Standby & bleeder resistor to 5F2-A

[lacrebob]

Can someone help me with a dumbed-down version of how to add standby switch and bleeder resistor to a Tweed Princeton 5F2-A?

I'm building a 5F2-A as my first build and I want to make those modifications. Since it is my first build, I'm hoping to get very specific help.

I usually read to add the bleeder resistor to the positive side of the first filter capacitor. On the layout, is the first filter capacitor the one on the left side?

[sliberty]

A very common way to install a standby switch in smaller Fender designs is to copy the approach Fender used in some of their bigger amps. Take a looka t the 5E5 Pro. What they did there was to install the standby switch on the center tap of the power trannie. Currently, your center tap does directly to ground. Instead, take the center tap to the siwtch, and the other leg of the switch to ground. When the connection is broken, the center tap is lifted and the no B+ will flow. When the connection is made by flipping the switch, B+ will flow, and the amp will come alive.

Some will argue that this is not the best way to implement a standby, and that a better way is to interrupt B+ as it flowsd from the recitifer tube to the filter caps. But since Leo did his amps this way, its probably appropriate for a Princeton build.

On the standard 5F2-A layout, the first filter cap is the one on the left. Place a 220K resistor across this cap as a bleeder.

[lacrebob]

I got it. That method may keep hum down.

What would the advantages be of installing the standby switch between pin 8 of the 5Y3GT and the first filter capacitor (16/450)?

[skyboltone]

I got it. That method may keep hum down.

What would the advantages be of installing the standby switch between pin 8 of the 5Y3GT and the first filter capacitor (16/450)?

Some designs do that. The tube has to fill up the filter caps instantly when you do it that way. You can approach the hot switching limits of the tube. Another approach is to connect a filter cap to pin 8 (- end to ground) then go to the standby switch. It's easier on the tube. It fills up that first cap on warm up, then when you throw the operate switch it's got a cushion between it and the "dead short" of your power supply rail. Problem is that the cap will stay "hot" for hours after turnoff without a bleeder. Use a 270K 1/2 watt end to end on the filter cap and it will discharge in about a couple of minutes. The amp will bleed off power on the balance of your power supply.

And anyway, get in the habit of using an alligator jumper clip from chassis to the power supply rail before sticking your hands in there. Put somethng like a 22K resistor in series with the clip if you want a quick drain with no sparkle.

[mhuss]

The tube has to fill up the filter caps instantly when you do it that way. You can approach the hot switching limits of the tube.

Same deal with the CT switch. It doesn't matter where you put the switch in a series circuit, same loads and stresses happen. I like putting a smallish (10-22 ohm) flameproof in series to take the "edge" off the initial charging current.

Another approach is to connect a filter cap to pin 8 (- end to ground) then go to the standby switch. It's easier on the tube.

...but harder on the switch. Most/all switches DCV rating is half the ACV rating.

And anyway, get in the habit of using an alligator jumper clip from chassis to the power supply rail before sticking your hands in there. Put somethng like a 22K resistor in series with the clip if you want a quick drain with no sparkle.

+1 !

--mark

[skyboltone]

...but harder on the switch. Most/all switches DCV rating is half the ACV rating.

I've don't really understood that. AC voltage has a peak to peak voltage of 1.71 times RMS. So wire rated for instance at 600VAC has to withstand 1026V on peaks. Same with any other component. But DC switching is harder on things? I dunno.

[txbluesboy]

The reason DC is harder on switches than AC is because AC current is continually going from positive to negative, doing so it is at 0 volts 120 times per second. When you try to stop the flow of current with the switch an arc is formed across the switch contacts. With AC this arc stops very quickly because alternating current passes through 0 volts very often, however direct current is continuosly positive ( or negative ) so the arc lasts much longer causing pitting on the switch contacts. The capacitors and choke make this situation even worse, being that their job is to keep current flowing at an even pace

[mhuss]

A second reason is that AC really *is* at or near zero volts for a significant part of the 50/60 cycle time, which is obviously easier on the switch (depending, of course, on the instant you flip the switch)

The arcing is a major issue at the higher voltages we tend to use around standby circuits.

--mark