Power scaling mosfet

Discussion in 'Shock Brother's DIY Amps' started by rainywish, Dec 9, 2019.

  1. rainywish

    rainywish TDPRI Member

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    Hi,

    I'm planning to do some experimentation with a variable voltage regulator in a cathode bias amp. Schematics I find online and commercial kits use a nte2973 mosfet. Unfortunately this one seems hard to find (in Europe at least). Mouser stopped selling them a few years ago. Are there any known alternatives?
     
  2. Guitarteach

    Guitarteach Poster Extraordinaire

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    I used a Toshiba K2968 MOSFET but that was three years ago.
     
  3. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I had a VVR in a Tweed deluxe. It didn't make the amp growl at a low volume, it just made it cleaner. I had better luck with the 1/4 power mod on Robrob's pages, only I swapped the resistors to make it one tenth power. It worked pretty well.
     
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  4. sds1

    sds1 Tele-Afflicted

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    Can you find STP20N90K5 or APT14F100B ?

    These components should meet the minimum specs of NTE2983. Price / availability may be better if you were to relax the specifications a bit. Specifically, I think the Vds could safely be < 900VDC...
     
  5. Bitsleftover

    Bitsleftover Tele-Meister

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  6. Ten Over

    Ten Over Tele-Meister

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    There's nothing magic about the NTE2973 and I don't even think it's a very good choice for a VVR. I have used STW12NK90Z and STW15NK90Z multiple times with good results. I've got a couple of STW12NK90Z's going right now re-forming some capacitors upstairs. I like the internal gate-to-source zener protection.

    At first it seems like the specifications are overkill. The STW12NK90Z lists 900V and 11A. Holy cow! Yeah, well, it won't do 900V and 11A at the same time because it is also rated at 230W. And the wattage derates to the tune of 1.85W per degree above 25 degrees. What those big numbers are good for is to generate a Safe Operating Area graph so that you can be confident that you will be well within that area at the conditions you will be operating under.

    If this is your first foray into VVR, there are a couple of important things. These suckers get hot, so they need to be heat-sinked (usually to the chassis). Most of the devices have the drain connected to the thermal pad on the back, so a thermal-conductive insulator needs to be used so that you don'e short your HT to the grounded chassis.
     
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