Trouble installing VVR - Help please

[Steve Conner]

Well, the MOSFET and JFET are the most tube-like of all the solid-state devices. They behave like pentodes, except they don't need a screen supply, the "screen grid" is some sort of quantum magic inside the chip.

N-channel JFETs are exactly the same as tubes with respect to "biasing".

The FET symbols on schematics are poorly standardized and confusing. Sometimes the guy who drew the schematic even gets the polarity wrong. I often have to figure it out from context. If you see a FET, it's usually N-channel, because they're cheaper.

The arrow in all semiconductor symbols represents a junction, and points towards N-type material. So a N-channel FET should have an arrow poking into the vertical bar of the symbol, which is supposed to represent the channel. In a JFET the arrow is on the gate pin, and that ties up with the existence of a diode junction there, whose polarity is as the arrow suggests if it were part of a diode symbol.

FET symbol trivia part 2: In a depletion (normally on) device, the bar is solid. In an enhancement (normally off) device it's broken into three segments. This is supposed to symbolize that the channel is normally closed, and you have to apply gate voltage to open it.

One gotcha with JFETs is that they don't have a source and drain. They have a gate and two other identical connections, which can swap roles as source and drain when they feel like it.

I should also mention that the IRFP460 is a 500 volt device, so it's not a general replacement for the VVR kit. However in the case of a 15 watt amp with a low B+ it would work. IXYS just launched some nice MOSFETs that are actually specified for this kind of use, anyone tinkering with power scaling may like to check out their "Linear L2" range.

On the original topic... Glad to hear you got it working

[clear and blue]

That makes sense. It helps to think of these things as old technology stuffed into smaller, more efficient packages. So in the VVR context the FET is used to control how much voltage is passed through and waste is passed through to ground through the drain? If this is right, the FET will be seeing ~350V constantly, so where I set the VVR pot won't affect how warm it gets.

The VVR is working fine now with the current MOSFET although it is worth bearing alternatives in mind for future projects. I think it is more useful than a MV so I would definitely be looking to implement it in any amp I might build. Although the next thing planned is a tube integrated stereo amplifier - don't think there is much need for output drive there. .

I think part of the fun of it all is that there is always something new to learn Enzo. Learning is the fun part - it is always better to admit holes in your knowledge and learn than let your pride take over to convince yourself you know it already.

Thanks to both of you for the help and advice, I really appreciate it!