heatsink...

How one can diagnosis a faulty 5AR4 rectifier tube?

Thank's for the message

JMFahey 1st March 2013 01:16 PM

I think your MosFets are overheating.

RuiR 1st March 2013 01:32 PM

Quote:

Originally Posted by JMFahey (Post 3391031)
I think your MosFets are overheating.

Thank you for your opinion, apreciated :)

Thermal paste and a thicker heatsink?

rknize 1st March 2013 03:38 PM

Are you using FETs in place of the CCS? If the CCS is doing it's job, the voltage will vary as the CCS maintains a constant current. What is the voltage *across* R13/23 and R14/24?

Apart from that, yes it does sound like a thermal issue. You are not using thermal paste? Have you adjusted the resistor values on your FETs so that you get the desired ~9mA of current through the tube?


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