managing inrush current advice

[R.G.]

Standby switches for high-powered amps are quite tricky. The safety approval ratings are a real issue, and even if they weren't, most switches are rated for AC and struggle to break high-voltage, high-current DC. I like to use a double-pole switch and put the two poles in series for better arc breaking, like Fender did in the 300PS. Putting the switch in the screen circuit would help, because the voltages and currents are smaller.

+1.

This is one reason I really like power MOSFETs as a standby switch. They are rugged enough to live in the high voltage environment, even with really big surges to fill filter caps, and do not arc at all. You get the high current stress off the standby switch entirely, and depending on the setup, may even be able to have the standby switch itself run with only low voltage DC on it.

One interesting setup that doesn't get much exploration is photovoltaic MOSFET drivers. With these widgies, you can float an N-channel MOSFET on top of B+ and drive the MOSFET gate from the current in an LED at the input to the MOSFET PV driver. These things naturally don't provide hard switching to the MOSFETs, so you get softer switching than you do with metal contacts, ramps and not cliffs.

[Steve Conner]

I remember seeing the MOSFET standby switch in some Yorkville amp. For my own MOSFET regulated amp, I just wired the standby switch up to disable the regulator, which causes the B+ to fade in and out over a few seconds.

However, many hobbyist amp builders are probably working with tubes because they couldn't get transistors to stay alive. High voltage MOSFET switches aren't necessarily a hassle-free addition to your amp. Voltage isn't necessarily a problem in itself though, 800V MOSFETs are fairly common and I've seen them up to 1200V.

The photovoltaic isolators are great if you can find them. You get ready-made "DC solid-state relay" modules that consist of a MOSFET driven by a PVI.

[R.G.]

I remember seeing the MOSFET standby switch in some Yorkville amp. For my own MOSFET regulated amp, I just wired the standby switch up to disable the regulator, which causes the B+ to fade in and out over a few seconds.

That works nicely.

High voltage MOSFET switches aren't necessarily a hassle-free addition to your amp.

I think "hassle free" and "high voltage" may be mutually exclusive.

I was trying to design switching power supplies back when only bipolars were available to do that. It was a real challenge to get bipolars to live in a 400V environment at all. Modern MOSFETs are a walk in the park by comparison. But you do have to understand that at high voltages, the inductance and capacitance of things like wiring makes a difference, and that inadequate layout and insulation can literally reach right out and bite you.

The photovoltaic isolators are great if you can find them. You get ready-made "DC solid-state relay" modules that consist of a MOSFET driven by a PVI.

Availability is getting better. It used to be only International Rectifier. Toshiba joined up, now Panasonic and Vishay are making them. Digikey stocks a variety of them. They're a little pricey ($2-$10 depending) but they do something which is otherwise very impractical.