Obsolete Mosfets

We all start somewhere
Post Reply
User avatar
pre65
Amstrad Tower of Power
Posts: 21367
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#1 Obsolete Mosfets

Post by pre65 »

I was looking at the Tubelab powerdrive circuit, and it seems the 2SK2700 mosfet is obsolete.

I found details of an STP4NA90 which is supposed to be a replacement, but none of the usual suppliers seem to stock it.

I saw a variation of a powerdrive type circuit that uses an IRF820, and some versions of that are still available.

Do you "experts" (Mike H ?) think the IRF820 will comfortably replace an 2SK2700 ?

Image
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#2 Re: Obsolete Mosfets

Post by Mike H »

Looking at the circuit, what might concern me is the gate threshold Voltage, which seems to be equal. Also the gate capacitance of the IRF820 is about half, which is doubtless an advantage.

Just leaves whether VDS of 500V will be OK for what is needed (compared to 900V), looks like it will be.

Also power rating of the two is comparable. So I would say yes. As usual, only one way to be sure! Try it and see. :D


.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Nick
Site Admin
Posts: 15694
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#3 Re: Obsolete Mosfets

Post by Nick »

STP3NK90 would be another candidate
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#4 Re: Obsolete Mosfets

Post by Mike H »

Datasheets here:

2SK2700T

IRF820


.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#5 Re: Obsolete Mosfets

Post by Mike H »

Nick wrote:STP3NK90 would be another candidate
^ Or what he said :D


http://www.alldatasheet.com/view_datash ... d=STP3NK90


.
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
pre65
Amstrad Tower of Power
Posts: 21367
Joined: Wed Aug 22, 2007 11:13 pm
Location: North Essex/Suffolk border.

#6 Re: Obsolete Mosfets

Post by pre65 »

Many thanks Mike & Nick.

Most helpful. :)
The only thing necessary for the triumph of evil is for good men to do nothing.

Edmund Burke

G-Popz THE easy listening connoisseur. (Philip)
User avatar
Nick
Site Admin
Posts: 15694
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#7 Re: Obsolete Mosfets

Post by Nick »

BTW, I would look at some gate resistance in that circuit, or at least check it with a fast scope.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#8 Re: Obsolete Mosfets

Post by Mike H »

Great minds think alike (?) I was thinking should I put a resistor in series with the gate, this would be just like a grid stopper resistor for a valve. Don't think it needs be a large value tho, what does Nick reckon?
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Nick
Site Admin
Posts: 15694
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#9 Re: Obsolete Mosfets

Post by Nick »

I would use at least a 1k carbon, maybe 10k to be safe.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#10 Re: Obsolete Mosfets

Post by Mike H »

That much, OK then. :thumbleft:
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
User avatar
Nick
Site Admin
Posts: 15694
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#11 Re: Obsolete Mosfets

Post by Nick »

Well, the mosfet is working as a source follower, so there won't be that much capacitance looking into the grid as long as the drain keeps away from B+ which it will. So 10k is not going to roll off that low down. Given the driving valve is CCS loaded, the entire thing has great potential for parasitics so best (IMHO) to plan ahead. Its the sort of circuit where I would want to watch it working, there are all sorts of possible dragons that may be lurking but not be easy to spot by listening to the output.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
User avatar
Mike H
Amstrad Tower of Power
Posts: 20156
Joined: Sat Oct 04, 2008 5:38 pm
Location: The Fens
Contact:

#12 Re: Obsolete Mosfets

Post by Mike H »

Yep I can see that.

So Phil, bit of 'scope practice might also be prudent. :D
 
"No matter how fast light travels it finds that the darkness has always got there first, and is waiting for it."
Post Reply