VVR Mosfet specs

[sdp1234]

I've been searching for a while now and I can't find the specs for the VVR Mosfet. I know that the NTE2973 is rated for 900V. Is 900 v the minimum? What about the recommended power dissipation... Are there any other specs that are important. Trying to see if I may have components on hand.

Thanks

[UR12]

I've been searching for a while now and I can't find the specs for the VVR Mosfet. I know that the NTE2973 is rated for 900V. Is 900 v the minimum? What about the recommended power dissipation... Are there any other specs that are important. Trying to see if I may have components on hand.

Thanks

There are a few specs that you need to look at , not just the power rating. Since most mosfets can easily handle the current that a tube amp demands from its B+ supply you need to look at the Max voltage and max power the mosfet can dissipate. As an example if we take an amp that has 400v B+ and pulls 200ma you will need a mosfet that is capable of at least 400v and can handle 80 watts (.2a x 400v = 80watts). So in theory any N channel Mosfet capable of dissipating 80 watts and has a voltage rating of over 400v should work. Having said that, I don't like running things at their max ratings and as a rule of thumb I like to see a minimum of at least 1.5 times the max ratings. That would put us in the 600v and 120 watts range for the mosfet. It is also important to take into account the voltage drop from drain to source (usually only a volt or two) as this comes into play with the current limiting design on the VVR. The voltage drop is dependent on the mosfets RDS rating or resistance between drain and source. I like using devices with no more than about 1 to 2 ohms as you loose very little B+ and it is easy to calculate the resistance needed for the 5 watt resistor for the current limiting.

I always ship the NTE 2973 in my kits because it works in a wide range of amps from 1w to 50watts and I don't always know what size amp it is going to be installed in. That doesn't mean that when you roll your own you need to go with that large of a Mosfet.

Not sure if that answers your question but I hope it helps.

[txbluesboy]

I have been using IRFP350PBF from Mouser on everything I have built. Its only rated at 400 volts, 16 amps, 190 watts. I know that some of my builds have B+ in the 450 volt range but so far I havent lost any.

[sdp1234]

I have 2 new IRF820s. Looks like they are borderline for my application. I'm looking at a single el34 at about 350v DC. The specs are 500v 80W and RDS <.3ohm. I think i will use this initially but upgrade when I put in another order.

Thanks