SB Switch

[C Moore]

I was reading Merlin's discussion on stand by switches and a had a few questions.
1. How many volts does the switch need to be rated at? In his example, he says the voltage is 300.
2. He says the switch is placed after the rectifier. I know Merlin is a real bright buy, and he knows tube amps, so he must be right. But to me it seems the switch is between the PT and the rectifier. Isn't the switch then Before the rectifier and not After?
Thank You

http://www.freewebs.com/valvewizard1/standby.html

[Cliff Schecht]

Using a standard high-current 125V/250V, 10-15A switch is usually enough. DC does break down switches much quicker than AC, but the fact that we are working with 100-300mA instead of 10-30A means that we aren't pushing the power limits of these switches by any stretch of the imagination. Some will eventually arc internally at too high a voltage level but usually as long as you use good quality stuff (Carling is about the best) then everything should be fine. I've even used the switches we usually use for bright switches as a standby switch and it worked fine.

There are a few places to put the standby switch. My favorite is on the HV secondary center tap. It's typically placed after the rectifier and either before or after the first filter cap. It's usually good not to put it before the rectifier caps though as the inrush current can also degrade switches over time. The worst, as Merlin states, is right before a choke where the flyback voltages can be quite high and cause the switch again to degrade and break down.

I don't use standby switches at all on my personal builds anymore. Just an extra, unnecessary complication. Just because Fender did and others copied blindly doesn't mean that you should! Directly heated cathodes do eventually breakdown from shoving HV on them before the heaters are warm, but indirectly heated receiver type tubes are tough little bastards in this respect. I do like to let the power tubes warm up before hitting them with full signal though. Don't know if it helps but it can't hurt..

[C Moore]

Thanks for the info Cliff.
In Merlin's example, would you consider that switch to be After the bridge rectifier?
Thanks

[Cliff Schecht]

In the second to last example he places the switch before the rectifier and only on one leg so that some current can flow. What's the issue you see with this?

[Phil_S]

FWIW, after reading several, actually many, discussions, here and elsewhere, I'm arrived at the notion that I don't really know nearly enough to have a proper opinion. I've determined, for better or worse, the Merlin, et. al. must know of what they speak. I have resolved that I won't be placing a standby in any more of my amp builds. One less hole, one less switch!

As to your question, with respect to having a standby, I don't see the point at all if you won't allow the first filter cap to charge when you turn on the main power. Usually, this means you allow power to flow to the rectifier and interrupt the B+ ladder after the first filter cap, between the cap and the output transformer. This is the traditional way you'll see the standby done on F or M brand schematics. Allowing that f/c to charge is what softens the blow when you close the standby (full power on).

I've got nothing against a switch that opens the CT. That's just not what I typically see from established brands.

[C Moore]

In the second to last example he places the switch before the rectifier and only on one leg so that some current can flow. What's the issue you see with this?

I do not have enough electronics knowledge to have an issue.
Merlin says the SB switch is AFTER the rectifier. You said it is BEFORE, and I see it as being BEFORE the rectifier. So what is Merlin seeing that you and I do not see? It is the second to last diagram.
Thanks