Switching power scheme

[catalin gramada]

Hello. I imagine a switching power scheme using double pole switches. My intention is not just to limit the inrush current but also to try to drop some voltage till tubes starts to conduct to protect a little my caps from preamp from overvoltage when power on. It is about 60-80v difference over. Voltage after rectifier is 580v.the caps for power stage and inverter are series stacked for 1000v rating but caps rating in preamp are 500v. What do you think about.please? Thanks

[Jazz P Bass]

One question: Why?

[J M Fahey]

Hello. I imagine a switching power scheme using double pole switches. My intention is not just to limit the inrush current but also to try to drop some voltage till tubes starts to conduct to protect a little my caps from preamp from overvoltage when power on. It is about 60-80v difference over. Voltage after rectifier is 580v.the caps for power stage and inverter are series stacked for 1000v rating but caps rating in preamp are 500v. What do you think about.please? Thanks

1) you forgot the secondary CT

2) you donīt need primary and secondary limiting resistors, pick one

3) VOX used a *rotary* OFF-Stby-On switch, which allowed a series resistor limit surge for smooth cap charging but then automatically shorted it when on the full ON position.

No way you could leave it in the path overheating and making your amp sag.

On the SS amps they used a similarly labelled one but in this case switched speakers OFF so no turn on or off pop could be heard.
Very clever.

[R.G.]

There are much more straightforward ways to do this.

For inrush, you can use a NTC inrush limiter as used in high frequency power supplies. This looks like a disk capacitor, but it's a resistor whose resistance goes down as it gets hot. It starts cold as a high resistor, limiting inrush current, then as the current heats it, it drops to a few ohms. Practically every high frequency power supply on the planet uses them, and they're intended for rectifying the 120 and 240V AC lines, so the voltage ratings are even about right.

That may be enough all by itself, but if it's not, why not use a zener or amplified zener to limit voltage to the preamp tubes? You don't show it but you almost certainly have a large resistance chain leading from your power amplifier to the various preamp tubes. Put a zener at the preamp side of that resistor chain that is rated at a higher voltage than the preamp tubes normally work, but lower than the start up voltage. The zener only conducts until the tubes start up and pull enough current to pull the voltage below it's conduction threshold.

It takes some learning and calculation to properly size these two components, but they are cheap, and will eliminate a lot of other parts, including some expensive switches.