what is hot switching?

[stingray_65]

I've seen this a few times now when reading rectifier tube data sheets:


Even ocational HOT-SWITCHING with CAPACITOR-INPUT CIRCUITS permits the flow of plate current having magnitudes which can adversly affect the life and reliabilty of tubes.

What is HOT SWITCHING and what is a CAPACITOR-INPUT CIRCUIT ?


Thanks
Ray

[Don Moose]

Hot-switching in general means operating a switch - like a standby switch - with power applied.

Capacitor-input current, in this usage, means the 'inrush' current a fully-discharged filter cap would demand of a hot-switched standby with, for example in excess of 300VDC standing on it. With several hundred microfarads (uF) of high-quality (low ESR) electrolytic this could be tens or hundreds of amperes for several tens of microseconds to tens of milliseconds. That spike, and the potential arc across the contacts as they get close, can kill a switch over just a few operations unless it was designed for that application.

That current spike goes to max almost instantly and falls off as quckly as the cap can charge through whatever resistance is in the switch and wiring. In some circuits with a lot of capacitance, you see a negative-temperature-coefficient resistor between the switch and the capacitive load. This resistor starts out at a higher resistance, limiting the inrush, and its resistance gets lower as the current flows and warms it up.

Hope this helps!

[Steve Conner]

In the context of a rectifier tube datasheet, it means switching the B+ on when the rectifier's filaments are already hot. And a capacitor input filter is one where a capacitor is the first thing after the rectifier: the standard hookup in most tube amps.

So what this means is that you shouldn't wire your standby switch in such a way that the rectifier tube has to suddenly charge the first filter capacitor when the switch is thrown. It will send the tube to an early grave.

Silicon diodes seem to take this kind of abuse all day, the concern only applies to tube rectifiers.

[EFK]

Resurrected! I'm trying to understand this concept - so for example, a JTM45 which places the standby switch b/t the rectifier output and the first 32uF filter cap will allow hot switching, correct? (In fact, now that I think about it, wouldn't one be hot switching a JTM every time it was taken off standby?) But if one were to place the standby switch *after* the first filter cap, it would protect against hot switching?

In a different scenario, how does this apply to an amp with no standby switch? In this case, if you turned off the amp, you would want to let the tubes cool down before turning it back on so that the first cap charged up slowly as the rectifier tube warmed up?

[Merlinb]

a JTM45 which places the standby switch b/t the rectifier output and the first 32uF filter cap will allow hot switching, correct? (In fact, now that I think about it, wouldn't one be hot switching a JTM every time it was taken off standby?) But if one were to place the standby switch *after* the first filter cap, it would protect against hot switching?

Correct.

In a different scenario, how does this apply to an amp with no standby switch? In this case, if you turned off the amp, you would want to let the tubes cool down before turning it back on so that the first cap charged up slowly as the rectifier tube warmed up?

Yeees, but the cathode probably cools down about as fast as the cap discharges, so it's unlikely to be a big deal.