I want to implement a source follower to drive a 6c33, I have in stock the IRF720 and DN2540, what will be better ?
DN2540 has half the input capacitance and will be easier to drive / exhibit less HF roll off.
The input capacitance of the 6C33 (30pf +/-7pf) plus stray should not be an issue for either device.
The input capacitance of the 6C33 (30pf +/-7pf) plus stray should not be an issue for either device.
You need to be a bit careful with the DN2540 as a follower- when I've used them as followers in tube circuits, they tend to die at an alarming rate. Try the FDPF5N50FT. Low Cdg (the important capacitance in a follower) and I haven't blown one up yet.
Ha ha ha, when I have used them as CCS load die at the same alarming rate.
Now the source is over 200v with soft start ( rectifier tubes ) but before with diode bridge at the start flying to 450v and ..... Later I change to 10m45s .
Now the source is over 200v with soft start ( rectifier tubes ) but before with diode bridge at the start flying to 450v and ..... Later I change to 10m45s .
Never lost one as a CCS. When I spent a day in Morgan Jones's lab doing source-follower vs cathode follower comparisons, we killed a dozen of them before deciding that perhaps we should choose a different device.
The FDPF5N50FT works great and is cheap. The IRF720 should also work, but the reverse transfer capacitance is higher (47pF vs 5pF) so if the circuit driving the MOSFET has a high-ish impedance, you could get HF rolloff problems.
Frankly, I'd just buy the better part. They're $1.13 in lots of 10, $1.46 in onesey-twosey.
Frankly, I'd just buy the better part. They're $1.13 in lots of 10, $1.46 in onesey-twosey.
Depletion mode devices are always on, which might result in funny conditions during (tube) start up. Enhancement mode mosfets are probably safer.
I blew up a number of DN2540s before I started tacking a 15V zener from the gate to drain.
I have not blown one up since then.
I have not blown one up since then.
I like enhancement mode MOSFETs, in DC coupled source follower service. A fair number of FETs are suitable. I like the ZVN0545A in combination with "wimpy" tubes, like the 12AX7 and EF86. For tubes with some real drive capability, the IRFBC20 get my nod. Both of the FETs mentioned exhibit low and stable reverse transfer capacitances, which is key in voltage follower service.
The remark about installing a protective Zener diode between gate and source is spot on. Voltages are not what you think they will be, until the tube conducts. The FET is ready to go at the click of the on/off switch.
The remark about installing a protective Zener diode between gate and source is spot on. Voltages are not what you think they will be, until the tube conducts. The FET is ready to go at the click of the on/off switch.
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.
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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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
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
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).
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.
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?
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
Shoog
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.
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.
If you stay below 200V VDS, Qt is below 2nC.
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
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