Standby Switches with SS Rectifier???

[ViperDoc]

I search the internet and this topic is beaten to a pulp, but I can't find a consensus on usage of standby switches with SS rectification. Say for a 50W Marshall-800-style amp with higher gain, B+ around 450-460 with 2x50 uF 500V filter caps, is it a problem to leave it out?

[TUBEDUDE]

Until your tubes warm up and start drawing current, the B+ will climb, possibly exceeding your filter caps rating. Look at the voltage on your filter cap when powered up from a cold start. If too high, make sure you have a bleeder, a thermistor, time delay relay, or some way to limit the initial voltage surge. Or filter caps with very high voltage withstand rating. Or just be comfortable knowing the caps lifetime may be shorter, and may die spectacularly.

[pdf64]

In the old days, ecaps intended for HT reservoir and decoupling applications had ‘working voltage’ and short term ‘surge voltage’ ratings, precisely in order to accommodate this issue.
Remember that valve guitar amps are the only receiving valve application ever that had a HT standby.
I’ve seen info for F&T ecaps that includes a separate surge voltage rating.

[xtian]

Leave the standby switch in the ditch. They cause more trouble than they solve.

[ViperDoc]

That's the plan...

[pompeiisneaks]

Leave the standby switch in the ditch. They cause more trouble than they solve.

this but turned up to 11.



~Phil

[didit]

Leave the standby switch in the ditch. They cause more trouble than they solve.

this but turned up to 11.



~Phil

111% !!

Best .. Ian

[ViperDoc]

Until your tubes warm up and start drawing current, the B+ will climb, possibly exceeding your filter caps rating. Look at the voltage on your filter cap when powered up from a cold start. If too high, make sure you have a bleeder, a thermistor, time delay relay, or some way to limit the initial voltage surge. Or filter caps with very high voltage withstand rating. Or just be comfortable knowing the caps lifetime may be shorter, and may die spectacularly.

I deleted the standby switch on the 5F4 I just built and noticed the B+ gets quite high initially. Those three 16 uF Atom caps are rated at 475 VDC. How is a thermistor best implemented? I’ve seen them on Express amps between the hot and neutral trafo side of the power switch if I’m not mistaken. Like here:

TWEXPRESSLAYOUT.pdf

[xtian]

I think you've seen a MOV in some builds, which is meant to protect against power spikes from the wall service.

I've never had any issues with high voltage on caps at startup. There's little heat created when there's no current running.

[ViperDoc]

Right. See the Express layout I just pinned to the last post. Looking up MOVs....

Edit: Metal Oxide Varistor. Surge protector mainstay looks like. No surprise that I’ve never used one. What would you use to buffer inrush current the right way?

[xtian]

What would you use to buffer inrush current the right way?

Never been concerned about it.

[pdf64]

... What would you use to buffer inrush current the right way?

Inrush current limiter https://www.amphenol-sensors.com/en/the ... -cl-series
One might be used in the primary, HT secondary, or both.
I’m not sure it would help the issue much though. If it is an issue.

[ViperDoc]

Thanks for the link.

[R.G.]

I would be remiss in my duty to promote solid state heresies in tube amps if I didn't point out some alternates.

The advice above is good - just don't put in a standby, or possibly make a switch labeled "standby" but really just a mute. If you really must do a tube standby, you can do a number of things to make this easier on the output tubes and the power supply.

One is to open the cathodes of the output tubes. This effectively leaves the cathodes hot, but no current through the tubes. At this point, grizzled old tube amp techs will start muttering "cathode stripping, sleeping sickness, etc." Those things do exist, but are probably not issues in the tube amps you'll work with. If you're worried about them, you can put a 10K resistor in the cathodes of the output tubes and short these resistors out when you want sound. This will leave some current, to avoid sleeping sickness, but make no sound come out. The resistors can be "shorted" by a high voltage MOSFET for ease in wiring the switching.

Another technique is to use a current clamp in the B+ circuit. A current clamp is a small circuit that conducts up to a certain current, then limits the current at that level. One of these between the rectifiers and first filter clamp will mean that suddenly turning on the high voltage secondary will not give a huge influx of current to the filter caps, only a smooth voltage ramp. You get to set the current level by component choice. A floating MOSFET current clamp can also be rigged to be completely shut off by an opto isolator, so you can use that to replace the high voltage standby switch wiring entirely if you just must use a high voltage standby.

It's a fertile field for innovation.

[Bergheim]

I switched from electrolytics to 800V film caps in the power supply. Never have to worry about surges or voltage limits ever again