what's this resistor for?

[Alex R]

Here's something (else) I'm not too sure about. This is the power amp schematic for a Simms-Watts 100w amp, big clean UK amp from the early 70s. They sound sweet and give the lie to the 'ultra-linear sounds clinical' malarkey. Anyhow, what's the 25ohm 10w resistor just after the bridge for? I know K O'Connor recommend a bigger resistor there for valve rectifier emulation, but what did they put it there for in 1970?

[Earl]

My 1st 'guess' would be that it limits the inrush current to the input filter when the switch is flipped.

[Enzo]

I'd agree.

The filter caps, when not charged, are a dead short for a brief instant when power is first applied. That can stress the rectifiers. The resistor limits the current through the rectifiers, but it is low enough to stay out of the way of the rest of the circuitry.

Let us say there is 500VDC at the bridge. A 25 ohm resistor across 500v to ground (dead short, remember) limits current to 20 amps. Rectifier diodes have a surge rating. For example, the common 1N4007 is a 1 amp diode, but it has a surge rating of 30 amps. That means it will stand 30 amps briefly now and then.

That 25 ohm resistor keeps current within surge ratings for the bridge rectifiers.

Yes, I doubt the transformer can provide 20 amps, but that 1 amp fuse takes a moment to blow - plenty time for diode to suffer. And that winding can throw a lot more current than you might think for the instant that matters. So we assume full voltage for determining the resistor.

[Bruce / Mission Amps]

My 1st 'guess' would be that it limits the inrush current to the input filter when the switch is flipped.

Agreed. Those solid state diodes will dump a boat load of current into the filter caps.

[Alex R]

Thanks, I see it now. Sounds like a good idea.