Help! Channel switching relay power supply going up in smoke

[KT66]

I just finished an amp for a customer of mine that has a footswitchable relay in it. It is basically a Plexi with 2 different front ends that can be selected with the footswitch, or if you flip a switch a cascaded gain stage is inserted when you engage the relay. It is a really cool amp, but I am having a problem with the relay power supply. I used a circuit that was posted on this forum by respected members that rectifies and regulates the filament supply to power the relay. I also use the rectified 5V to power an LED on the front panel to let the user know that the "B" channel is engaged. When I was testing it, the FWB I made from 4 1N4007s went up in smoke. I replaced it with a bridge rectifier I had in my junk drawer that I think had a 25 amp rating, and it went up in smoke too. I will try and post a schematic, if anyone can clue me in as to what bonehead mistake I am making I would greatly appreciate it, other than this problem this amp totally rocks and I need to figure this out soon so I can ship it to my very patient customer.

[paulster]

I'd say almost certainly that you connected DC ground to the chassis (Switchcraft jack perhaps?) and also had the heater centre-tap connected to the chassis for your AC ground.

Each half-cycle this shorts out one half of the bridge rectifier and the result is it smokes.

You either need to keep the DC ground isolated from the chassis and all other B+ derived DC grounds, or forego the heater centre-tap and rely on the DC side giving you your ground reference for the heaters.

Since you're not using the rectified DC for preamp heaters and only for switching I'd isolate this from the chassis and leave the heaters as they were. You'll need to use an insulated jack as well for your footswitch.

[KT66]

The jack is a plastic jack and is isolated from the chassis. The negative lead from the FWB is only connected to the regulator, the LED and the relay, it is not connected to the chassis or ground in any way. The heater center tap is connected to ground. When I had 4 sepearate 1N4007 configured as a FWB, the 2 that toasted were the 2 that went to the ground side of the bridge from the 6.3V AC input.

[Aurora]

Is the regulator isolated from the chassis? Most 3-pin regs have the tab connected to the center pin , i.e. minus in this case....

[KT66]

Ya, the regulator tab is isolated from the chassis, I bolted it down to the tagboard, you can see my setup in the pic below. I'm begining to think it has to do with the heater CT. Does anybody know how the Weber relay board is wired up ?

[Merlinb]

You can at least simplify the problem by getting rid of the regulator- you don't need a regulated supply just for a relay. Should make it easier to track down the problem.

[ampdoc1]

Lift the 6.3 center-tap. I'd also dump the VR. I'm using a FWB rectifier tied to the heaters AC to provide ~6VDC for my heaters and a relay.

Check the simple heater supply on this amp.

ampdoc1

[paulster]

Lift the 6.3 center-tap.

I would try this as part of the fault-finding process but you only need to do this if you've chosen to use the DC side for the ground reference though. That's the odd thing here since the OP says the DC ground is isolated. Since the preamps are on AC heaters, grounding the AC supply via the centre-tap will yield a quieter preamp.

I'd also dump the VR.

I'm with you here. Not needed for a simple relay circuit and it's another potential failure point.

[Tonegeek]

disconnect the DC connections to your bridge and test for a short to your chassis on each of those wires. It should read open on your meter for both of them. If thats OK then double check that you wired the bridge correctly or that there is no short in your circuit board that the bridge is sitting on.

[KT66]

I took your suggestion Tonegeek and lo and behold, there was a connection to ground to the chassis from the negative side of the bridge. After pulling my hair out for a few minutes trying to figure it out, I realized what had happened. I had connected the foot pedal to the jack with alligator clips because I didn't have the phono plug on it yet, and the alligator clip was shorting the negative from the bridge to the chassis. I replaced the bridge and it works fine now, thanks for everybody's help.

[Tonegeek]