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Source follower IRF720 or DN2540
Source follower IRF720 or DN2540
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Old 16th February 2015, 02:43 PM   #1
Celsius is offline Celsius  Spain
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Default Source follower IRF720 or DN2540

I want to implement a source follower to drive a 6c33, I have in stock the IRF720 and DN2540, what will be better ?
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Old 16th February 2015, 03:10 PM   #2
TheGimp is offline TheGimp  United States
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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.
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Old 16th February 2015, 03:25 PM   #3
SY is offline SY  United States
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Source follower IRF720 or DN2540
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.
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Old 16th February 2015, 03:39 PM   #4
Celsius is offline Celsius  Spain
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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 .
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Old 16th February 2015, 03:48 PM   #5
SY is offline SY  United States
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Source follower IRF720 or DN2540
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.
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Old 16th February 2015, 04:00 PM   #6
Celsius is offline Celsius  Spain
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What were your experience as source follower, what can I hope ?
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Old 16th February 2015, 04:06 PM   #7
SY is offline SY  United States
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Source follower IRF720 or DN2540
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.
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Old 16th February 2015, 05:18 PM   #8
goldenbeer is offline goldenbeer  Germany
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Depletion mode devices are always on, which might result in funny conditions during (tube) start up. Enhancement mode mosfets are probably safer.
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Old 16th February 2015, 05:26 PM   #9
TheGimp is offline TheGimp  United States
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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.
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Old 17th February 2015, 03:08 AM   #10
Eli Duttman is offline Eli Duttman  United States
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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.
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