65 Twin Reverb Re-issue: what are these things?

[rockman627]

This had 2 scratchy volume pots so of course I had to remove a circuit board just to spray the pots. Now the volume on the Vib channel is very low. But that's not my question. What is the pot near the pilot light? And why does this have a ground switch that is actually connected? The Deluxe Reverbs I've seen have just have an unconnected switch .

[Helmholtz]

The pot looks like a "humdinger", i.e. a heater balancing pot. This can only work if the heater winding has no ground connection, so make sure.

The ground switch I dunno. Probably something stupid.
Best make a drawing of its wiring.

[Mick Bailey]

Agreed, it's a 100 ohm heater balance pot. The service manual shows the ground switch, but the terminals are labelled NC (not connected). It's odd that your switch is wired.

[g1]

The service manual shows the ground switch, but the terminals are labelled NC (not connected). It's odd that your switch is wired.

Can you upload that one? I have one that shows the hum balance pot, but no ground switch.

Edit, never mind, found one with both showing ground switch connected.

[rockman627]

I've heard of humdingers, this is the first time I've seen one. I thought ground switches were obsolete. In this day of 3 wire AC cords what does this ground switch do?

[Mick Bailey]

Here's the schematic showing the ground switch disconnected;

[SoulFetish]

I thought ground switches were obsolete. In this day of 3 wire AC cords what does this ground switch do?

The simple answer is in most guitar amps, ground switches are obsolete, and any existing capacitor should be removed.
However, there is a special class of capacitor designated "Class Y" which can be safely installed between AC line or neutral and ground. Class Y is specifically designed to go open in the event of catastrophic failure, and should not present a shock hazard. These can be helpful in suppressing RFI and EMI, in certain applications.

[Tom Phillips]

I just finished work on a Twin Reverb Reissue that was manufactured in 1996. It has an unconnected ground switch installed and there is no evidence that it ever was connected. It's just there to fill the hole. I have seen this before too.

[g1]

ground switch installed and there is no evidence that it ever was connected. It's just there to fill the hole. I have seen this before too.

We liked to think of those as coming equipped with a spare switch.
I'm guessing it may have been a violation of certain safety certifications, and may have been connected in some countries and not others. As there seems to have been a bit of back and forth with the schematics since then (connected/not connected), maybe the safety requirements also changed.

[Tom Phillips]

Let's call it a placebo switch. Or re-label it Talent ON-OFF to really make it interesting.

[The Dude]

Maybe it's not rated high enough to be a "Talent" switch.

I do find it interesting, in these times, that a company would install an unused switch. They won't usually give you an extra half inch of wire to lift up a circuit board.

[Mick Bailey]

Maybe they thought it added authenticity to a reissue.

[rockman627]

I mentioned the vib channel had low volume. I spent a whole workday trying to figure why there was continuity between lugs 1 and 3 on the volume pot. It turns out if the tone pots are off, the volume goes way down. Just before quitting time I remembered reading about some amps need the tone controls on to hear anything. That was it, I closed it up, tested again, and told my boss I figured out the problem.

[Tom Phillips]

I mentioned the vib channel had low volume. I spent a whole workday trying to figure why there was continuity between lugs 1 and 3 on the volume pot. It turns out if the tone pots are off, the volume goes way down. Just before quitting time I remembered reading about some amps need the tone controls on to hear anything. That was it, I closed it up, tested again, and told my boss I figured out the problem.

And that first hand experience is the way we learn things that we will never forget. There have been more than one customer who brought me an amp with a problem caused by the same situation. This applies to all Fender type tone stack and similar designs that include treble, middle and base controls. To help the user remember I ask them "What have you got if you completely turn down the treble, middle and base." Answer = basically nothing. They get that concept. Situations like this are why I like to do a sound check with the customer when they drop off the amp. Sometimes this happens when someone other than the owner has used the amp.