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Q:DN2540 CCS protection diodes

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Posted on January 5, 2017 at 18:06:34
deathtube 667
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Joined: November 25, 2010



I had built some DN2540 CCS plate loads a while ago and used them without protection diodes.

I'd like to add some circuit protection before trying them out again.

Built a simple 6SN7 preamp circuit with back to back zeners from gate to source in LTSpice4 and it ran OK.

All I have are 5.1V zeners, so I used a string of them.

If I can find a 15V - 18V zener at Rat Shack, that might be easier implement.

Let me know if this is good enough or not.

dt 667

 

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RE: Q:DN2540 CCS protection diodes , posted on January 5, 2017 at 19:10:11
Chip647
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Why are you putting them back to back? You only need to limit the gate to 12 volts above the source.

 

RE: Q:DN2540 CCS protection diodes , posted on January 6, 2017 at 00:29:38
Michael Samra
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Why doesn't he set R1 with a pot as opposed to a fixed value because current can change with different tubes of the same type?

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Q:DN2540 CCS protection diodes , posted on January 6, 2017 at 00:31:38
Michael Samra
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DT
12v to 18v Zeners are very common but you need to get them from Mouser or Digikey or McShane.

"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Q:DN2540 CCS protection diodes , posted on January 6, 2017 at 03:55:28
Alex M
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Location: Hampshire
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Nikos Salas uses 12V zeners with the DN2540s in the CCS in his HPHV shunt regulator pretty much as your diode strings, but with the difference that each of the zeners is on the opposite end of the gate stop resistor from the gate of the MOSFET. It probably doesn't make any practical difference, but I would consider this better practice, as it isolates the gate from anything that could send RF noise to the active device.

As already noted, you don't need back-to-back zeners.

Alex

 

RE: like this ..., posted on January 6, 2017 at 08:00:29
deathtube 667
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Posts: 1270
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A gate stop resistor and a 12V zener.

Thanks.

dt 667

 

RE: Q:DN2540 CCS protection diodes , posted on January 6, 2017 at 08:02:41
Tre'
Industry Professional

Posts: 11950
Location: So. Cal.
Joined: February 9, 2002
The current through the circuit is set by the CCS, not the tube.

I build and set the current of my CCS on the bench with a low voltage bench supply and a 100 ohm resistor in place of the tube before I install them.

Once installed, even though there is now a tube and a lot more voltage, the current is always the same as it was when I set it.

That proves that the CCS is working.

The only thing that will change when you sub in a different tube (of the same type) with different transconductance (do to age or condition) is the voltage at the plate.

If you want a certain voltage at the plate you don't change the current through the circuit, you change the bias condition of the tube by changing the value of it's cathode resistor. When you do, the current through the circuit stays the same.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: like this ..., posted on January 6, 2017 at 08:16:21
Alex M
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That's weird! I would have posted the Salas schematic but remembered that on this forum you can't upload files. I had forgotten I'd put that up on my own website...

Alex

 

RE: like this ..., posted on January 6, 2017 at 08:37:57
Triode_Kingdom
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Yes, you can upload files. There's an Upload Image dialog in the reply box.



--------------------------
Buy Chinese. Bury freedom.

 

Likewise for the K&K CCS kits, posted on January 6, 2017 at 08:59:53
Lew
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You can, and I do, set the current with a 9V battery source on the bench top, before installation, and it remains constant in the actual circuit.

 

RE: like this ..., posted on January 6, 2017 at 09:21:49
Alex M
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Location: Hampshire
Joined: February 9, 2001
OK, that's good to know.

Alex

 

RE: 12V zeners available at RS ..., posted on January 7, 2017 at 08:48:14
deathtube 667
Audiophile

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They had 12V IN4742A zener diodes in stock.

 

RE: Likewise for the K&K CCS kits, posted on January 8, 2017 at 04:55:12
PakProtector
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Joined: May 14, 2002
It is useful to battery bias even the depletion mode MOSFET's. 4x of the 3.3V button cells will do. The voltage across the cathode resistor is then larger and a single 1% resistor can be used instead of a pot dialed in to set it as precisely. This does require two 9V batteries to check the current set level though...:)
cheers,
Douglas

also, at high currents it would also be of use to use one of the 10M400( or the higher voltage units ) bits since they have a larger bias voltage, and the lower FET does better with more voltage across it. Watch the gate stopper value on them, as with more voltage across the DN3545 or DN2540 the capacitance is lower and you'll probably find the value needs to be higher.


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Q:DN2540 CCS protection diodes , posted on January 8, 2017 at 07:14:41
PakProtector
Audiophile

Posts: 11236
Joined: May 14, 2002
You need back to back zeners if you battery bias to use a higher set resistance.

cheers,
Douglas


Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

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