Removeing the groundswitch in a Fender ab763

[überfuzz]

I'm rewiring parts of a Fender Twin Reverb using a 3 wired power cord. If I understand this correctly I don't need to use the ground switch if I ground from the power cord. However, I'm not sure of the capacitor from lead to ground.

Anyone familiar with Fenders AB763 circuit who can point me in the right direction about the 0.047 uF capacitor going from lead to ground in the ground switch. What do I do with the capacitor now when the ground switch is gone?

[Enzo]

Get rid of it.

[überfuzz]

OK, thank you!

It's just that I'm the nervous kind, well, I am when it comes to my amps and high voltage. Is there a theory or an explanation to the removal of the capacitor.

Cheers, have a nice weekend!

[Enzo]

In a two-wire system, neither side of the power line is ground. The chassis needs to be at "ground" potential for noise reduction if nothing else. That cap coupled noise to chassis, and the switch selected which side of the incoming mains was the one.

Now you have a wire from ground to your chassis. You don;t need that cap to reference either side of the mains to chassis. The cap won't do you any good, and it has the potential to cause problems.

[überfuzz]

Mhmm, I don't quite follow you... But it seems awkward to use a cap from ground to lead in a cord grounded system. I'll get rid of it. Thank you!

[acorkos]

looky here...

[Kazooman]

I'm confused

I've seen this drawing here before and I am confused (Although I have successfully converted my '60's era Bandmaster and Reverb, several Dynacords amps and Echolettes tape echos to three wire plugs).

I agree with the schematic representation of the wiring (although I would like to see the switch before the fuse), but the pictorial version is confusing. I'm not certain where the black wire from the cord even comes into the picture. Presumably it runs to the switch, but that wire is under the label "black wires to transformer". Is the white wire from the cord connected to the outlet and then to the lug on the old ground switch and then to the transformer?

Why not get rid of the cap and the ground switch? Neither are needed. Black in goes to switch, then fuse, and then to transformer (and outlet small lug if desired). White in goes to transformer (and outlet large lug if desired). Green in goes to chassis. The outlet really should be replaced with a three prong version as well.

How does this look? Right angled junctions are either wire nut connections or soldered to the appropriate lug.

[pontiacpete]

I put the xformer (black) on the far side of the switch not the fuse. The other xformer wire I do like you do, directly to the neutral wire from power cord.

[überfuzz]

Either way the capacitor is going. I'll post a picture of my power cord wiring next time I'm at home.

[Kazooman]

Fuse Position

With the fuse in front of the switch the fuse holder is always hot when the amp is plugged in. Someone might try changing the fuse by flipping the switch off with the plug still in and then pulling the old fuse. It is possible (not easy, but possible) to end up touching a live part on the fuse holder (especially if the fuse holder is wired poorly with the line attached to the sleeve and not the end lug). With the switch first in line either pulling the plug or flipping the switch cuts power to everything, including the fuse holder. It's kind of like wearing both a belt and suspenders to hold you pants up . Not a big issue either way.

Come to think of it, I used the original drawing and modified it for my version. I am not certain just what is meant by the representation of the fuse holder, but I would wire the hot in side to the end lug, not the sleeve.

[Kazooman]

Hey Uberfuzz

Just disconnect the cap and leave it attached to the amp chassis. Someone looking for an authentic vintage amp might want to have the cap back. It will be just fine sitting by itself. Likewise, leave the ground switch in place even if you wire around it. And, unless you really need the extra outlet, I would leave the original in place and not hook it up. Save the original cord and the amp can be restored to authentic vintage condition.

[Enzo]

What was missing from the drawing, and I believe is your point of confusion, is the mains cord. On the original two wire, the cord came into the chassis and was wired to the convenience outlet. And from there, we see the scematic presented. To convert to three wire, we can mount a three wire cord, ground the chassis to it, and wire hot and neutral to the same convenience outlet.

Given that the mains cord is terminated at that outlet, the rest of the new/old schematics should fall into place.

I want my mains wired to the fuse first, in case the switch fails. Switch innards fall apart, which they can do, and a short to frame takes out the fuse. A properly wired fuse holder has the hot lead on the tip, not the cap end. If you look at those combination IEC power inlet and fuse holders, you will find the mains wired direct to the fuse.

[Kazooman]

I Get The Original Scheme Now

Thanks for the explanation. I agree that if the line in is connected to the outlet then the diagram is fine. Still not very clear in my book.

I understand your reasoning on the fuse/switch thing also, but I have to say that after just under five DECADES of service my original Bandmaster on/off switch is still going strong. If it were to fail with a short to the frame it would trip the circuit breaker on the line. Not a big problem.

[Enzo]

Oh a crumbling switch is certainly not a common problem, but in my decades of running a service bench, I have repaired exactly that.

This is a philosophical point that can be debated with nice rationalizations for doing it either way. In my own way of thinking, I want the fuse protecting as much as possible, and I don;t much see the need for the switch protecting the fuse.

We can debate esoterica all day, but the bottom line is that mostly what will happen is a power tube will fail and the fuse will blow. And for that, it doesn;t much matter what order the parts are in.


Now you want a real argument, let's talk about which way to hang the end of the toilet paper roll.

[JHow]

Now you want a real argument, let's talk about which
way to hang the end of the toilet paper roll.

Whatt!!! You hang it on something?!