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Source follower IRF720 or DN2540
Source follower IRF720 or DN2540
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Old 17th February 2015, 06:00 AM   #11
bwaslo is offline bwaslo  United States
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Source follower IRF720 or DN2540
I reduced the death rate of DN2540 with a small 1n4148 signal diode from gate to source. But the DN2540's voltage rating is still not terribly comfortable.

I'm starting to use TK5A60W: 600V, high transconductance, flat Crss of 1.5pF, even an insulated tab package for easy heatsinking. No results to report yet, but it looks good on paper.
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Last edited by bwaslo; 17th February 2015 at 06:06 AM.
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Old 17th February 2015, 07:12 AM   #12
jan.didden is online now jan.didden  Europe
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One often overlooked issue with the 2540 is that its Id(peak) spec is nothigher that its Id(cont) spec. Both 500mA.
Most devices, FET of BJT, can handle a higher current than the continuous spec for a short period which often saves them at switch-on or transient overload. The 2540 just dies.

I had a lot of them die as power regulators in a regulated supply. The supply had to deliver only 100mA or so, but at switch-on there was a largisch load pulse into an uncharged capacitor at the output and pooof!

Jan
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Old 17th February 2015, 07:37 AM   #13
Merlinb is offline Merlinb  United Kingdom
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I use the STP11NK40Z, mainly because it is available in a plastic package. It has Cdss = 30pF which is quite sufficient. You don't need especially low Crss MOSFETs for source followers, unless the source impedance is particularly high, more than 30k say (e.g. 12AX7 or pentode).
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Old 17th February 2015, 10:50 AM   #14
piano3 is offline piano3  United Kingdom
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I have managed to blow up a number of FQP2N60 when used as source followers despite using zener protection on the gate. They typically fail after a few months, obviously on either switch on or off. I am always very careful during construction to wear a wristband. Are there any other precautions I must observe? The failures have been in a unit with a very simple non-regulated power supply with quite a bit of C but not much R; the source-followers come after a switched attenuator and selector switch. I must say I have been at my wit's end trying to figure out the problem.
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Old 17th February 2015, 11:12 AM   #15
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by piano3 View Post
I must say I have been at my wit's end trying to figure out the problem.
Do you have a large output coupling cap? Perhaps the charging current at start-up is enough to push the FET outside its SOA for a while. This could perhaps be protected by using a 10V zener and putting in a small source resistor, say 22 ohms, to create a simple current limiter?
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Old 17th February 2015, 11:53 AM   #16
Shoog is offline Shoog
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The IXYs chips seem to be much more capable of handling switch on conditions without self destructing. I don't think I have ever had one let me down.

Shoog
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Old 17th February 2015, 12:12 PM   #17
piano3 is offline piano3  United Kingdom
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The coupling caps to the next (valve) stage are only 0.22 uF with 1M grid leaks. I already have 11 volt zeners from gate to source; the followers have CCS from a subsidiary negative supply and the gates are at ground via 680k "leaks". The preceding volume control is fairly low impedance and is coupled to by 10uF caps which are PIO (and I would imagine somewhat leaky). This might worry me if I had not installed the gate zeners. I had imagined voltage transients from the supply(ies) being the problem because of low series R (some 10's of ohms) but perhaps this is rather fanciful.
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Old 17th February 2015, 01:45 PM   #18
TheGimp is offline TheGimp  United States
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I've been using MTD1N40 and MTD1N50 with good success. Crss drops by about 10Volts Vds and is specified at 12pf max at 25V.

If you stay below 200V VDS, Qt is below 2nC.
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Old 17th February 2015, 03:20 PM   #19
Celsius is offline Celsius  Spain
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Yesterday I put the IRF720 as SF very easy with 25k to ground the current is lock at 10mA the mosfet adjust automatic by itself.
The preamp stage is a 6ac7 in pentode mode, I don't like this tube, but the amp use it, the impedance of the 6ac7 is 1M

I am waiting the FDPF5N50FT that SY said and next week will try with the DN2540 with zenner .

Now the Bass are 1000 times better than before but I loose 60% of the tube warm sound, I need more time to intimate with the IRF
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Old 17th February 2015, 06:45 PM   #20
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by piano3 View Post
The coupling caps to the next (valve) stage are only 0.22 uF with 1M grid leaks.
Do the valves get preheated before applying HT?
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