active mosfet screen voltage dropping resistor

[imaradiostar]

I'm planning to use a mosfet as an active screen voltage regulator- not so much a regulator as an active resistor. Take a look at this:

http://www.triodeel.com/gromm260.gif

I have some mosfets with a drain-source breakdown voltage of 450 volts- less than the intended B+ of the amp I'm building. I'll set the gate voltage with a divider like the grommes schematic. In order to avoid exceeding the breakdown voltage during startup I'll have a divider at the source as well- maybe a pair of 220k 1 watt resistors. Once running I'd want the source to be at about 350 volts where it would power screens, PI and preamp.

Will it make smoke or sound?

jamie

[Steve Conner]

What do you mean, a divider at the source as well? I think you want to rethink that.

It might work, since the MOSFET only sees the difference between the screen voltage and the plate voltage, and most newer MOSFETs have some avalanche capability (think a 450V zener with a rating of a few watts between drain and source)

But you can easily get 800V MOSFETs, they're used in switchmode power supplies that have to run off 240V line.

A 12V zener between gate and source can stop mysterious MOSFET blowouts in circuits like these. If the MOSFET is going to go, it'll fail short and dump the full plate voltage into your low voltage rail.

A capacitor between gate and ground will have its capacitance "multiplied" by the gain of the MOSFET. A 1uF cap can look like an electrolytic the size of a bean can. This is an easy and cheap way to get a very well smoothed screen supply.

If you're really adventurous, you can add a current limiter to stop your power tubes from melting their screen grids in the event that you forget to connect a speaker.

[imaradiostar]

I guess I was just trying to get by without using the proper part. I thought the divider at both terminals of the mosfet might help prevent me from exceeding the max voltage from gate to source. My mosfet is a 450 volt part and the amp in question has plate voltage around 520 volts- so not too far out.

Thanks Steve!

jamie