Mosfet breakdown

[RuiR]

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

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 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

[Harleyjon]

Do they recover when cold?

[RuiR]

Originally Posted by Harleyjon

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


Thank you

[Harleyjon]

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]

I don't know what i'm doing wrong

Well, for starters you are posting no schematic
WHAT are you talking about?

[RuiR]

Originally Posted by Harleyjon

...
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]

Originally Posted by JMFahey

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]

I think your MosFets are overheating.

[RuiR]

Originally Posted by JMFahey

I think your MosFets are overheating.

Thank you for your opinion, apreciated

Thermal paste and a thicker heatsink?

[rknize]

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?