B+ switch

[Ned]

I have learned one way to provide a standby switch is to install a switch in the B+ immediately after a SS FWB rectifier. I have been looking to purchase a switch for this use and have visited tube amp supply vendors web sites. I find a switch they say will work, go to the Mfg.'s site check the spec's and always find out the switches are not rated at a high enough DC voltage. Am I missing something? Does anybody know what switch will work? Thanks.

[tubeswell]

I use switches rated at 250VAC 10A. 2CW

[defaced]

Check this out: The Valve Wizard

[tedmich]

Apexjr has some beefy switches for cheap:
SPST Standard size toggle switch, 10A 277V 20A 125V $1.25 each


replace a few time when noisy,

or try HV DC arc suppression tricks used on relays:

Contact Protection and Arc Suppression Methods for Mechanical Relays

[R.G.]

There is a simpler way. Use an 800V-1kV power MOSFET to switch the ground side. That makes the switch be a low voltage one, as all it carries is the small voltage to the gate. You could also use a big beefy looking one of any kind you like as long as you put enough (unnecessary!) current through it to make the contacts stay clean.

[Steve Conner]

I've used both methods, the MOSFET and the regular switch, and both are still working. The MOSFET has lasted 10 years, the switch about 4 so far.

Switches aren't rated for such high DC voltages, but in practice they seem to take it fine. To be sure, I once destroyed a cheap rocker switch trying to break DC, but it took 600 volts DC at several amps, with a large inductor in series. I was testing a big DC power supply with a couple of hairdryers in series as the load, and I foolishly switched one of the dryers off. It failed to break the arc and the switch melted.

You can increase the voltage rating of a switch by getting a double-pole one and connecting the contacts in series: it helps to break the arc.

[R.G.]

The MOSFET has lasted 10 years

That's not too big a surprise.

MOSFETs are primarily used as power switches in off-line power supplies. In this application, they common are used to switch perhaps several amperes of 400V (rectified 240Vac) at between 50kHz and 1MHz for years without giving up. Switching amplifier B+ a dozen times a day for a year is about 4380 times, or less than a tenth of the power switching done in power supply switching duty in one second.

Essentially all of the trouble to be expected with MOSFETs switching B+ in a tube amp will be preventing transients and noise from getting to and killing the the gate insulation.