Mosfet switching

[lowell]

I'm pursuing incorporating mosfet switching in the power supply of my new amp. It's purpose is to switch the PA from full power to "tweed mode." I've attached a schem if someone would please review it, I could use some help as I'm new to FET switching.

Also I'd like to switch negative feedback in and out around the PA. How do you wire negative feedback w/ a cathodyne PI? Finally, will a series MOSFET switching arrangement work?

[defaced]

Cathodyne with NFB.

[lowell]

thanks.

[loudthud]

Your diagram is a little confusing because the labels don't make sense. Example: Gate and Drain of M6 both go to "B". That doesn't make any sense.

You have to remember that MOSFETS typically have a Gate to Source voltage rating of 20V. If you exceed 20V they short out. M11 shunts across the tube rectifier so it's gate voltage will need to be higher than B+ to turn it on. You might want to use a P channel MOSFET there but finding one with >400V breakdown is tough.

[lowell]

whoops.. ok here it is. i cleared up the control voltage labels. So a mosfet won't work for the rectifier... how about a Vactrol? It seems a VTL5C3 can handle the voltages... but 40ma of current isn't gonna work is it? I may be left with a relay as my only option there.

[loudthud]

You have to consider what the Source voltage is going to be, then you know what the control voltage (Gate) needs to be in order to switch the MOSFET on and off. Look at M8. If the control voltage is zero volts (to ground), M8 will be off. To turn M8 on, the control voltage needs to be 5 volts higher than the expected voltage across the 55 ohm resistor. It would be better to ground the Source and connect the 55 ohm in series with the Drain. Then the control voltage can be zero and +5 to +20.

The same applies to M6. If the B+ is 400V, then the Source of M6 will be about +200V. M6 will be off if the control voltage is +200 to +180. If it's lower than +180, M6 will be destroyed. But when M6 is on, the control voltage needs to be +405 to +420. It's gonna take some level shifting circuitry to make that happen. Possibly an opto isolator or some form of solid state relay. Much more complicated than just using a relay.