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Standby Switch

AL1

Member
Messages
84
Would anyone be so kind as to give me explicit directions on how to hook up a standby switch on a black face princeton reverb. The amp has a new multicap can rated at 475 vots. I have the switch in place but not sure what to do. Thanks in advance.
 

wilder

Member
Messages
763
I don't have a PR handy to look at, but I could send you pics of the same area in my BF Deluxe Reverb. Email me if you need them.

Chris
 

John Phillips

Member
Messages
13,038
Break the circuit after the first filter stage. If you do it before, you'll overload the rectifier with the charge-up draw when you flip to 'on'.

Find the wire that runs from the rec socket to the corner of the tag board, and unsolder it at the board end. Remove the wire that runs from that eyelet to the cap can completely. Connect the wire from the rec socket to the same tag on the cap can (you may want to shorten it). Now, run two new wires from this tag on the cap can and the eyelet on the corner of the tag board to the standby switch.

NB: be absolutely certain that you know what you're doing with cap discharging - the Princeton is one of the amps that doesn't have bleed resistors on the caps.
 

AL1

Member
Messages
84
Hi John, thanks for your reply. Should I put a large resistor or a capacitor or a line snubber across the standby switch as others have suggested? If so, what value. Thanks.
 

John Phillips

Member
Messages
13,038
I wouldn't bother, personally. None of the Fender amps with standby switches have them, anyway.

Although in theory it might help reduce switch-contact arcing, I've never noticed a significant number of dead standby switches on old Fenders... it's usually the power switch that breaks if anything.

FWIW, I actually like hearing the switch pop! It lets you know the amp is working properly, without having to play anything. Useful on stage :).
 

AL1

Member
Messages
84
John, what about the issue of the first filter cap voltage going too high when the power switch is thrown? Should I put a large resistor across the standby switch to prevent this? If so, what value? Someone suggested a 250K ohm 2 watt resistor across the standby switch. I put a new multicap can rated at 475 volts.
Thanks John, for all your help.
 

John Phillips

Member
Messages
13,038
Originally posted by AL1
John, what about the issue of the first filter cap voltage going too high when the power switch is thrown?
It won't. The rectifier tube warms up slowly and begins to conduct only a little at first, so allowing the first cap to charge up slowly. That's exactly why you want the first cap connected directly to the rectifier, before the standby switch. If you connect a well-warmed-up rectifier to an empty cap, the initial current will be well over the rectifier rating.

Any resisitor small enough in value to pull down the voltage on the cap significantly would negate the effect of switching off the standby, anyway...

Leo & Co. knew exactly what they were doing! In general, if they did something a certain way, it's the right way. They weren't infallible (there are a few strange oversights, like the BF/SF Champ bias issue) but tens of thousands of old Fender amps have been working reliably for decades, and that must mean something! Actually, including most Champs... :)

Basically, as Chris said, you need to look at the standby switch layout in the DR and copy it.
 




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