Can I Parallel 2 Mosfets in VVR?

[bcmatt]

So, this is the regular VVR I use in Cathode biased amps:

VVR.jpg

If power handling is a worry and I'm worried about that MosFET, can I just add a second MosFET in parallel?
Like this?:

Parallel MosFET VVR.jpg

Do MosFETs work like that, or does that screw up the circuit?

I imagine that the heat dissipation is the major issue, but with 2 mosfets, I could separate them more and give each their own heatsink....

[FUCHSAUDIO]

No reason you can't. I would add a 100 ohm series gate resistor on each, and also keep the 10 ohm source resistors on each, the input drain leads can simply tie to the same point.

[bcmatt]

No reason you can't. I would add a 100 ohm series gate resistor on each, and also keep the 10 ohm source resistors on each, the input drain leads can simply tie to the same point.

So, like this?

2Mosfet.jpg

[Stevem]

They also need to be mounted right next to each other so that all wire length used is the shortest.

Also I consider the leads of semiconductors as part of its built in heat sink and as such contributing to the items wattage rating.

For Zener diodes used in tube amps ( especially if near the output tubes and there heat ) I do this mod if I have to greatly trim back there lead length.

I get a few non insulated crimp on lugs and cut them so I have just the crimp section left and solder one on each diode lead close to the body.

[bcmatt]

They also need to be mounted right next to each other so that all wire length used is the shortest.

So, this is the part that also makes me wonder if 2 parallel mosfets even buys any more wattage rating since they would have to be close enough to each other that they would end up sharing the same large heatsink. So, how much wattage handling would be added by the second mosfet, if it's going to need to sit right next to the first on this same heatsink with a fan blowing across the fins?

20240511_054511.jpg

Or, would a few inches of lead not be so bad in order to get a second heatsink next to it?

Also I consider the leads of semiconductors as part of its built in heat sink and as such contributing to the items wattage rating.

For Zener diodes used in tube amps ( especially if near the output tubes and there heat ) I do this mod if I have to greatly trim back there lead length.

I get a few non insulated crimp on lugs and cut them so I have just the crimp section left and solder one on each diode lead close to the body.

That's a handy idea for sure. I'll try that.

[FUCHSAUDIO]

No reason you can't. I would add a 100 ohm series gate resistor on each, and also keep the 10 ohm source resistors on each, the input drain leads can simply tie to the same point.

So, like this?
2Mosfet.jpg

YUP.

[FUCHSAUDIO]

They also need to be mounted right next to each other so that all wire length used is the shortest.

Also I consider the leads of semiconductors as part of its built in heat sink and as such contributing to the items wattage rating.

For Zener diodes used in tube amps ( especially if near the output tubes and there heat ) I do this mod if I have to greatly trim back there lead length.

I get a few non insulated crimp on lugs and cut them so I have just the crimp section left and solder one on each diode lead close to the body.

I agree on diode bridges and zener diodes. I don't think the leads of a FET should be counted as a heatsink really. By paralleling FET's each device shares the load. You cannot get rid of wattage (laws of physics and ohms law and such), but you can make each device do half the work by paralleling devices. As far as heat sinking goes, a fan, a bigger sink, spreading out devices a bit, all can't hurt. I like aluminum chassis, as they are "free heatsinks".

[TUBEDUDE]

That is one hellified heatsink! With a fan across it, it should create frost on your FET.

[bcmatt]

That is one hellified heatsink! With a fan across it, it should create frost on your FET.

Haha! Ya, I think it definitely needs the fan. The fins are so thin, they don't have any mass on their own. I think it really depends on that heat transfer to the air that is being blown off. I've had this sitting in my parts bin for like a decade from somebody's old computer. I always salvage old cpu heatsinks, but I remember thinking this one might be needed if I ever re-attempt VVR on a my 2204 clone or some other 50-ish watt amp.

I've heard of power-scaling on 100 watt amps, but I wonder how that is pulled off...

[sluckey]

If power handling is a worry and I'm worried about that MosFET, can I just add a second MosFET in parallel?

I can't imagine a tube amp needing parallel MOSFETs for VVR. Thes devices are rated for high power/high current with proper heatsink. For example, the NTE2973 that is commonly seen in many VVR schematics is rated for 900V, 14A, and 275W. If I needed a beefier VVR than that I'd be too scared to build it!

https://www.nteinc.com/specs/2900to2999/pdf/nte2973.pdf

[bcmatt]

If power handling is a worry and I'm worried about that MosFET, can I just add a second MosFET in parallel?

I can't imagine a tube amp needing parallel MOSFETs for VVR. Thes devices are rated for high power/high current with proper heatsink. For example, the NTE2973 that is commonly seen in many VVR schematics is rated for 900V, 14A, and 275W. If I needed a beefier VVR than that I'd be too scared to build it!

https://www.nteinc.com/specs/2900to2999/pdf/nte2973.pdf

So really, it seems like it is more useful to keep one mosfet with very short leads very aggressively heatsinked.... and that heatsink well ventilated.

[sluckey]

For an 18W amp in an aluminum (not steel!) chassis I'd just bolt to the chassis. Don't forget heatsink grease/paste or one of those fancy silicone pads. For a 50W amp I'd use a real adequate heatsink.

For a larger amp I'd just get an 18W amp.

[bcmatt]

For an 18W amp in an aluminum (not steel!) chassis I'd just bolt to the chassis. Don't forget heatsink grease/paste or one of those fancy silicone pads. For a 50W amp I'd use a real adequate heatsink.

For a larger amp I'd just get an 18W amp.

Amen Brother!

I'm just trying to keep options open for when I have Iron kicking around or already have a bigger amp that could really use a non-MV VVR situation.

[bepone]

connecting mosfets in parallel can easily lead to a silicone pop-corn

[Kagliostro]

Just for knowledge

At the begin I didn't understand well why you want to use a pair of Mosfet in parallel for VVR

for the same reason ....

I can't imagine a tube amp needing parallel MOSFETs for VVR.

however reading your post I remembered a circuit a friend sent to me some years ago

Double Mosfet VVR.jpg

in this circuit you have 2 mosfet that aren't in parallel, they are in series

the first Mosfet (on the left) act as Voltage Regulator the second one (on the right) works like a short

if the first Mosfet blows and go short the second Mosfet replace the first as Voltage regulator

Why this is wanted ? Because if you use the output as B+ or use it to supply a G2 (especially in a tube that don't like High Voltage as B+ on G2 and require a less voltage to feed it) if the Mosfet burns you easily burn also the tube, instead, this way, if one mosfet burns you have the other that act as safety component

Franco