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Source follower IRF720 or DN2540
Source follower IRF720 or DN2540
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Old 17th February 2015, 06:57 PM   #21
Celsius is offline Celsius  Spain
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question: The source follower (irf720) need grid-gate stopper
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Old 17th February 2015, 07:55 PM   #22
piano3 is offline piano3  United Kingdom
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Merlin, yes; heaters switched on first followed by +285 and -15 together.
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Old 18th February 2015, 01:09 AM   #23
Eli Duttman is offline Eli Duttman  United States
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Originally Posted by celsius235 View Post
question: The source follower (irf720) need grid-gate stopper
Yes, like tubes (which they resemble), FETs need gate stoppers. A 1 Kohm carbon comp. part should be quite adequate on an IRF720's gate.
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Old 18th February 2015, 04:27 AM   #24
dhsettim is offline dhsettim  New Zealand
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Default Depletion FETs as Source Followers

I adapted a K&K cascode depletion CCS board (IXYS IXTP01N100D and DN2540) to use a cascode source follower to drive the EL519 (A2) in my venerable EAR 859 amp. I added back to back (important) 15V zeners. It replaces the PCC88 cathode follower. I built the PCB into an adapter socket so I can use the valve or swap in the FETs. Been using the FETs for 2--3 years now with no issues and I routinely turn off (and on) my amp when not in use.
Everyone thinks the source follower sounds "better" but each to their own.
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Old 18th February 2015, 07:37 AM   #25
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by piano3 View Post
Merlin, yes; heaters switched on first followed by +285 and -15 together.
Hmm... with the following valve preheated the coupling cap could have a low impedance charging path into the grid (does it have much of a grid stopper?), which will pull the MOSFET source down to a low voltage while the current is highest. Possibly pushing the MOSFET momentarily outside its SOA? Just a thought.
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Old 18th February 2015, 08:25 AM   #26
Celsius is offline Celsius  Spain
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Quote:
Originally Posted by dhsettim View Post
I adapted a K&K cascode depletion CCS board (IXYS IXTP01N100D and DN2540) to use a cascode source follower to drive the EL519 (A2) in my venerable EAR 859 amp. I added back to back (important) 15V zeners. It replaces the PCC88 cathode follower. I built the PCB into an adapter socket so I can use the valve or swap in the FETs. Been using the FETs for 2--3 years now with no issues and I routinely turn off (and on) my amp when not in use.
Everyone thinks the source follower sounds "better" but each to their own.
tim
The original desing of my amp don t use cathode follower and drive directly the 6c33 with pentode 6ac7 (6sj7) in my case when I use the Source follower I think that the sound is better than before but I need to listen more time to be sure. This weekend I will change to dn2540
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Old 18th February 2015, 08:42 AM   #27
piano3 is offline piano3  United Kingdom
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As far as I recall (it is my father's amp, 500 miles away), I used grid stoppers of at least 300 ohms. Is it important whether the gate stopper and protection zener are in series or parallel?
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Old 18th February 2015, 02:48 PM   #28
Merlinb is offline Merlinb  United Kingdom
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Originally Posted by piano3 View Post
Is it important whether the gate stopper and protection zener are in series or parallel?
No that shouldn't matter.
A 300 ohm stopper wouldn't limit the charging current very much. You would have to put a small resistance between the MOSFET source and zener to create a definite soft limit, e.g. like in this image:
Click the image to open in full size.
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Old 18th February 2015, 03:38 PM   #29
piano3 is offline piano3  United Kingdom
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Thank you Merlin; is the implication then that the abrupt appearance of the high voltage with preheated valves is a bad idea in this case? Would a slow ramp-up remove the problem?
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Old 18th February 2015, 07:32 PM   #30
bwaslo is offline bwaslo  United States
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Source follower IRF720 or DN2540
Just in case you were asking whether running two zeners in opposed series or opposed parallel with each other makes a difference - YES. A Zener clamps voltage to its zener voltage in one direction, and to a diode drop (about 0.7V) in the other! So the pair should be in opposed series if you want to clamp the voltage within the zener's voltage (and an additional 0.7V). Otherwise, it will just be the same as running two regular diodes in opposed parallel, clamping to within 0.7V.
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