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Mosfet breakdown

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R

RuiR

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Joined 2011
#1
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 😕 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
 
#4
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.
 
#7
JMFahey said:
Well, for starters you are posting no schematic 😉
WHAT are you talking about?
Click to expand...

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
 
#10
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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#11
rknize said:
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?
Click to expand...

I already taken them out 🙁 I had the time and done it because i will be busier.
*across*? It had about 325v after R14/R24 and the same voltage before R13 and (i can't be certain) after R13

rknize said:
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?
Click to expand...

I did use thermal in the first set, the one that take longer.
yes i did use 330Oh resistors with the IXCP10M45S - about 9.4mA and 185Oh when i used the DN2540 - ~10mA

Thank you Russ, i learned a lot from your posts when i was building it 1 year ago. I implemented the variable resistor switch 😉
 
#13
rknize said:
By across, I mean putting a probe on each side of the resistor and measuring the voltage across it. That will tell you what the current is and how it may be drifting. I assume you were measuring relative to ground?

Others have used the DN2540 with good results. Maybe it is oscillating?
Click to expand...

Thank you.

Yes, i measure to the ground, newbie but not that much 😀

The first set i used was DN2540's

This is how i assemble them:

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.



The two nuts are to gain hight so it can be pressed against the board
 
#14
Lost.. need help please

Hi

I lost count the times i already dismantle this thing trying to spot why it doesn't work 🙁

I'm ending with a voltage after R14/R24 equal at B+ in R4, ~450/460VDC.😕

Before R13 and R14 i'm getting 12VDC at one side and 2VDC in the other 😕 With no voltage or a residual only at R10/R20

I'm starting to despair and i might pick a shovel and bury it 😡

With the mosfets changed i managed to lift their tracks, i replaced them with wire and the circuitis fine.... i looked so many times to them and i can't see any problem there

Please any thought? Even the the most odd one is welcome😉
 
#20
This wiring is true for constant current sources (like LM336) but isn't true for MOSFET's. You will need to rewire your circuit to use P channel MOSFET's as CCS or BJT's. You need to generate a voltage of about 3-4V between gate and source (depending of the polarity of the MOS transistor) to turn it on (Class A biased), and then adjust source resistor to get drain current you need. Power in it will be directly I * E across the mos, then multiply it by the thermal resistance from die to air, and you will get temperature rise in the MOS without the heatsink. If this rise is too high, use a proper heatsink needed to reduce temperature to normal (< 70 deg Celsius) value.

But, why not to use the other half of the 'AT7 as a SRPP?
 
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