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[John Roberts {JR}]

Without getting into modifying classic equipment, my thoughts are:

a] Both the switch and the fuse should be on the Hot conductor.
b] The Safety Ground/PE/EGC should connect to the chassis near where the power cord enters the chassis.
c] The connection needs to be very robust. It may need to carry 150 Amps or more until the circuit breaker trips.  Some say that a simple solder connection is not enough while in the next breath they say that a nut & bolt connection needs to be vibration proof.
d] The circuit/power supply common (ground) should connect to the chassis near the input jacks (not near the Safety Ground).

I guess I will be repeating myself a lot in this thread. 

The hazard I am trying to protect against is when a guitar player gets between two mains power drops (like back line and FOH) and one of those two drops has an energized safety ground (like RPBG).

That has happened and the musician was killed by modern properly ground bonded equipment.

I concede this is an obscure fault, but that's why it's not simple. 

JR

[Kevin Graf]

I am not optimistic about UL blessing an outlet strip that lifts the ground bond, but with the GFCI they might allow it with a skull and cross bones marking.
I like the idea of sensing for ground current and breaking all 3 lines... not as cheap and easy but might protect against more potential hazards.
JR

Bill Whitlock writes aabout a plug-in balanced isolation transformer:

Bill Whitlock, 9/4/2012 Overview of Audio System Grounding & Interfacing page 203

The “Ground Lift” Feature
Legally Disconnects Safety Ground … BUT
Because this unit has the required GFCI on its output outlets, it is legally
allowed to interrupt safety ground. Paradoxically, the same can’t be done
when a premises AC outlet is replaced with a GFCI type. NEC says that, if a
safety ground is available to the outlet, it MUST be connected … even if the
outlet is a GFCI protected one. Although this unit claims to eliminate ground
loops via the “LIFT” switch, there’s a catch ...

He never explains just what the catch is.

http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

[John Roberts {JR}]

An isolation power transformer is very expensive.

JR

[Ned Ward]

Bob - thanks for starting this post on what is a critical mod/upgrade to older amps. I've had the following done on all my Fenders, and believe something similar was done on my Showman...:

http://www.mylespaul.com/forums/squawk-box/103068-death-cap-2-prong-power-cords-caution.html

The only question to add to the mix - what about the courtesy AC outlet on the back? May be good to include that in wiring solutions (or if there's a strong reason not to, explain).

The ground switch left over leaves a great way to add if needed any changes without marring a vintage amp - no need to drill another hole in the chassis. While I haven't done this, I could see that being far more beneficial than drilling ugly holes every which where. Mine stay in for cosmetics only.

[Bob Leonard]

Any amps I've done for you Ned have been modified as needed, but the reason I don't disconnect or modify the accessory plug is because the only way to get a 3 prong cable into it would be to break off the ground. You're correct though, this is also an area that should be addressed. I think I'll work with the suggestions from the thread here and make some changes when I get the Twin Reverb on the bench next week. Maybe JR would even send me a schematic of the change he's proposing (HINT).

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........

[Mike Sokol]

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........

Does this help?

[Stephen Swaffer]

I like the idea of sensing for ground current and breaking all 3 lines... not as cheap and easy but might protect against more potential hazards.

Cost could still likely be comparable to a good surge protector-which begs the question which would you be willing to spend more money on-protecting the guitar player or his gear?

[Keith Broughton]

The only question to add to the mix - what about the courtesy AC outlet on the back? May be good to include that in wiring solutions (or if there's a strong reason not to, explain).

Disconnect from AC and leave it in for cosmetic reasons.

[John Roberts {JR}]

Any amps I've done for you Ned have been modified as needed, but the reason I don't disconnect or modify the accessory plug is because the only way to get a 3 prong cable into it would be to break off the ground. You're correct though, this is also an area that should be addressed. I think I'll work with the suggestions from the thread here and make some changes when I get the Twin Reverb on the bench next week. Maybe JR would even send me a schematic of the change he's proposing (HINT).

I finished a Pro last week, and man does it sound good. You need one, you are getting sleepy, repeat after me, Ned needs a Pro Reverb.........

Bob: here is a mark up of what I am proposing for use in combination with a GFCI power source.

My speculation is that connecting the chassis to ground through a stinger cap "should" protect human from death from RPBG and external hot ground hazards. It does not protect against an internal HV PS fault inside chassis.

BTW is that amp UL approved? Ungrounded chassis (2-wire)  products generally use higher insulation standards inside transformer primary, etc. 

JR

[Keith Broughton]

Been looking at the schematics ...what exactly does the stinger cap do and is it required when doing a 3 wire grounded AC cable mod?

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