the standby switch

[Randall]

What are the merits of different standby wiring schemes? In many Fender circuits it is switching hot B+. In an Ampeg circuit I am studying, it switches the PT CT to ground. The latter seems safer to me. Has anyone heard of a Fender standby switch failing and electrocuting anyone?

[bob p]

Has anyone heard of a Fender standby switch failing and electrocuting anyone?

I've heard stories of famous musicians being killed by electrocution, but I've never heard any details discussed where the exact cause was clearly identified. (I guess nobody who's been killed by electrocution has lived to tell the story.)

later Fender-style standby switches DC. That's harder on the switch than switching AC. With CT switching you have to be careful about runaway bias. In other words, cathode bias = yes, fixed bias = no (unless you think things through).

[R.G.]

The devil makes me want to say that it's even safer to make the standby switch be a high voltage, high current MOSFET from the cathodes of the power devices to ground. You can switch that with a reed switch.

Even safer, use a P-channel MOSFET in the B+ line, driven by a LED-to-PhotoVoltaic isolator and get 4kV of isolation in the isolator, then drive the LED side from the heater supply.

[Dave H]

And the devil wants me to say that you don't need a standby switch. What use are they anyway? If you want to take a break just pull the input jack half out to short the amp's input.

[g1]

Has anyone heard of a Fender standby switch failing and electrocuting anyone?

Not sure about the history on that, but with modern wiring, 3 prong plugs and grounded chassis, it should no longer be possible (unless there is failure of the chassis ground/earth ground wiring).

[Tom Phillips]

Consider that there is a growing collection of information to support that a standby switch isn't really necessary at the voltages we use in guitar amps. That's why, in your thread about the small chassis you are planning, I suggested that you could just eliminate the standby switch.

[Enzo]

I had to think about that switch failure, not sure what he has in mind. It is not likely the standby switch could fail in any way to make the chassis hot. Those ways would instantly blow a fuse. But perhaps if you interpret it in the sense the switch is defective and won't turn off. Some unsuspecting novice tech assumes the power is off because the switch is down, but the circuit remains live. But Darwin ought to weed out potential (pun intended) techs who rely on just looking at a switch to determine if a circuit is live.

But the switch is only one place where high voltage exists, there are many other equally deadly exposed (inside) high voltage points.

[bob p]

Even safer, use a P-channel MOSFET in the B+ line, driven by a LED-to-PhotoVoltaic isolator and get 4kV of isolation in the isolator, then drive the LED side from the heater supply.

Hijack: what I don't understand is why the people who do power scaling haven't been bright enough to isolate it that way. To me it just seems insane to run B+ through a pot.

[Randall]

I was thinking about the possibility of making the toggle hot without having first had a close look at a switch first. And I was also thinking 50's wiring. But mostly I was wondering the merits of B+ vs CT switch methods.

[d95err]

Hijack: what I don't understand is why the people who do power scaling haven't been bright enough to isolate it that way. To me it just seems insane to run B+ through a pot.

Kevin O'Connor's DC Powerscaling circuits use a control voltage of about 10-15V on the pot or optical isolation.

[Mick Bailey]

The safest, most reliable standby switch is one that stays in its packaging.

At best they make working on an amp more convenient. At worst they're responsible for many abrupt component failures that wouldn't otherwise occur if the amp had warmed up by itself. They're a nice addition if you like replacing tube rectifiers. Just ask Korg.

You only have to take a look at that initial shock the power tubes get when the standby is flipped in any amp to realize they're not a good idea.

[Randall]

Mick Bailey, educate me please. I have several 60's amps with the original tube rectifiers with standby switches. I have never had to replace a failed rectifier tube in any amp, whether my own or a bench repair.

I am also interested in the debate of whether leaving a tube on standby during a break to maintain constant temp is better or worse than switching the mains on or off and having to cool down/ heat up each time.

[Mick Bailey]

That's why; Original tube rectifiers. Theyr'e not made like that any more. I get plenty of modern amps with failed rectifiers, especially when the 1st filter cap size has increased by the designer to reduce noise in higher gain amps. You've never had a modern Vox with a failed rectifier? Must be a different spec to the ones supplied to Europe.

Dave H has the answer to 'standby', or just turn the volume down. If that switch is important I'd rather see it shorting the input to ground for a standby. Or wired with a soft start resistor.

[pdf64]

I was surprised that the DrZ EZG50 has the standby between the 5AR4 and the reservoir cap (50uF).
At least it would be 'less worse' to put it after the reservoir cap.
Pete

[Steve Conner]

The standby switch is the guitar amp's equivalent of an appendix.

Personally I feel it is kind of useful: If you want to take a break from playing, it lets you cut off the current to the power tubes so they can cool down a little, and you're not burning through your tube life quite as fast. But with the heaters still on, the tubes don't cool down completely, so the thermal shock isn't as great when they get going again.

It also reduces the thermal shock on power-up because you can let the heaters warm the tubes up before hitting them with plate dissipation. I won't get into the debate about "cathode stripping".

However as explained above, the standby switch needs some care in implementation. It's all too easy to create a current or voltage surge that would actually reduce the lives of some components.