Standby Switch Wiring in Sentura II

[patlaw]

The standby switch on a Sentura II is between the rectifier tube and the first filter capacitor. On the Fender Twin, it's between the first filter capacitor and the choke. What practical difference does it make?

[Chuck H]

I can only speculate. With the switch between the rectifier and the first filter the rectifier will be warm and capable of delivering at it's maximum when the switch is flipped. That means any capacitors instantly connected by the switch will try to charge and create considerable inrush current. With the switch placed after the filters It's possible that the inrush surge would be softened by the gradual heating of the rectifier cathode. I might be full of S on this.

[pdf64]

'The standby switch on a Sentura II is between the rectifier tube and the first filter capacitor'

The use of such a switch is best avoided, as it puts stress on the rectifier tube (hot switching).
Just use the power switch and allow the rectifier to ramp the VB+ up smoothly.
Find some other way to mute the amp, if necessary.

[patlaw]

pdf64, while I agree with your suggestion, I'm restoring my amp and am trying to decide whether to wire it according to the Sunn schematic or according to the Fender schematic. Given the choice of the two, which would you select?

[Chuck H]

My vote is for the Fender design.

[g1]

Agree. Go with the Fender circuit. Especially considering the state of modern rectifier tubes.

[Leo_Gnardo]

I can only speculate. With the switch between the rectifier and the first filter the rectifier will be warm and capable of delivering at it's maximum when the switch is flipped. That means any capacitors instantly connected by the switch will try to charge and create considerable inrush current. With the switch placed after the filters It's possible that the inrush surge would be softened by the gradual heating of the rectifier cathode. I might be full of S on this.

No reason to doubt yourself Chuck, you have a good point. When the standby switch is closed, the current drawn to charge all the caps beyond the first one will be shared by the pre-charged first cap and the rectifier tube. So - less of an overload to the rectifier tube when switching to operate mode. You just have to make sure the voltage rating of the first cap (or combination of 2 in series) is adequate to stand extra high voltage that will develop as there's no current load on the high voltage supply while in standby mode.

There is a case to be made for not using a standby switch if you're using 5AR4/GZ34 rectifier. There's a built in slow start which lets the power supply voltage ramp up after maybe 10 seconds or so warmup. However this works only on cold starts. Lots of old hi fi amps ran like this.

The point's been made, in guitar amps standby switches aren't so much needed, except perhaps to keep amps quiet while the band is taking a break. In hi power equipment like big radio transmitters, tubes that cost hundreds or thousands of dollars are at stake. You definitely want a standby system there. System that is, because a simple toggle switch would probably vaporize.

[patlaw]

You just have to make sure the voltage rating of the first cap (or combination of 2 in series) is adequate to stand extra high voltage that will develop as there's no current load on the high voltage supply while in standby mode.

The Fender wiring makes more sense to me, too. Therefore, the first cap will be two 450 volt caps in series, so voltage is not a problem. The unloaded voltage is about 605 volts.

[Chuck H]

350V caps are a lot cheaper and a 700V totem will have some periodic over voltage margin it won't even need.

[patlaw]

350V caps are a lot cheaper and a 700V totem will have some periodic over voltage margin it won't even need.

That's true, but I already have the caps. If I don't use them here, they will probably dry rot.

[Enzo]

Let me be the devil's advocate. We can rationalize why one way might be "better" than the other, but neither circuit is known for blowing up rectifiers. I can't say I have replaced more rectifiers in Sunns than in Fenders, percentage wise. In other words, in the practical real world, I don't think I much matters.

[patlaw]

Probably not, but there is a pedagogical aspect to refurbishing old equipment, as well. That's part of the fun for me.

[Enzo]

Point being that whatever decision you make, it won't be "wrong."

[patlaw]

By the way, where are you getting your electrolytic caps? I like to use axial to replace axial, when they're available. What good sources are there besides Digikey, Mouser, and Jameco? Digikey is not very price-friendly for low (repair) quantities. Mouser is a good choice. Jameco has a good selection of axials, but who knows what the quality of their private-labeled parts is? Weber has some Richey high-voltage caps.

[Chuck H]

I use to just recommend the Sprague Atoms. I've had too many problems with them to ever order them again. Illinois seem to work well but die young (no flames please, this is just what I've seen). I've never tried any of the boo-tiki brands or amp supplier house brands. I think the axials I've had the least trouble with are the cheap Xicons. What I use now are radial caps and I've been really happy with this brand and product series so far. When I need to install them axial I just add lead wire to one of the leads, shrink tube insulate it, wrap that lead over the top and either shrink tube or zip tie it to the can. Only a little unsightly and I know I have a good cap in there. But that's not what you asked for. I've read that the F T caps are good.